Electronics > Projects, Designs, and Technical Stuff
How to add a thermocouple to hot water tank
rfdes:
I have a desire to use a PID heater controller to regulate the temperature of my hot water heater and am looking for ideas how to adequately mount/attach the thermocouple to the tank in a robust manner. Has anyone on the forum does this sort of thing and if so what tips can you offer.
Thanks
Jim
Ian.M:
It depends a lot on the tank. Basically, for an external temperature sensor, you need to get right down to the tank surface where it either isn't insulated or where you can make a hole in the insulation, above the heating element or heating loop and away from any cold-water inlet. Thermally conductive epoxy would be suitable for bonding most sensors to the tank. You'll need to thermally insulate over the sensor and surrounding area to get a true tank surface temperature.
Consider copying an external tank thermostat - small box for the sensor and connectors, with a spring to hold the sensor against the tank, sat in a square opening cut through the insulation, held in place with a low-stretch strap right round the tank over the sensor box.
dom0:
Heating systems generally use cheap PTCs (KTY10 series) for measuring the temperature (since these have a couple kΩ they get away with a two-wire measurement). These are available COTS with a PVC cable bonded to a stainless steel capsule of the correct diameter (8 mm or so) to be inserted into a stub tube that screws into threaded studs of your water tank. So you can change the sensor without draining the tank.
Thermocouple seems unnecessarily complex and expensive for this application. (Unless your hot water heater is actually the primary hot water loop of a pressurized water reactor: PTCs aren't suitable to the higher temperatures and radiation, so thermocouples would be the way to go).
james_s:
Most of the cheap PID controllers are designed to use thermocouples so I suspect that's why it was chosen. They are not really any more expensive than PTCs so that isn't really a factor.
Attaching one directly to the outer surface of the tank would probably work. You could also drill and tap a hole for a threaded sensor however that creates the potential for a leak.
Brutte:
Unless this is a copper water tank I would strongly discourage drilling holes and damaging internal surface protection.
The water changes density with temperature so if your tank is slim there should be a significant difference in temperatures along its height. It might be tricky to make that work satisfactory with PID (especially with D).
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