rq3
Senior Member
Last year I built a controller for my fridge/freezer based on a Picaxe 20M2 and three DS18B20 temp sensors. It's been working great BUT:
One of the sensors monitors the temp of the fridge evaporator, so it is exposed to below freezing temps, and then thaws to above freezing temps (to ensure that the evaporator defrosts). This is how the original fridge controller worked, with two sensors, one to monitor the defrost of the evaporator, and one to monitor the actual temp of the fridge contents. If both were warm enough, the compressor would turn ON.
The evaporator temp sensor has now failed twice! The second attempt involved potting the DS18B20 in a small stainless steel tube with urethane having a glass transition of -72C. This material is specifically designed to pot components exposed to extreme temperature variations, as it stays relatively flexible to prevent stress to the potted components. I use it extensively in a commercially available product with no problems over many years.
I can confirm that the DS18B20 cannot tolerate being exposed to freeze/thaw cycles for very long, having experienced failure of this component twice in those conditions. The failure seems to be failure of the internal wire-bonds within the TO-92 package. Whether this is due to moisture (not likely, given the urethane potting) or thermal expansion (likely), is unknown.
The DS18B20 units that remain frozen (freezer control), or remain above freezing (refrigerator control) seem fine. I "fixed" the fridge by shutting off the evaporator sensor in software (thanks to the picaxe!) and just rely on the remaining "fridge contents" sensor to control the fridge compressor.
Just thought this might be of use to anyone using the DS18B20 sensors.
One of the sensors monitors the temp of the fridge evaporator, so it is exposed to below freezing temps, and then thaws to above freezing temps (to ensure that the evaporator defrosts). This is how the original fridge controller worked, with two sensors, one to monitor the defrost of the evaporator, and one to monitor the actual temp of the fridge contents. If both were warm enough, the compressor would turn ON.
The evaporator temp sensor has now failed twice! The second attempt involved potting the DS18B20 in a small stainless steel tube with urethane having a glass transition of -72C. This material is specifically designed to pot components exposed to extreme temperature variations, as it stays relatively flexible to prevent stress to the potted components. I use it extensively in a commercially available product with no problems over many years.
I can confirm that the DS18B20 cannot tolerate being exposed to freeze/thaw cycles for very long, having experienced failure of this component twice in those conditions. The failure seems to be failure of the internal wire-bonds within the TO-92 package. Whether this is due to moisture (not likely, given the urethane potting) or thermal expansion (likely), is unknown.
The DS18B20 units that remain frozen (freezer control), or remain above freezing (refrigerator control) seem fine. I "fixed" the fridge by shutting off the evaporator sensor in software (thanks to the picaxe!) and just rely on the remaining "fridge contents" sensor to control the fridge compressor.
Just thought this might be of use to anyone using the DS18B20 sensors.