The next test was to see if the dehumidifier would maintain a set humidity. The room had temperature and humidity sensors. The dehumidifier was set to 30 percent RH. During testing, the room sensor was showing humidity as low as 10 percent RH, but the dehumidifier was still running, and its sensor showed 30 percent. When the dehumidifier shut off, its sensor quickly dropped to about 10 percent, to match the room sensor.
My first suspicion was that the enclosure the dehumidifier sensor was contained in was conducting the outside temperature and preventing the sensor from accurately following the room conditions. The enclosure had extra strain relief openings that were plugged. I removed the plugs to allow better air circulation, but it did not make any difference.
My next suspicion was that there was a voltage problem. I wondered whether the sensor supply voltage was low when the unit was running. In order to attach test leads to the sensor, the sensor had to be taken out of its enclosure. The voltage metered correct, and the sensor more closely followed the room sensor. So somehow the enclosure was affecting the sensor.
The dehumidifier originally had a terminal strip to attach the sensor wires. I had run the wires in flexible conduit to protect them. I had also replaced the terminal strip with a hole punched into the sheet metal side and a strain relief.
The wiring connections were not sealed from the air plenums, so the conduit was directing air from inside the dehumidifier to the sensor enclosure. I used some weather stripping to block the air flow through the conduit. The dehumidifier sensor then followed the room conditions similarly to the room sensor, and the dehumidifier cycled properly.
This entry was submitted by Glenn Aitchison and edited by Rob Spiegel.
Glenn Aitchison’s first field service job was in 1987. Since then, he has worked in robotics, automotive, and industrial automation and machinery. Glenn has also received his certificates of qualification as an industrial electrician and as an industrial mechanic (Millwright). His business cards have included field service technician, technologist, and engineer.
Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.
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