Hot Tub Temperature Probe

[DavidK]

mackrackit
That is a great link. Now I know what to ask for, thermowell. The web link sure is good reading plus lots of links to research. Guess I was not to far off when I was thinking of the test tube.

My goal was to get a fairly accurate reading, but maybe that is not as important as I figured. Most people never take a temperature reading before they get in the shower, it is all relative.
Thanks for the tip
David

[duncan303]

Hi David

My goal was to get a fairly accurate reading

With a Hot tub the pipework remains all fairly close within the integrated unit, and could be arkward to find a place to cut a tee into, unless you have a separate Plant room. If you attempt to install an ABS TEE then you might need to make sure you have enough room for a slip coupling, careful if you are going to use a solvent cement, you only get a couple of seconds alignment. Much better to fix a "strap boss" and reduce down to your pocket. Consider mounting the entry to the pipe line from below horizontal thus reducing the requirement for venting(the pocket could fill up with air). Choose a place (any place, you can calibrate to any temperature) that has the most consistent water temperature ie before any heat exchanger or inline heater. So you are really looking at the area at the return drain. Do not worry about setting the pocket "into" the flow as turbluence will take care of uniformity, plus you will find that there will be less wear on the pocket itself, believe it or not but you actually have a water based sand blaster going on. If you have a cartridge or sand filter then you can place the pocket after them.

Then of course the fun starts by integrating the PIC to optimise the heating circuit to reduce overshoot and hysterisis in the heating circuit.


HTH

Duncan

[DavidK]

Duncan
Thanks for the input. Where the water comes into the filter reservoir there is a cover that I can mount the Thermowell to and drop my DS1820 right in. So I really don’t need to tap into the water lines. I think this is what you suggest, keep the sensor it in the return part of the process.

This afternoon I opened up the doors where the controller is and found a thermowell for the existing temperature sensor. Now it looks like I might be able to fit the DS1820 in the same well, but that probably is not the best place. My overall goal of this project is to only monitor the water temperature remotely. We have frequent power bumps in our area and my GFI trips occasionally, so I want to catch the trip by alarming the water temperature. The other reason is that my wife wants to make sure the tub is up to temperature before she runs outside. We have turned into real pansies.

Did a search on EBAY for a “thermowell.” Surprised me, found lots of them.

As far as replacing the pump/heater/light controller it might not be too bad. My tub is vintage so all I have to worry about is time-of-day, minimum temperature, run temperature, pump high/low speed and the light. I would probably use 24 VAC controlled contactors for the 30 amps of 240 VAC current, then use the PIC to control everything else. Not ready to go there yet, but it sounds like a fun concept to design.
David

[duncan303]

Hi David

I responded to your post as a spur of the moment, but have been thinking a little more.

Maybe in your particular situation, the temperature is not critical as in an industrial chemical process; you simply have a fairly large body of water that you anticipate bringing up to and maintaining a temperature within say +/- 2 DegC. Although the thermal characteristics of ABS pipework are very poor you could use this to your advantage, why not simply attach the 1820 directly to the outside of the pipework, AND insulate heavily around the section. You could mount the 1820 directly onto a wide metal band as a type of thermal heat plate.

Better still why not adapt the idea of a bog standard floating pool thermometer. Mount the 1820 at the bulb location; open up the float ball place circuitry, power supply, and a small RF unit inside, even turn the ball into a sexy multicolour “Light glow ball” to reflect the readiness of the tub! Have the RF transmitter send to a LCD or Oled display that you keep in the house, integrate another RF unit into the control panel and Viola. Hook another RF up into a USB dongle and keep an eye on things online, or have it send text updates to your mobile.

What a neat little run of projects all easily achievable and conjunctive, without having to use MPLAB IDE!

Now I wonder if the humidity phase boundary at the surface of the water would excessively interfere with the RF? Would 2.4Ghz be more reliable than 433 etc. I can’t off-hand remember.

I am probably not helping at all here, but am having fun.

Duncan