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Figure 5. Si7022 Functional Block Diagram
Rev. 1.7 5 3. System Design Considerations 3.1. Power Requirements The Si70xx devices are inherently low power if the heater is not used. For example, Si7013 power consumption during a conversion is 150 µA typical, and the power consumption in idle mode is less than 100 nA. This low power consumption means that there is no significant self-h eating in the conversion process. Because the Si7007, Si7022, and Si7023 PWM output parts do humidity and te mperature conversions twice a second to update the PWM output the power consumption if these parts is high er at 180 µA typical. Generally this still does not cause significant heating. 3.1.1. Battery Life Considerations Normally, battery life is rated in milliamp -hours (mAh). For example, an Energizer ® E91 battery is rated for approximately 2500 mAh for light loads when allowed to discharge to 0.9 V. Considering the case of an Si7013, which has a V DD range of 1.71 to 3.6 V, two AA batteries in series provide 3.0 V when charged and 1.8 V when fully depleted, so this is acceptable. Again, for the case of an Si7013, a humidity and temp erature conversion typically consumes 150 µA. In normal mode, the total time for an RH and temperature conversion is 8.4 msec, and, in fast mode, this is reduced to 5.5 msec. A temperature conversion is performed every time an RH conversion is done (to allow for temperature correction). If a conversion is performed once per second, then 31.5x10 6 conversions occur per year. The number of milliamp- hours is as follows: Plugging in values, this translates to 11.1 mA-hours for one year in normal mode and 7.2 mA-hours for fast mode. The 100 nA quiescent current of the Si7013 will consume an additional number of milliamp hours, as shown below: At 100 nA quiescent current, this is 0.876 mA-hours per year. Even when targeting a 10-year battery life, the sensor itself would consume less than 120 mA hours of charge in normal mode. This means that only about 5% of the available 2500 mAh is consumed by the sensor. Of course, in a practical system, there are many other drains on a battery, and, often, coin cells, which have much less capacity than AA cells, are used. On the other hand, it is generally acceptable to reduce the conversion rate well below once per second, meaning that a sensor, such as the Si7013, will generally consume a small portion of the available battery life. milliamp hours (mAh) number of conversions conversion time (msec) conversion current (mA) milliamp hours (mAh) quiescent current (nA) 8760 (hours/year) 106 nA/mA
6 Rev. 1.7 3.2. Temperature and Humidity Sensor Placement The following are general guidelines for sensor placement: For accurate humidity and temperature measurement, keep heating from other components to less than 0.2 °C, or compensate for heating. For rapid response to air temperature changes, keep the thermal mass that is attached to the sensor low, and insulate the sensor from larger system thermal masses. For rapid response to humidity changes, keep the sensor exposed to the ambient, or make the opening comparable in size to the cavity in which the sensor resides. Do not use materials, such as soft plastic, glue, or wood, in the vicinity of the humidity sensor since these can absorb or emit moisture as well as give off fumes that affect the senor reading. Protect the sensor against liquids and dust using the factory-installed ePTFE filter or similar cover. Protect against ESD with exposed ground metal. For the humidity sensors, if the sensor is not in a cavity, use the ePTFE filter cover to prevent ESD directly into the sensor area. Use conformal coating material on the leads, or use ESD diodes on all pins. Unused pins may be connected to VDD for ESD protection. For situations in which it is not possible to completely insulate the se nsor for the system, insulate as much as possible and use a temperature sensor connected to the system to allow compensation of residual heating. If a thermal model can be developed for the overall system, it is also possible to apply an inverse filter and speed up the response of the overall system to changes in the ambient. These are discussed in more detail below. 3.2.1. Place the Sensor Away from Heat Sources As air is heated or cooled, humidity will be reduced or increased by appr oximately 5% per degree. That is, increasing temperature by 1 °C will reduce the relative humidity of 100% humid air to 95%RH or 50% humid air to 47.5% RH. This “rule of thumb” is useful for estimating the effect of small temperature increases. The first consideration in trying to measure humidity or te mperature outside of an enclosure is to place the sensor away from any heat sources internal to the enclosure or to thermally insulate the sensor from the internal heat source so it is better connected to the ambient envir onment than internal heat sources. For accurate humidity measurement, heating from other sources should be limited to no more than 0.2 °C. In some cases, where the amount of heating is known or can be measured, it is possible to compensate for the heating or cooling. For larger temperature increases, the “Magnus” equation can be used to more accurately calculate the change of humidity for a change in temperature (see also appendix B). 3.2.2. Thermal Mass and Thermal Resistance When the IC is soldered down, it becomes thermally connected to the printed circuit board on which it is mounted. The printed circuit board is, in turn, thermally connected to the system it is mounted within. The time constant to respond to changes in ambient air temperature depends on the effective thermal mass the device is connected to as well as the effective thermal resistance. If the entire system (such as a thermostat) can be expected to go up or down in temperature along with the ambient it is measuring, then separating th e sensor from the system is not necessa ry, but response time will generally be slow. A general model for this is shown in Figure 7.
Figure 7. General Thermal Model for Sensor Placement with some specific numbers is discussed in "Appendix E— Thermal Model for a Sensor on a Paddle" on page 38. affected by the following factors. ambient temperature in order to accurately measure humidity. Generally, the opening should be comparable in size of the cavity in order to avoid slowing of the response. Porous materials within the cavity can absorb or emit humidity and dramatically affect the response. plastic, metal, or hard rubber. Any sealants should be fully cured. ePTFE is often known under the name brand Gore-Tex®.
