Relative Humidity and Temperature Sender Module - RH #166

The RH #166 Relative Humidity sender unit provides the capability for a single relative humidity and temperature measurement.

It may be interfaced with our TM #128, IOM #135E, IOM #136 and IOM #142A modules. All of these modules provide multiple Dallas 1-W runs, each of which may accommodate up to 16 Dallas 1-W devices of various types including the DS2438 temperature and A/D sensor.

The RH #166 RH Sender Unit consists of a Honeywell HIH-4000-001 and a Dallas DS2438 temperature and A/D sensor.

When using the TM #128, IOM #135E, IOM #136 and IOM #142A modules the data from a Dallas DS2438 is returned in the form of;

   1 04 26 09CE 23.81 3.17 5.03
where the first digit identifies the bank, the second field is a sequential number identifying each 1-W device as it is found. The third field identifies the device as a DS2438 (code 26) and field four is an abbreviated serial number which uniquely identifies the Dallas device. The last three fields are the temperature in degrees C (Tc), the voltage appearing on the V_ad input of the DS2438 and the supply voltage (V_dd). (For some designs the format of the bank and the sequential number may differ slightly).

In the example above, Tc = 23.81 degrees C, V_ad which is the output of the HIH-4000 sensor is 3.17 VDC and the supply voltage V_dd is 5.03 VDC.

The relative humidity may then be calculated as;

   RH_raw = 161.29 * V_ad / V_dd - 25.80
   RH_true = RH_raw / (1.0546 - 0.00216 * Tc)
In this case;
   RH_raw = 161.29 * 3.17 / 5.03 - 25.80
          = 75.8 percent

   RH_true = 75.8 / (1.0546 - 0.00216 * 23.81)
           = 75.6 percent

Note that the RH #166 RH Sender Unit in conjunctiom with the TM #128 or other IOM module returns only the temperature and the V_ad and V_dd voltages. The calculation of the relative humidity described above is performed by the PC or similar interfacing unit. That is, it is for you to use your creativity and skills with Visual Basic, Visual C++, Delph, etc to perform these calculations.

Note that the RH #166 Sender Unit is powered (+5 VDC and GRD) by the master unit; TM #128, IOM #135E, IOM #136 or IOM #142A. The RH #166 includes a 47 uFd electrolytic capacitor and a 0.047 uFd ceramic capacitors to supress noise on the power leads.

I suggest running two twisted pairs to the remotely located RH #166 unit; DQ and GRD and +5 VDC and GRD. The GRD conductor associated with the DQ lead should only be connected at the master to avoid ground loops. This is easily implemented using CAT 5 cable. The total cable associated with any Dallas 1-W run should not exceed 200 feet.

Note that multiple RH #166 sender modules may be accommodated on the same 1-W run.

The RH #166 unit is hand assembled on a small protoboard developed for another application. Four 1/2 inch nylon spacers and associated #4-40 screws are provided.

Mounting a RH measurment device such as this outdoors poses a challenge. On the one hand you want to avoid rainfall and on the other you want free air movement. A cylindrical bird feeder (no food) appears an inexpensive approach.

PC Com Port Interface Program

I have developed sample code which illustrates how a PC (Windows) may be interfaced with a TM #128 using an RS232 Com Port to measure temperature, relative humidity and dew point. This is written using Liberty Basic. This may be readily adapted to other modules, including the IOM #135, IOM #136 and IOM #142.