14 April, 2021

Learning the hard way that plastic TO-92 is affected by humidity

I've had my APRS temperature station running since 2012. It is based on a DS18S20 sensor that just sticks out of a window. However, after some time it started to give too high readings, and after having replaced it several times I found out that I needed to waterproof it better. It seems that the TO-92 plastic housing, shown here to the right, somehow was influenced by humidity.

I wasn't able to find waterproofed DS18S20 sensors, but I could find ones with DS18B20. I got one and connected it, but it only showed -1 C regardless of weather. The software in the Opentracker USB was not able to read it, although I have Arduino programs that can read both.

What to do then? Use the housing and replace the sensor. That meant drilling out the filling in the original DS18B20 water-proof housing. I couldn't get the TO-92 housing out, so the old sensor is still there in the tip. Everything else was easy to remove. Then solder the DS18S20 sensor to the wires, as shown above, and put it into the tube with a generous amount of epoxy. When cured, the sensor looks just like the original one as seen in the second image, but hopefully this time it will be able to withstand changes in temperature and humidity for years to come.

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