8 Rev. 1.7 Most of the Si70xx humidity and temperature devices are offered with a factory installed cover that is rated IP67. The cover is solder resistant (it can withstand a peak tem perature of 260 °C) and has a pore size of 0.25 µm, so it blocks all dust while passing water vapor. The cover will block liquid water at pressure equivalent to over one meter depth. As a practical test of the effectiveness of the cover, Si7021 devices were subjected to a cigarette smoke test. In this test, Si7021 devices were placed in a 3 liter jar with 20 lit cigarettes. During the time the cigarettes were burning, a small air gap was allowed so the cigarettes would not extinguish. After burning was complete, the air gap was removed, and the parts were allowed to sit in the smoke for 24 hours. Use of the cover reduced the maximum shift from this extreme exposure from almost 40% RH to ab out 6% RH. In another test in which 100 cycles of condensation were allowed to form on the parts, the cover reduced the shift in reading from an average of 2.0% to 1.57%. The cover has a minimal effect on response time. 3.2.5. Compensation for Heat Sources and Optimizing Transient Response In some cases, it is not po ssible to completely isolate the sensor from the system for aesthetic reasons, and it is desirable to still obtain a fast an d accurate measurement of the am bient temperature and humidity by compensating for the effects of the system. In these cases, it is best to optimize the sensor loca tion as much as possible using the above guidelines. To compensate for the system effects, either the system mu st have a known effect, or an additional temperature sensor (typically a thermistor) is ne eded. The thermistor should be placed to measure the temperature of the system in the place where it is having the most influence on the humidity and temperature sensor. In these cases, the Si7013 se nsor is a good choice because it has an auxiliary A/D for digitizing the thermistor voltage and a linearization engine for converting this to temperature. 3.3. Dealing with Condensation and High Humidity Prolonged exposure to high humidity will cause gradual drift of th e humidity sensor readin gs. All members of the Si70xx family have on-chip heaters that can be used to heat the chip to counter local high humidity and reduce this drift. Condensation will also cause erroneous readings. If the co ndensation forms on the pol yimide film, there can be permanent shifts in sensor accuracy due to residue left after evaporation. The hydrophobic filter prevents liquid water from penetrating, so condensation on the outside of the part will generally not result in condensation on the polyimide sensing film of the Si70xx. However, if condensation forms on top of the filter, readings will be high until it evaporates. Also condensation on th e PCB can affect reliability and signal integrity. Turning on the heater will reduce the chance of condensation forming and will also evaporate condensation. However, turning on the heater will affect the local relative humidity (see also “Estimating RH with Heating” on page 28. For example, turning on the heater with the control setting 0x3 heats the sensor about 5 °C (depending on PCB design and airflow), which results in a ~30% drop in local RH. However, due to variability in air flow and heater current, the Si70xx heating can vary ±2 °C making RH readings with the heater on unreliable. Depending on the nature of the application, there are several ways of dealing with this: The amount of heating can be measured or characterized. For example, turn the heater on and off a few times, and use the on-chip temperature sensor to measure the amount of heating. The air ambient temperature can be sensed with a separate sensor, and RH can be calculated While the RH reading is not accurate, the dew point reading is fairly accurate (although generally about 1 °C low) with the heater on. If dew point is the only concern, it can be calculated from the humidity and temperature and then subtract 1 °C. The Si7006, Si7013, Si7020, Si7021, and Si7034 have opti ons for increasing the heater current up to 94 mA with VDD = 3.3 V. Depending on the PCB layout and thermal design, it is possible to get junction temperatures well in excess of 100 °C. Shifts in sensor readings from previous exposure to high humidity can be reversed by turning on the heater with a sufficiently high setting to get the chip temperature over 100 °C for approximately 24 hours.
Rev. 1.7 9 3.4. PCB Layout The Si70xx should be thermally isolated from the equipmen t connected to it to prevent heat from the equipment from affecting RH. The Si70xx should be thermally immersed in the ambient environment it is intended to sense. One strategy for accomplishing this is to put the Si70xx on a small PCB and run a ribbon cable to the host processor. The small PCB should be placed away from heat sources and should be placed in the ambient environment as much as possible. That said, even with the hydrophobic filter, keeping dust, liquids and cleaning agents away from the sensor is required. 3.5. Design and Bring-Up Checklist Be sure the sensor is placed away from heat sources and exposed to the environment being measured. Prevent the active area of the sensor from being exposed to liquids, dust, and other contaminants as well as sunlight or other UV sources. The optional filter cover available with Si70xx parts serves this purpose and is compatible with soldering. Generally, avoid the use of ground planes around the Si70xx, which could conduct heat from external sources. Route the ground connection. Do not connect unused pins. Make sure the CSb pin of the Si7015 is low prior to starting I2C communications. The Si7005 should not be on the same bus as other I2C devices when it is active. It acknowledges data bytes that match its address. This issue has been resolved with other members of the Si70xx family Be sure to meet all of the timing and level requirements of the device. The Si7005 can tolerate SDA or SCL higher than VDD and has 8.5 mA drivers. The Si7006, Si7013, Si7020, and Si7021 have 2.5 mA drivers and do not tolerate I2C pins higher than VDD. The Si7034 is a 1.8 V part but can tolerate 3.3 V on its inputs. It's drive strength is 1.5 mA. See also application note, “AN883: Low-cost I2C Level Translator” for a low cost I2C level translator circuit. Route the I2C signals away from analog nodes and noisy digital nodes. Use 0.1 µF bypass capacitors on VDD placed close to the sensor. Pay careful attention to I2C protocols, such as start and stop conditions, the repeated start of a read transaction, and proper treatment of the Acknowledge bit. Allow adequate time for initialization (per data sheet). If the optional thermistor sensing of Si7013 is used, make sure the thermistor is thermally isolated. If there are long leads to the thermistor, use a twisted pair. Avoid noise pick up; use either a shield or capacitive filter. 3.6. Si70xx Self Test The following steps define a reliable test of the Si7006, Si 7013, Si7020, Si7021, and Si 7034 family that uses the integrated heater: 1. Read and write all I2C registers checking for expected values and capability of modifying where appropriate 2. Perform an RH and temperature measurement. 3. Turn on the heater and wait 60 seconds. 4. Check for delta temperature with heater on. This can be adjusted changing the heater setting. A setting of 0x3 will give over 3 °C. 5. Check for delta RH is > RHinitial 4x (delta temperature in °C).
