c15_142 (Влагомеры)

REFERENCE AND APPLICATION DATAHumidity SensorsHIH SeriesHUMIDITY SENSOR THEORY AND BEHAVIORSENSOR CONSTRUCTION: Relative humidity sensors use anindustrially proven thermoset polymer, three layer capacitanceconstruction, platinum electrodes and except for high temperature versions (shown bottom), on-chip silicon integrated voltage output signal conditioning.

(RHIC Sensor).In operation, water vapor in the active capacitor’s dielectric layerequilibrates with the surrounding gas. The porous platinumlayer shields the dielectric response from external influenceswhile the protective polymer over layer provides mechanicalprotection for the platinum layer from contaminants such as dirt,dust and oils. A heavy contaminant layer of dirt will slow downthe sensor’s response time because it will take longer for watervapor to equilibrate in the sensor.VOLTAGE OUTPUT: The RHIC sensor linear voltage output is afunction of Vsupply, % RH and temperature. The output is ‘‘ratiometric,’’ i.e. as the supply voltage rises, the output voltage risesin the same proportion. A surface plot of the sensor behavior fortemperatures between 0°C and 85°C is shown in the graphbelow.

This surface plot is well approximated by a combinationof two equations:1.A ‘‘Best Fit Line at 25°C,’’ or a similar, sensor specificequation at 25°C. The sensor independent ‘‘typical’’ BestFit Line at 25°C (bold line in graph) is:Vout J Vsupply (0.0062 (% RH) + 0.16)A sensor specific equation can be obtained from an RHsensor printout. The printout equation assumes Vsupply J5VDC and is included or available as an option on everysensor.2.A sensor independent equation which corrects the % RHreading (from the Best Fit Line Equation) for temperature, T:True RH J (% RH)/(1.0546–.00216 T); T J °COr True RH J (% RH)/(1.093–.0012 T); T J °FTEMPERATURE & HUMIDITY EFFECTS: The output of allabsorption based humidity sensors (capacitive, bulk resistive,conductive film, etc.) are affected by both temperature and%RH.

Because of this, temperature compensation is used inapplications which call for either higher accuracy or wider operating temperature ranges.When temperature compensating a humidity sensor, it is best tomake the temperature measurement as close as possible to thehumidity sensor’s active area, i.e. within the same moisturemicro-environment.

This is especially true when combining RHand temperature as a method for measuring dew point.Industrial grade Humidity and Dew Point instruments incorporate a 1000 ohm Platinum RTD on the back of the ceramic sensorsubstrate for unmatched temperature compensation measurement integrity. No on-chip signal conditioning is provided inthese high temperature sensors.The equations above match the typical surface plot (Best Fit Lineat 25°C) or the actual surface plot (sensor specific equation at25°C) to within the following tolerances:±1% for T>20°C±2% for 10°C<T<20°C±5% for T<10°COur dewpoint instruments account for the sensor specific version of the surface plot directly via a look up table.NOTE: Convert the observed output voltage to % RH values viathe first equation before applying the second equation.142Honeywell 1 MICRO SWITCH Sensing and Control 1 1-800-537-6945 USA 1 F1-815-235-6847 International 1 1-800-737-3360 CanadaREFERENCE AND APPLICATION DATAHumidity SensorsCONDENSATION AND WETTINGCONDENSATION occurs whenever the surface temperature ofthe sensor’s active area drops below the ambient dew point ofthe surrounding gas.

Condensation forms on the sensor (or anysurface) even if the surface temperature only momentarily dropsbelow the ambient dew point. Small temperature fluctuationsnear the sensor can unknowingly cause condensation to formwhen operating at humidity levels above 95%.While quick to condense, water is slow to evaporate in highhumidity conditions (i.e. when the surface temperature of thesensor is only slightly above the ambient dew point.) Because ofthis, a sensor’s recovery period from either condensation orwetting is much longer than its normal time response. Duringrecovery, the sensor outputs a constant 100% RH signal regardless of the ambient RH.HIH SeriesINTEGRATED SIGNAL CONDITIONINGAll RH sensors quickly recover from condensation or wettingwith no shift in calibration. However, after 24 hour or longerexposures to either high >95% RH or continuous condensation,an upward shift of 2% to 3% RH may occur.

This shift is repeatable and can be reversed by placing the sensor in a low 10% RHenvironment for a 10 hour period.Silicon integrated humidity sensors (RHIC – relative humidityintegrated circuit) incorporate signal conditioning circuitry onchip with the sensing capacitor. These ‘‘RHIC’’ humidity sensorsare laser trimmed so that at Vsupply J 5 V, the output voltagetypically spans 0.8V to 3.9V for the 0% RH to 100% RH range at25°C. (Sensor specific calibration data printouts and best fitlines at 25°C are either included or available as an option onevery sensor.)When an application calls for continuous monitoring of RH athumidity levels of 90% and above, take steps to avoid intermittent condensation. Some strategies are:1.Maintain a good air mixing to minimize local temperaturefluctuations.2.The HIH-3602-A and -C use a sintered stainless steel filter toprotect the sensor from splashing. A hydrophobic coatingfurther suppresses condensation and wetting in rapidlysaturating and de-saturating or splash prone environments.3.Heat the RH sensor so that the active area is hotter than thelocal dew point.

This can be done through an externalheater or by self heating of the CMOS RH chip by operatingit at a higher supply voltage.NOTE: Heating an RH sensor above ambient temperaturechanges its calibration and makes it sensitive to thermal disturbances such as air flow. When contemplating such an approach,MICRO SWITCH recommends selecting an HIH-3602 type sensor and getting application technical support.The HIH-3602-C incorporates a RHIC humidity sensor, a 1000Vplatinum RTD and anti-static protection in a single TO-5 can.RHIC based sensors are factory calibrated, micro-power devices with either individual calibration and/or good unit-to-unitinterchangeability.

These features help OEM manufacturersavoid in-house humidity calibration costs and extend battery lifein portable instruments. Improved accuracy can be obtained bytuning system electronics to account an individual sensors BestFit Line at 25°C.Reference/IndexHoneywell 1 MICRO SWITCH Sensing and Control 1 1-800-537-6945 USA 1 F1-815-235-6847 International 1 1-800-737-3360 Canada143.