RHTD relative humidity/temperature probe

(c) 1998 ,2001 EME Systems
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The RHTD from EME Systems is built around the Honeywell/Microswitch/HY-CAL HIH3602C monolithic IC humidity sensor. This sensor element is exceptionally rugged, well suited for applications in harsh outdoor environments. It is protected by a hydrophobic sintered metal filter that can be scrubbed clean with water and a stiff brush. Under the filter is a sensor element constructed with muliple protective layers of polymer and monolayer platinum. The IH3602C also contains a RTD temperature sensor in intimate proximity with the humidity element, to facilitate temperature compensation and calculation of derived quantities such as dew point and vapor pressure. The EME RHTD improves upon the IH3602C by mounting it in a polycarbonate enclosure along with a stable power supply and circuitry to condition the humidity and temperature signals for voltage outputs. The IH3602C sensor element is replaceable. Both the humidity and temperature signals are voltages compatible with many data loggers, including the EME Systems OWL2c It is an excellent choice for any kind of agricultural or environmental research.

Photo of RHTDEnclosure, polycarbonate 2.5"l x 2"w x 1.4"h plus gland nut for cable entry. Weight, 4 ounces. Mounting template. The IH3602 replaceable sensor element is protected by a sintered hydrophobic metal filter, and projects through an O-ring in the polycarbonate case. The calibration constants are noted on a calibration label on the side of the case. The sensor element is socketed and can be easily replaced if necessary. The calibration certificate from HyCal is to be found inside the enclosure.

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Humidity output:
Temperature Output:


The block of four green screw-down terminals inside the enclosure allows connections to a data logger or contoller. There are connections for the +6 to 18 volt power supply, the humidity and temperature output signals, and common. The cable passes through a gland nut in the side of the case.

The humidity signal produced by the RHTD is simply a buffered replica of the signal from the IH3602 sensor element. The signal is not amplifedor shifted. A calibration equation of the form,

%rh = S* (volts-Vo)

is printed on the calibration label on the outside of the RHTD enclosure. This equation with its constants particular to the individual RHTD unit, and should be entered in your software. An temperature correction is necessary for the best accuracy, especially in humid situations. This correction is of the form,

%RH := %rh*(90.2 + 0.4605*T - 0.00307*T2 )/100

where the %RH on the right hand side is the uncorrected value from the first equation, and where T is the temperature in degrees Celsius. That applies for temperatures above freezing, T>0.

When T is less than freezing, -25<T<0, a different equation applies:

T=ABS(T)   ' make T positive
%RH := %rh*(90.2 + 1.314*T - 0.018*T2)/100

More information about the temperature compensation is found on a separate technical page on programming and temperature compensation.

The RHTD is calibrated so that the output voltages will be 1.00 volts at 0.0C and 3.00 volts at 50.0C. The first-order equation for calculation of temperature is
 C = Volts * 25 - 25
If the RTD characteristic were exactly linear, then -25C would fall at 0.0 volts and +100C would fall at 5.0 volts. But since the RTD characteristic is slightly nonlinear, there will be small errors at all temperatures except 0.0C and 50.0C. The error is less than 0.2C for temperatures between 0.0 to 50.0, but it increases rapidly outside that range. The instructions that come with the RHTD suggest a quadratic correction formula where best accuracy is needed

The RHTD manual comes with information on operation, programming the equations, recalibration and troubleshooting.

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