10 Rev. 1.7 3.7. ESD Considerations It is desirable to expose the Si70xx sensor to the environment. For the sensor to respond to the environment there must be a way for the air being sensed to reach the sensor (the environmental access port). This means that the sensor may also be exposed to ESD as specified in IEC 61000-4-2 with ESD peak voltage of up to ±15 kV. For the humidity sensors, when the cover is not used and the ESD source is directly over the package opening, it is possible for ESD to arc into the sensor area and cause damage. This ca n be avoided by using of the Silicon Labs filter cover or by placing the Si70xx so that the sensor opening is offset from the environmental access port. The above approaches will prevent ESD discharge into th e sensor area, but ESD discharge to the leads may still be possible. The best practice for ESD protection of the leads is to arrange the sensor placement and environmental access so that high-level ESD events will preferentially be directed to ground (i.e., have an exposed ground trace or ground shield closer to the environmental access port than the sensor). If grounded, a metal case is used; this is also effective for ESD protection. If it is not possible to protect the leads of the device from ESD, unused leads should be connected to VDD. High- quality ESD protection diodes can be used on leads that have signals on them. The ESD protection device should be rated for more than 15 kV immunity and should limit ESD voltage peaks to less than 10 V. Some examples that have been tested include Vishay MSP3V3 and COMCHIP CPDQ3V3U-HF.
- Humidity and Temperature Sensor Special Handling Considerations
humidity sensor readings. Even very small particles can have a significant effect. Generally, materials that outgas or give off an odor have the potential to affect sensor performance. be effective if the fume concentration is not high. Use low volatile organic compound (VOC) materials. Immediately cure the material in a well-ventilated environment. different vendor, it will be acceptable for the Si70xx devices. soldering can shift senor accuracy outside of data sheet limits. Avoid the use of hot air rework tools. Figure 8. Limit Solder Rework to Five Seconds or Less filter cover is not used, Kapton tape will serve the same purpose, although it has to be removed after soldering.
liquids and should not be removed after soldering. Figure 9. Do Not Remove Si70xx Protective Cover compounds or solvents. If installed, do not remove the white filter cover from the devices. contaminate the sensor area. Inert dust (e.g., talc) is essentially benign. Excessive amounts of dust can slow response. Contaminants or particles embedded in the polyimide can affect the RH accuracy. Certain polyethylene bags will outgas and damage the sensor. Bleach, hydrogen peroxide, ammonia, and other chemicals can affect or damage the sensor. recommended for the device to fully recover its accuracy.
Sensor Accuracy Including Hysteresis,” shows the result of a typical sweep. Figure 10. Measuring Sensor Accuracy Including Hysteresis Contamination of the sensor by particulates, chemicals, etc. Variations due to temperature. RH readings will typically vary with temperature by less than 0.05% C.
14 Rev. 1.7 4.7. Hysteresis The moisture absorbent film (polymeric dielectric) of the humidity sensor will carry a memory of its exposure history, particularly its recent or extreme exposure history. A sensor exposed to relative ly low humidity will carry a negative offset relative to the fact ory calibration, and a sensor exposed to relatively high humidity will carry a positive offset relative to th e factory calibration. This fact or causes a hysteresis effect illustrated by the solid trace in Figure 10. The hysteresis value is the difference in %RH between the maximum absolute error on the decreasing humidity ramp and the maximum absolute error on the increasing humidity ramp at a single relative humidity setpoint and is expressed as a bipolar quantity relative to the average error (dashed trace). In the example of Figure 10, the measurement uncertainty due to the hysteresis effect is ±1.0%RH. 4.8. Prolonged Exposure to High Humidity Prolonged exposure to high humidity will result in a gra dual upward drift of the RH reading. The shift in sensor reading resulting from this drift will generally disappear slowly unde r normal ambient cond itions. The amount of shift is proportional to the magnitude of relative humidity and the length of exposure. In the case of lengthy exposure to high humidity, some of the resulting shift may persist indefinitely under typical conditions. It is generally possible to substantially reverse this effect by baking the device as described in the following section. 4.9. Bake/Hydrate Procedure After exposure to extremes of temperature and/or humi dity for prolonged periods, the polymer sensor film can become either very dry or very wet, in each case the result is either high or low relative humidity readings. Under normal operating conditions, the induced error will diminish over time. From a very dry condition, such as after shipment and soldering, the error will diminish over a few days at ty pical controlled ambient conditions, e.g., 48 hours of 45 ≤ %RH ≤ 55. However, from a very wet condition, recovery may take significantly longer. To accelerate recovery from a wet condition, a bak e and hydrate cycle can be implemente d. This operation consists of the following steps: Baking the sensor at 125 °C for ≥ 12 hours Hydration at 30 °C in 75% RH for ≥ 10 hours Following this cycle, the sensor will return to normal operation in typical ambient conditions after 48 hours.
