About a week ago I managed to snag several HMP35As off of eBay. While sure these sensors are obsolete, the Vaisala capacitive polymer H chip within are compatible with the HMP35C. Beauty of this is we can harvest the usable parts from the old sensors and repurpose them toward the ones we will use operationally. Since the HUMICAP-H chips are of such high quality, their lifespan is usually far greater than anticipated. With a rated “less than 1% per year,” I’m fairly confident we will get years of accurate dew point measurements out of these probes.

For temperature, the 35C uses a Campbell Scientific 107 thermistor probe. The 107 offered by CSI individually uses a 100kA1IA thermistor with two resistors, a 249kohm 0.1% 10 PPM/C and 1kohm 0.1% 10 PPM/C. Since the HPM35C by default uses the 107, replacing it is easily accomplished. Since the required circuitry is already embedded within the sensor, all that is required is the 100kA1IA thermistor and 249Kohm resistor.

Since most of the thermistors are bad in the probes we have, I’m planning on replacing all of them so they are ready for when we need them to be calibrated and installed. Stay tuned.