How do I get an accurate temperature reading?

citivasAugust 8, 2011

My new pool (25,000 gallons) has an automated system that has its own temperature gauge in the plumbing at the equipment pad, plus the heater has a temperature gauge and I keep a floating thermometer in the pool. Since day one they have never agreed with each other. The two gauges at the equipment are off from 1-3 degrees from each other, but they can vary wildly from the gauge in the pool. The pool builder and servicing company say to ignore the "cheap" gauge in the pool and trust the ones at the equipment. But we�re now having doubts about that. The pool wasn�t swimmable until early July so it was peak season, very warm and we get all-day full sun exposure with no shade. In that context the difference was 6-7 degrees between the equipment and the pool gauges, with the pool thermostat always lower. Now we�ve had some overcast days in the low 80�s and the difference is severe. The pool says 72 degrees with the equipment saying 85/87 (and we have the heat pump set to stop at 85).

The thing is, there is no way it feels remotely like 85 degrees when you swim in it. It felt almost warm to get into before and now everyone is a cold. So while it may not be 72, of the dozen people who were in it in the last week not one would say it is remotely close to mid-80�s, or even 80.

I bought a more expensive digital floating thermostat to see if that made a difference and it too is registering meaningfully lower temps than the equipment if not exactly the same as the previous floating one.

Bottom line, is it true that the equipment readings should be more accurate than the pool readings or is it possible the readings in the equipment could be skewed hot (like the way readings of outside air in your car are always higher than reality when the car has been sitting outside in the sun on a hot day)? And is there any way to get a truly reliable measurement of the pool temp independent of my equipment?


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That is too funny, I have almost the exact same situation (down to the size of the pool and equipment). In our case, the floating thermometer had an air bubble in it, so we just threw it out. Haven't gotten a new one. Our Salt Cell has the lowest temperature of the various pool equipment ones and the salt cell is out of the sun by the most, so I trust it the most. I find that the gauge used for determine when to run the heaters is off by between +2 and +7 degrees of the of the salt cell gauge based on where the sun is. Sort of a bummer, if I really want my pool to be 86 F, I have to set it at 90....Which is pretty dumb, IMHO....Again, pool builder said the same stuff....

To give you another point of reference, the other day our pool equipment read 82F and our friends pool thermometer read 80F (they don't have a heater or any equipment that knows the temperature). The difference felt more like 10 F different, not 2.....So, in their case, their thermometer in the pool was reading high...Fun, fun..

    Bookmark   August 8, 2011 at 7:25PM
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Automation systems can have the temp sensor re-calibrated at the controller.

The system needs to be running at least 15 minutes for the sensor to provide a steady state temp.


    Bookmark   August 8, 2011 at 10:30PM
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They did supposedly calibrate the automated (Jandy) control system temp, but they were basically calibrating it to supposedly agree with the heater (even though it still ends up off by a couple degrees). They wouldn't calibrate it to the pool thermostat because they said it was wrong and to ignore it. I don't think it is dead-on right either, but the equipment is definitely overstating the temp.

What I find odd is that it is getting further and further off as the weather cools -- though a new heatwave is coming on so it will be interesting to see what happens. The pool is easily at least 6-7 degrees cooler now that it was a couple weeks ago despite the equipment saying it is the same.

Are there any particularly accurate portable or floating thermometers?

    Bookmark   August 9, 2011 at 10:45PM
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You want a thermometer that sinks. The water temperature of a pool is typically several degrees cooler 6" to 12" down.

Water coming from the pool is going to go through buried lines. The water may cool some before it gets to the temp sensor.

The pool heater's thermostat should be set to it's highest level, typically 103 or 104 F. The heater's safety circuit should be connected to the automation system. A low voltage relay there opens and closes, telling the heater when to fire up or shut off.

I am less inclined to care what the heater thinks if the pool or spa get to the comfort zone a homeowner wants.


    Bookmark   August 10, 2011 at 7:46AM
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The heater is set to 104 -- the maximum setting. I am only referencing what it says the current temp is to indicate it is close to agreeing with the Jandy automated system temp sensor, versus the floating thermostat.

If the floating sensor is already too high versus lower in the pool, then the problem is even bigger since it is already showing the pool is 12-15 degrees cooler than the equipment. And I'm confident (by feel) my pool is not in the 60's. As I've said, I believe the water is warmer than the floating thermostat but much cooler than the equipment temp sensors.

    Bookmark   August 10, 2011 at 7:07PM
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What automation system do you have?
What heater?

Lets get the automation system right since its the one that matters most.


    Bookmark   August 11, 2011 at 3:32AM
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Weird. I answered this before and it seems to have disappeared from the site.

The automation system is Jandy. The remote is the PDA-6. Not sure the model # of the control system itself,

    Bookmark   August 12, 2011 at 4:10PM
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