Rev. 1.7 15 5. An Introduction to Humidity Atmospheric air normally contains water vapor and can be thought of as a mixture of ideal gasses. Dry air (no moisture content) is the combination of approximately (on a mole basis) 78.09% N2, 20.95% O2, 0.93% Ar and 0.03% CO2 and trace elements. The amount of water vapor found in air depends on available liquid water (or ice), temperature, pressure and ranges from nearly zero to the point of saturation called “dew point (frost point)”. Water vapor enters the air by evaporation due to the vapor pressure of water or ice. 5.1. Vapor Pressure There are a few key concepts to keep in mind when disc ussing vapor pressure. Pure water vapor pressure, p, is due to water vapor pressure over water or ice without the presence of other gases such as air. In combination with air, the actual water vapor pressure is increased by a dimensionless factor referred to as a water vapor enhancement factor, f. This factor is a weak function of temperature and pressure and is approximately 0.47% at sea level and 20 °C. The actual vapor pressure of water vapor, p', is the pure water vapor pressure, p, multiplied by the enhancement factor, f. Dalton's law states that the total pressure of a mixture of gasses is equal to the sum of the partial pressures of each component gas and assumes the combination of gasses behaves like an ideal gas. In an ideal gas mixture such as air, the total pressure is the sum of the partial pressures of each gas. The pressure of air at any point can be calculated as follows. Note that pH2O is the actual vapor pressure of water in air referred to as p'. There are several useful equations when working with vapor pressure. The best one to use depends on the available information you have about the problem you need to solve, the range of conditions for the problem the degree of accuracy required and the computational re sources available. These equations and their range of application are discussed in detail in appendix B. Pressures referred to in this document are absolute (not gauge) unless otherwise specified. 5.2. Temperature The relative humidity value can change significantly with even slight variations in temperature. For example, a 1 °C change in temperature at 35 °C and 75% relative humidity will introduce a –4% RH ch ange. A higher temperature increases the ability of air to absorb moisture and a lower temperature decreases the ability of air to absorb moisture. Temperature changes can introduce moisture variations in an air mass if condensation occurs or through secondary impacts such as changing the moisture absorption or desorption of environmental materials in an enclosure. For humidity sensors that respond in proportion to relative humidity and not absolute humidity, the issue of temperature measurement error is not significant unless conversion to dew point, absolute humidity or any other measurement of water vapor in the air is required. In th e case of a dew point calculation, a 1 °C error in the measurement of the temperat ure will produce approximately a 1 °C error in the calculation of the dew point. This temperature dependency not only emphasizes the import ance of accurate temperature measurement, it also highlights the necessity of thermal stability, which can be di fficult to achieve. Even if the temperature and humidity measurements are taken in relatively close proximity, there can be considerable differences in corresponding levels of humidity and temperature. To achieve the most accurate measurement it is best if the humidity and temperature measurements are taken as close as possible to each other—ideally co-located on the same chip. p pf p 1.0047== pp N2 pO2 pH2O pAr pCO2++ + +=
have enough energy to escape the attractive forces holding the water together and evaporate into the atmosphere. place at the surface of the liquid while boiling can take place throughout the volume of the liquid. Figure 11. Water in Open Container
18 Rev. 1.7 6. How Humidity is Quantified Humidity represents the amount of water vapor contained in the air and can be quantified in many ways. Several of the terms describing humidity are defined differently for meteorology applications and thermodynamics or chemical engineering applications. For this reason, it is important to understand the context of the application. The following is a brief description of the most common terms for quantifying humidity. 6.1. Absolute Humidity Absolute humidity in the context of meteorological applications, sometimes referred to as “volumetric humidity”, is defined as the mass of water vapor dissolved in a total volume of moist air at a given temperature and pressure. Typical units are g/m3 or kg/m3. The value of absolute hu midity defined in this manner changes with temperature and pressure and is inconvenient to use in many engineering applications. Absolute humidity for use in thermodynamics or chemical engineering applications is defined as the ratio of water vapor mass to dry air mass. Typical units are kg/kg. Other names for this ratio include mass mixing ratio, humidity ratio, mass fraction or mixing ratio. This quantity is simpler to use and more accurate for mass balance or heat calculations. Due to the conflicting definitions of absolute humidity, caution is required when using this term. 6.2. Specific Humidity Specific humidity, Yw, can be defined for meteorology a pplications as the ratio of water vapor mass per mass of moist air expressed as g/kg or kg/kg. Specific humidity is constant with changes in temperature and pressure for conditions above the dew point and is a useful quantity in meteorology. The rate of evaporation of water is directly proportional to specific humidity. 6.3. Relative Humidity Relative humidity is the ratio of actual water vapor pres ent in air with the amount of water vapor that would be present in air at saturation, expressed as a percent age. The official symbol for relative humidity is although RH, %RH, rh, or %rh are commonly used. Relative humidity can be expressed as the ratio of the actual vapor pressure, p', to the saturation vapor pressure, ps’, at a constant temperature over a plane of liquid water. 6.4. Dew Point Upon heating, the capacity of air to absorb moisture increases. Consequently, the relative humidity of air decreases as the air is heated. Conversely, as moist air is cooled, its capacity to absorb moisture decreases and relative humidity increases. The dew point is the temperature, assuming constant pressure, moist air is saturated (reaches 100% relative humidity) and cannot absorb additi onal water. As the temperature is decreased past the dew point, moisture condenses until the air is saturated (reaches 100% relative humidity) at the new lower temperature. 6.5. Frost Point Frost point is the same as dew point over solid ice where the condensate is frost instead of liquid water.
