What remote sensor do I have?

I fielded a call a few days ago from a customer who was having difficulty with the installation of a remote sensor. The customer reported that his HAI controller was not able to read the humidity level provided by the sensor, but yet the controller had no problem with reading the temperature. The customer had already checked the integrity of his wiring, so any wiring issue was ruled out.

When I asked him for the part number of the remote sensor, he couldn’t tell me for he couldn’t find one on the sensor. Strange, I thought, that there was no part number. So I went to our warehouse and pulled out the two types of HAI remote sensors to see for myself. HAI has a remote Temperature Only sensor, which is p/n HA31A007, and a Temperature and Humidity sensor, p/n HA31A008. Sure enough, there was no visible part number on either sensor but there was something that caught my eye that distinctly differentiated between the two.

On the temp and humidity sensor, there’s an additional small circuit board and humidity canister installed on one corner of the sensor. These items are not present on the temp only sensor. To add to the confusion of not having a printed part number, both sensors have terminals marked with “H” and “T,” with the “H” terminal operational only when you have the additional hardware installed. Take a look at the image below, and you’ll be able to identify which sensor you are working with.

Getting back to the customer, it turned out that he had the temperature only sensor, so of course, the “H” terminal is not usable at all. At least the customer had already installed the 4-conductor cable so in future, he could easily replace his existing sensor with the temp and humidity version, or just have the unused wire there, ready to be used as a spare.