How to add a thermocouple to hot water tank - Page 1
Forum section: electronics / projects

author: rfdes
author_rank: Regular Contributor
timestamp: October 22, 2019, 03:30:54 pm
content: 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.ThanksJim

author: Ian.M
author_rank: Super Contributor
timestamp: October 22, 2019, 04:10:04 pm
content: 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.

author: dom0
author_rank: Super Contributor
timestamp: October 22, 2019, 05:26:17 pm
content: 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).

author: james_s
author_rank: Super Contributor
timestamp: October 22, 2019, 05:42:25 pm
content: 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.

author: Brutte
author_rank: Frequent Contributor
timestamp: October 22, 2019, 07:20:08 pm
content: 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).

author: james_s
author_rank: Super Contributor
timestamp: October 22, 2019, 08:55:15 pm
content: That's a good point, I forgot about that.I do wonder what the value of this is, the plain old bimetallic thermostats seem to work pretty well, I never had an issue with them back when I had electric hot water.

author: mikerj
author_rank: Super Contributor
timestamp: October 22, 2019, 09:03:47 pm
content: Quote from: Brutte on October 22, 2019, 07:20:08 pmYep, typically a bimetallic thermostat for a cylindrical hot water tank will be mounted about 1/4 to 1/3 up from the bottom of the tank.

author: james_s
author_rank: Super Contributor
timestamp: October 22, 2019, 09:37:00 pm
content: Most of the tanks I've seen have two elements each with their own thermostat as I recall, one lower and one upper. It's been a while since I've dealt with one though, my current house has always had gas.

author: Brutte
author_rank: Frequent Contributor
timestamp: October 23, 2019, 08:44:28 am
content: Quote from: james_s on October 22, 2019, 08:55:15 pmOne can imagine a plethora of control laws based on the current and/or the past state of all the parameters in a heating system. Zillions or more of them out there. The on/off based on temperature at some distinct point in tank is just one way to skin a cat. But you can have a control law also based on the past of the temperature (like I in PID). For D you can make the drive more agile. So, simplifying, a controller with D will give you similar performance with smaller tank than the one without D. Once it will sense the change, it starts heating instead of waiting till the temperature hits the low temperature threshold.I think the observer-based controller could give even better perfomance. If there is a cold water flowing into the tank (based on flow meter) then it is a clear sign we need to pump in some energy and not wait till the temperature drops down. Then even smaller tank would do the job.

author: beanflying
author_rank: Super Contributor
timestamp: October 23, 2019, 08:57:47 am
content: A fairly common option in Industry is to use a Thermowell. That way you can remove the issue of electrolysis or chemical attack with the sensor.The issue with wanting to PID ( I assume sealed?) a boiler is the thermal gradient and the general lag of that closed system to respond to a heat input. You will still need to run the PID fairly loose if it is a typical tall boiler. So not really a lot to be gainedIf it is an open vessel at or near boiling like an Urn or Kettle then there is generally move vertical movement in the generally shorter water column so it can be held tighter. I run a small Urn for Brewed Coffee that will hold very close to a set point at the outlet.