Rev. 1.7 19 7. Humidity Measurement For relative humidity measurements, it is not necessary to measure the ambient temperature unless you are using a psychrometer. However, to determine the dew point or absolute humidity, the ambient air temperature is required. Accurate air temperature measurements can be a significan t challenge, since air is a poor thermal conductor and the temperature at any given point can be impacted by air currents and temperature gradients. It is very important to understand the dynamics of your measurement system and the dynamics of the environment being measured. Before taking a measurement, you always need to wait long enough to ensure the temperature and humidity are stable and the sensor(s) of the measurement instru ment are in equilibrium with the ambient conditions to be measured. A wide range of techniques are employed to measure humidity. These range from simple mechanical indicators, to highly complex and expensive analytic al instruments. In general, measuring humidity (dew point, absolute humidity, specific humidity, mixing ratio, relative humidity or equivalent wet bulb temperatur e) is not a trivial task. Many of the instruments currently available have poor accuracy, narrow-bandwidth, contamination issues, hysteresis and measurement drift over temperature and time. Regular calibration is required on some instruments, which is both inconvenient and expensive. Some instru ments are large, awkward, and expensive. Discussed below, are different methods of humi dity sensing. The new generation of humidity sensor technology used in Silicon Labs’ solid state humidity sensors provide superior accuracy, minimum drift, low cost, low power, small size and ease of use. 7.1. Psychrometer A psychrometer is the oldest method for measuring humidity more commonly known as the wet bulb/dry bulb method. A psychrometer consists of two thermometers, one with an ordinary dry bulb and the other with a moist cloth covering the bulb (wet bulb). Evaporation from the moist cloth lowers the wet bulb thermometer's temperature. The wet-bulb thermometer shows a lower temper ature, Twb, than the dry-bulb thermometer, Tdb, if the air is not saturated with water vapor. The temperatures are the same in saturated air. The amount of evaporation is dependent on the relative humidity of the air (more evaporation and lower Twb with lower RH). Given the dry bulb and wet bulb temperatures the relative humidity is looked up on a psychrometric chart. For example, at sea level if Tdb = 25 °C and Twb = 18 °C, RH = ~50%. Looking up the %RH on a chart for every measurement is both time-consuming and cumbersome but can be automated with a microcontroller. A psychrometric sensor can achieve good precision with %RH resolutions of 0.01% for humidity ranges from 10–100% at temperatures from 0 to 60 °C, and accuracy of 1%. The disadvantages of a psychrometric sensor are a slow response time, large physical size, the need to ke ep one thermometer bulb wet, the need to have airflow around the wet bulb, and high cost. 7.2. Chilled Mirror Hygrometers The chilled mirror hygrometer is considered the most accurate and reliable hygrometer. Chilled mirror hygrometers use a cooled mirror with an optoelectronic mechanism to detect condensation on the mirror surface at an accurately measured temperature. The system is confi gured to reflect LED light off a mirror at an angle of approximately 45 degrees with a photo- transistor detecting the reflected lig ht. The temperature of the mirror is electronically controlled, typically with a Peltier-effect device. The system works by cooling the mirror's surface below ambient temperature until condensation forms, causing the LED's light to scatter resulting in a sudden drop in the output of the photo-tr ansistor. The surface temperature of the mirr or is read using an accurate temperature sensor such as a thermistor. The temperature at which condensation forms is the dew point. All humidity values can be calculated from the dew point. The mirror temperature can be controlled with a feedback loop to continuously track the dew po int. The chilled mirror is the most stable an d accurate method to determine relative humidity. However, it is crucial to keep the mirror cl ean, provide a method of clea ring the condensation and to ensure that the temperature sensor and mirror are of high quality. This method operates over the full humidity range (0-100%RH) and can be used for numerous gases at many pressures. Ch illed mirror hygrometer instruments are bulky and very expensive.
20 Rev. 1.7 7.3. Mechanical Hygrometers Mechanical hygrometers use sensing elements relying on a mechanical property of the sensor varying with humidity. The most common example is the animal hair hygrometer, which uses a piece of animal hair kept under tension. As humidity increases the hair becomes more flexible and stretches. A strain gage monitors the displacement caused by the hair stretching with a change in the moisture content of the air. The output of the strain gage is directly proportional to the relative humidity and is usually indicated on a mechanical meter. Mechanical hygrometers are generally compact, light weight, reliable, and inexpensive. Accuracies, however are in the ±10% range. 7.4. Electronic Humidity Sensors Electronic humidity sensors typically use either a change in resistance or capacitance to measure humidity. These sensors have become a popular choice because technology advances have made them accurate, compact, stable and low power. A capacitive sensor consists of two electrodes, separated by a dielectric. As the water vapor in air increases or decreases, the sensor's dielectric constant changes producing a higher (or lower) capacitance measurement corresponding to the humidity level. A resist ive sensor consists of two electrodes, separated by a conductive layer. As the humidity in the air increases (or decreases) the conductivity of the sensing layer changes, altering the resistance between the two electrodes. New techniques for producing thin films have made these types of sensors, accurate, stable, and easy to manufacture in large quantities. The choice of material assures fast response times with little hysteresis. The accuracy of electronic sensors is limited by their drift over time caused by wide variations in temperature and humidity or the presence of pollutants. The Si70xx humidity senors use a MiM capacitor as the re ference and perform a high-accuracy 24-bit conversion using a sigma delta conversion approach. Each part is fa ctory calibrated for capacitance to RH with a minimum offset and slope correction. Later me mbers of the family include non-linearity correction and temperature compensation. Silicon Laboratories humidity sensors use the capacitance change due to moisture absorption of a polyimide film to sense humidity. The polyimide film is deposited over a metal finger capaci tor and exposed to the ambient via an opening in the package. The polyimide material and sensing capacitor has been selected for excellent stability. An optional expanded polytetrafluoroethylene (ePTFE) hydrophobic filter provides protection against dust and most liquids. The polyimide film is thin (<5 µm) and very responsive to humidity (response time of less than 10 seconds). The hydrophobic filter has little impact on the response time. While the Si70xx sensors are largely conventional, mixe d-signal CMOS integrated circuits, relative humidity sensors in general, and those based on capacitive sensin g using polymeric dielectrics in particular, have unique application and use requirements that are not common to conventional (non-sensor) ICs. Chief among those are: The need to protect the sensor during board assembly, i.e., solder reflow, and the need to subsequently Rehydrate the sensor. The need to protect the senor from damage or contamination during the product life-cycle. The impact of prolonged exposure to extremes of temperature and/or humidity and their potential effect on Sensor accuracy. The effects of humidity sensor “memory”. Each of these items is discussed in more detail in the following sections.
- Humidity Control for Thermal Comfort
between 40–60% relative humidity in the occupied space. evaporative cooling. The body may overheat, resulting in discomfort. low relative humidity allowing for effective cooling. Table 1. Human Reaction to Humidity* Simple Conversion and Applications”, American Meteorological Society February 2005.
is calculated with the following equation2.
- www.shorstmeyer.com/wxfaqs/humidity/humidity.html
speeds greater than 4.8 kilometers/hour (3.0 mph). Wind chill is calculated by the following equations4.
- http://web.uvic.ca/~eos340/wind_chill.pdf
Table 2. Human Reaction to Heat Index, HI, in Shady Light Wind Conditions*
collectively as “apparent temperature” or “relative outdoor temperature”. winter in centrally heated buildings, may lead to desicca tion of some materials causing them to become brittle. splits in the wood. The ideal temperature and relative humidity will vary depending on the material and application. during shipment or storage for food and a variety of other materials. Table 3. Wind Chill Temperature Chart* *Note: The shaded area indicates a danger of frostbite.
24 Rev. 1.7 APPENDIX A—I NDUSTRY S PECIFICATIONS AND GUIDELINES A.1 ANSI/ASHRAE Standard 55 A.2 BS1339 BS 1339-1:2002 Part 1: Terms, definitions and formulae BS 1339-2:2009 Part 2: Humidity calculation and tables - User guide BS 1339-3:2004 Part 3: Guide to the measurement of humidity U.K. National Physics Laboratory Guide to RH Measurement http://www.npl.co.uk/publications/good-practice-online-modules/humidity/ The following topics can be found on Wikipedia: Relative Humidity Dew Point I2C IP Rating Polymers Wave Soldering Si70xx certificate of compliance (with web link)
Rev. 1.7 25 APPENDIX B—E QUATIONS FOR V APOR P RESSURE AND HUMIDITY C ALCULATIONS Many equations have been developed to express humidity parameter relationships. They can generally be broken down into two groups, those developed from thermodyna mic principles and equations empirically derived from experimental data. The Clapeyron and Clausius-Clapeyron equations will be pr esented as examples of the first group and the Sonntag, Magnus and Antoine equations as examples of empirically derived expressions. The empirically derived expressions are generally easier to us e but have limitations to the range of use and accuracy. Approximations may be useful to further simplify calc ulations and conversions between humidity parameters as long as the limitations imposed by the approximations are fully comprehended. The use of these equations and approximations will be discussed below. Clapeyron Equation The Clapeyron equation is based on on e of the Maxwell thermodynamic relationships contains no approximations and provides an exact solution. It considers saturati on pressure and temperature, the change of entropy associated with a change of phase and the change in volume of the two phases and represents the slope of the vapor-pressure curve. The Clapeyron equation can be expressed as follows: The Clapeyron equation is valid for all phase transitions (s olid/liquid, solid/gas and liquid/gas) and represents the slope of the phase change boundaries. The parameters in this equation that can be directly measured are temperature, pressure and volume. Entropy and enthalpy can only be measured indirectly in terms of the other parameters. p where: p Saturation Vapor Pressure T Temperature in °K s Entropy Change between the two Phases v Volume Change since: s h where: h Enthalpy Change between the two Phases The Clapeyron Equation can be rewritten as follows:
26 Rev. 1.7 Clausius-Clapeyron Equation The Clausius-Clapeyron equation modifi es the Clapeyron equation with two simplifying approximations that make this equation useful for ice to water vapor and liquid wate r to water vapor transitions. The first assumption is that the change in volume from liquid water to gas (water vapo r) or solid (ice) to gas (water vapor) is approximately equal to the volume of the gas (water vapor). The second approximation is the gas (water vapor) can be treated as an ideal gas. Incorporating these assumptions in the Clapeyron equation yields the following. Rearranging: Integrating as an indefinite integral and assuming h is constant: Where: h = enthalpy change for phase change which varies between 2.501 x 106 and 2.257 x 106 J/kg in the range of 0–100 °C R’ = universal gas constant R = specific gas constant for water which is 461.5 J/(K – kg) Rearranging: Accuracy is best around the temperature used to calculate C2. For example C2 = 2.53 x 1011 Pa at 0 °C VV gas Vliquid– Vgas VV gas Vsolid– Vgas= Vgas nRT p h TnRT RT2 dp h pd T--- C1+= p C2e h– T---
Rev. 1.7 27 Humidity-Related Calculations While the above expressions are physically based, they are difficult to solve and manipulate. Hence, many approximate formulas have been developed. The two most common are the Antoine Equation and the Magnus equation. Many of these formulas contain coefficients that can vary depending on their source, its age and, in some cases, the context of the equations use. Antoine Equation This equation calculates saturation va por pressure. Additionally; coefficients are available for this equation for a wide variety of vapors other than water. The coefficients used in the following equation are for an air-water system and are optimized for use over the temperature range 0 to 100 °C. Magnus Equation This equation calculates saturation vapor pressure as does the Antoine Equation. It has the advantage that it can be easily manipulated to find the dew point (tdp) or frost point. Where: P’s is pressure in Pascals Nm–2 T is temperature in °C Over the range of –40 to +50 °C, the best fit constants are as follows: For air with a vapor pressure P’, the dew point is defined as the temperature at which the water vapor would be saturated. Thus, Utilizing the relationship RH = 100 x P’/P’s A1 B1 C1 Reference Approximation of Saturation Vapor Pressure. J. Appl. Meteor., 35, 601–609 ps 23.19 3830 exp= tdp 3830 Where: Pressure in Pascals Nm 2– Temperature in °K (°C + 273.15) PsC 1e A1T Td B1 In P A1 P ln–
28 Rev. 1.7 Equation Comparison The following graph compares the above equations: Mark Lawrence Rule of Thumb* This easy to use equation provides an estimate of change in dew point for a change in relative humidity or can be easily reversed to estimate a change in relative humidity for a known change in dew point. This approximation is valid for RH>50%. This rule of thumb says that the de w point temperature decreases approximately 1C for every 5% decrease in RH starting at tdp = t and RH=100%. This relationship is very handy if very little computational capability is available and the accuracy limitations and range of applicability are acceptable. *Note: See Lawrence, Mark G., “The Relationship between Relative Humidity and the Dewpoint Temperature in Moist Air - A Simple Conversion and Applications”, American Meteorological Society February 2005. Td B1 In RH A1T A1 In RH – A1T
Rev. 1.7 29 Estimating RH with Heating Equation Development The Magnus equation for partial pressure of water in air is: for a given relative humidity RH in percent and temperatur e in ºC. Typical values for A1, B1, and C1 are 17.625, 243.04, and 610.94, respectively. If the air is heated, the partial pressure does not change, and the apparent relative humidity drops according to: or this can be simplified to: P RH C1e T P RH C1e T T+ RH RH e A1 T e T T+ RH RHe A1– B1 T
30 Rev. 1.7 Linearization The above equation is still too complex to be useful in simple systems; however, it can be noted that, over a narrow temperature range, the relative humidity error is fairly linear with relative humidity for a given amount of heating and ambient temperature. The error is linear with RH and increases by about 5% RH per °C (thi s is the familiar “Mark Lawrence Rule of Thumb”). Thus, the actual RH can be estimated fairly accurately by: The accuracy of this estimate can be improved by meas uring the temperature of the RH sensor and correcting for heating to get the ambient temperature. For 5 °C heating, the correction factor varies from 0.0598 at 0 °C to 0.0435 at 50 °C ambient or and finally, RH RHmeasured Tambient Tmeasured T–= CF 0.0598 0.000346– Tambient= RH RHmeasured
Absolute vapor pressure— A measure of the actual amount of water present in the air. were cooled; meaningful as an indicator of comfort. Hydrophobic— Water repellent/resistant. digit represents level of protection against liquids. Oleophobic— Oil repellent/resistant. Relative Humidity— Absolute_Vapor_Pressure ÷ Saturated_Vapor_Pressure; expressed as a percentage. Table 4. Common Pressure Unit Conversions
Table 5. Humidity Terms and Definitions
Rev. 1.7 33 APPENDIX D—N ONLINEAR C ORRECTION OF VOLTAGE INPUTS WITH THE Si7013 The Si7013 has the capability to apply a lookup-tab le-based non-linear correction to voltage measurements. This correction is invoked by writing a “1” to bit 5 of user register 1. Note that humidi ty measurements should not be performed when this bit is set. In the discussion below, “input” refers to the A/D voltage measurement result, which is a 16-bit signed integer, and “output” refers the output after the non-linear correction, which is assumed to be a 16-bit unsigned integer. The non-linear correction is based on a 10-point table lookup linearization. Each point consists of the ideal output for a given expected A/D measurement result. Table 6 is st ored in the Si7013 memory, which must also have the slope at points 1–9. Slope is multiplied by a scaler of 256. Only nine of the input/output pairs nee d to be in the table because the 10th pair is determined by the slope equation. Overall, the Si7013 has 27 16-bit numbers in its table (54 bytes). This table is stored in non-volatile memory of the Si7013 and must be programmed based on the desired look-up table. The actual output is determined by extrapolation: If in >in2, out = out1+slope1 x (in-in1)/256 Else if in >in3, out = out2+slope2 x (in-in2)/256 Else if in >in4, out = out3+slope3 x (in-in3)/256 Else if in >in5, out = out4+slope4 x (in-in4)/256 Else if in >in6, out = out5+slope5 x (in-in5)/256 Else if in >in7, out = out6+slope6 x (in-in6)/256 Else if in >in8, out = out7+slope7 x (in-in7)/256 Else if in >in9, out = out8+slope8 x (in-in8)/256 Else out = out9+slope9 x (in-in9)/256 slopeN 256 outputN 1 outputN– +
non-linear correction data is saved to memory, it cannot be overwritten. As an aid to calculation of the table, several tools have been developed. Table 6. Memory Location Descriptions
The values highlighted in gray would be the table entries for the Si7013. Table 7. Example Non-Linear Correction to Thermistor Voltage Measurements
Table 8. Example Non-Linear Thermistor Correction Entries into Si7013 Memory
Rev. 1.7 37 The command code, 0xC5, is used for programming; so, for example, to program a Si7013 at slave address 0x40 with the values above starting with 0x4C to memory location 0x82, one would write: <Start Condition> 0x40 W ACK 0xC5 ACK 0x82 ACK 0x2E ACK <Stop Condition> <Start Condition> 0x40 W ACK 0xC5 ACK 0xB7 ACK 0xD6 ACK <Stop Condition> After writing all values, the chip should be reset by a power cycle or I 2C reset command before attempting to use the correction mode of the voltage-sensing circuit. It is not possible to change the values once written. Ho wever, to verify the values were written properly, use command 0x84. In the above example, to verify that 0x2E was written to location 0x82, use: <Start Condition> 0x40 W ACK 0x84 ACK 0x82 ACK <Start Condition> 0x40R ACK 0x2E NACK <Stop Condition> where 0x2E is the expected return value of the read transaction. How to Choose Linearization Points The table must be arranged in order of decreasing values for the input. The slope can be positive or negative, and it is assumed that only positive output values are desired. Generally, the input/output pairs should be chosen around the area where greatest accuracy is desired. If the transfer function is highly nonlinear and the more closely spaced the po ints, the less error there will be from interpolation. However, if accuracy is desired over a wi de range, the input/output pairs should be spaced evenly over the range of interest. The spreadsheet allows estimation of errors from the extrapolation process. Other Register Settings The voltage measurement configuration is determined by user register 2 of the Si7013. For thermistor measurement, it is assumed that the A/D input buffers will be used and that the A/D reference is VDD. The buffers are needed since the thermistor impedance is fairly larg e. VDD is used as a reference because the thermistor is biased relative to VDD. Thus, for a “hold master” measurement (SCL is held low during the measurement phase), Register 2 would be set to the following: Noise and AC Pick Up The A/D of the Si7013 is a delta sigma type converter, an d the input is not sampled. Thus, it is assumed that the input voltage is constant over the measurement period. Generally, some amount of analog filtering prior to the A/D input is desirable. In the standard application circuit, this is accomplished with 0.1 µF capacitors. These capacitors will form a filter at about 30 Hz, which is adequate for high -frequency noise pick up (e.g. am radio signals) but not for 60 Hz. If 60 Hz filtering is desired, these capacitors can be increased to >1 µF, or the result could be digitally filtered (average of several measurements). If the sa mpling can be synchronized to 120 Hz (use the faster conversion time for this), then an average of just two samples would reject 60 Hz. In the standard biasing circuit, the bias can be turned off between measurements to save power (change bit zero of user register 2). If this is done, allow adequate settling time between enabling the bias and making the measurement (use approximately 100 msec for the 0.1 µF filter, which has a time constant of 30 msec). D7 D6 D5 D4 D3 D2 D1 D0 00 1 0 01 1 0 / 1 Not Used Hold Master Enable Linearization 7 msec Conversion Time Not Used Enable Buffer VDD is Reference Switch High or Low to Bias the Thermistor
Rev. 1.7 39 The thermal impedance to air for a standard FR4 PCB is about 1000 C/W per cm 2 of PCB area. With two sides exposed to the ambient a total of 18 cm2 is connected to the ambient. This makes R1 55.5 C/W. The mass of 9 cm2 of PCB material is about 2.5 g (the specific gravity of FR4 is 1850 kg/m3 and assuming 1.5 mm thickness) and the specific heat capacity of PCB material is 0.6 J/(g-C), so the heat capacity is 1.5 J/C. The time constant R1 x C2 is 1.5 J/C x 55.5 C–second/J = 83 seconds. This time constant is independent of the PCB area— more area means lower thermal impedance but higher thermal mass. To improve the time constant beyond this thinner PCB material would have to be used or there would need to be fins or airflow to reduce the thermal impedance. Turning our attention to R2 the thermal conductivity of FR 4 material in plane is around 1Watt/meter-C. So, for the example of the connection of the sensor area to the rest of the system by 1cm wide 3cm long 1.5mm thick FR4: Conductance = thermal conductivity x area/length = 1 Watt/meter- °C x 10–2m/cm x 1 x 0.15/3 = 0.05 x 10–2W/°C Or thermal impedance (1/conductance) is 2000 °C/W (this is R2). This is assuming minimal copper routing on the connector material. With this design since R2 is 36 times R1, so the system heating will have a fa irly minor effect. For example if the system heating is 10 °C the sensor temperature would only increase 0.3 °C. If more thermal connection is tolerable the connector area could be made shorter or wider or the PCB area the sensor is connector to could be made smaller. This example was intended to illustrate the thermal design considerations for good response to ambient conditions and insulation from the system. In some cases, it is not possible to place the sensor in sufficient thermal contact with the environment to shield it for the thermal mass and heat sources in th e system. In this case, it is often possible to compensate for the system by placing an addi tional temperature sensor in the system. However, in all cases, the thermal contact of the sens or to the environment should be maximized, and the thermal contact of the sensor to the rest of the system should be minimized.
40 Rev. 1.7 DOCUMENT CHANGE LIST Revision 0.1 to Revision 1.0 Updated storage, handling, and assembly instructions. Corrected Table 4, “Common Pressure Unit Conversions,” on page 31. Revision 1.0 to Revision 1.1 Multiple updates to include Si7013, Si7020, and Si7021 parts. Revision 1.1 to Revision 1.2 Added "4.2.Use of Conformal Coating and Under-Fill Materials" on page 11. Corrected simplified Magnus equation on page 27. Revision 1.2 to Revision 1.3 Added link to Si7013 Thermistor Correction Calculation Table on page 31. Revision 1.3 to Revision 1.4 Updated section 3.3. Added Appendix E. Revision 1.4 to Revision 1.5 Revised to include Si7006, Si7007, Si7022, and Si7023. Revision 1.5 to Revision 1.6 Updated title to include temperature sensor. Updated to include Si7050, Si7053, Si7054, and Si7055. Revision 1.6 to Revision 1.7 Added Si7034.
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