Does anybody really know what temp it is?

Brett S

Well-known member
Mar 15, 2019
724
Orlando
Does anyone really care?

So a couple of years ago I bought a little sinking thermometer from Amazon and tied it to the side of my pool so it would sit a foot or two below the surface of the water. I’ve learned that if the thermometer says 72 then I can swim, but it’s awfully cold. If it’s anything below 72 then it’s too cold for me to swim. If the thermometer reads closer to 80 then the water is starting to get pretty comfortable. And it’s downright warm by the time it gets up to 82 or 84. I never really had any reason to doubt the readings on that thermometer and I’m still fairly new to the whole pool thing, so I guess I don’t really even know what normal water temps are.

Last week I got a circupool RJ-30+, which has it’s own thermometer and temperature display and I was surprised to see that it was way off from what I was reading with my pool thermometer. The RJ-30+ was showing temps that were a good 12+ degrees higher than the thermometer in the pool. I know that everything has it’s tolerances, but 12+ degrees seemed a little too much. I talked to DSP to see if there was a way to recalibrate the thermometer in the RJ-30+ and they are sending me another temperature sensor for it. It should get here in a few days.

However, I got to thinking about it today and it occurred to me that I really have no reason to trust my $5 thermometer from Amazon and for all I know the SWG could be accurate. I grabbed a couple of cheap little digital thermometers from my box of random aquarium equipment and they are both within .2 degrees of each other and right in the middle. They were about 6 degrees above the pool thermometer and 6 degrees below the RJ-30+.

I think no matter what the RJ-30+ is pretty high. When I first connected it the pool thermometer was showing 82 and the RJ-30+ was reading 94. The water was definitely pretty warm and I’d even be willing to believe that it was in the upper 80’s, but I can’t imagine that it’s was really in the mid 90’s. It could be that the aquarium thermometers are the most accurate. I’ll see what the new temperature sensor for the RJ-30+ says when it gets here later this week.

So after all that, is there a good source for a reasonably accurate pool thermometer? And even if there is, in the end does it really even matter? I’ll learn (or, I guess re-learn) what temps are worth swimming in.
 

Newdude

Gold Supporter
TFP Guide
Jun 16, 2019
11,240
NY
Well you will have several temperatures across the various areas/depths of the pool as well. Not 14 degrees apart, but 5-7 could be possible. Then a +/- 2 or 3 degrees on each device and you could legit have 10-12 degrees off and its all ‘right’. Several devices testing the same spot should be fairly close.
 

Brett S

Well-known member
Mar 15, 2019
724
Orlando
Yeah, you would think so. When I tested with the aquarium thermometers I put the probes within an inch or two of the pool thermometer, so they should have been pretty close. Obviously the temp sensor on the RJ-30+ isn’t near them, but even after the pumps have been running for a while and things should be pretty well mixed I’m seeing vastly different temperatures reported.
 

RDspaguy

In The Industry
Mar 21, 2020
1,409
Cabool, Mo
Floating, or sinking, thermometers are notoriously inaccurate. Often the scale is on a plastic housing and the tube can be slid up and down inside it, changing your reading with the touch of a finger. Given that it was $5 on amazon, I would not place alot of trust in it's reading. Same goes for fishtank thermo too, though I admit I am not experienced with those.
Your sensor in the salt cell is down stream from your equipment. Your equipment produces heat, even if the heater is not on, from friction, pressure, and residual heat from pump and even salt cell. I would not trust that as an accurate pool temperature either.
Add in localized temperature variances as newdude mentioned, and you have a whole bunch of guessing going on. Even with a controller and temp sensor before the pump you are pulling cold water from the drain and warmer water from the skimmers, so may not accurately show the average temperature of the water. Want to know if the water is warm? Stick your foot in. That is your best thermometer.
 
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Brett S

Well-known member
Mar 15, 2019
724
Orlando
Well, just to follow up on this, I got the new temp sensor from DSP and installed it. It’s reading about 1.5 degrees lower than the original sensor. I have also decided that my cheap amazon pool thermometer is reading very low as well.

I played around with my digital aquarium thermometers because they have a probe on a few feet of wire, so I can put the probe in different areas to see what the temp is. I also discovered that when the aquarium thermometers are just sitting in my house that their temperature is within a degree of my Nest thermostat. I imagine that has a reasonably decent temp sensor and should be fairly accurate.

So I put the probe for the aquarium thermometer right next to the pool thermometer this morning. The pool thermometer was reading 77.5 and the aquarium thermometer was reading 84.5. At the same time, the new temp sensor in the RJ-30+ was reading 86.0. I then moved the aquarium thermometer and stuck the probe into the return an inch or two figuring that this water should be fairly close to the temp that the RJ-30+ temp sensor was reading. In that case the aquarium thermometer was reading 85.0, or just a degree less than the RJ-30+

So after all this I have decided that knowing the actual water temp is definitely not an exact science. The temp does vary from one location in the pool to another and each thermometer could be off by a couple of degrees (or even more than a couple of degrees in the case of my cheap pool thermometer).
 
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Katodude

Silver Supporter
Aug 22, 2017
1,718
West Palm Beach/Florida
Pool Size
15000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Hayward Aqua Rite (T-15)
More importantly, what guy in Florida swims in 72 degree water. We wont even consider out pool until it is in the 80s. Now I understand if you live in the northern climes that is a fact of life, but Brent you live in FLORIDA!!!
 

Brett S

Well-known member
Mar 15, 2019
724
Orlando
More importantly, what guy in Florida swims in 72 degree water. We wont even consider out pool until it is in the 80s. Now I understand if you live in the northern climes that is a fact of life, but Brent you live in FLORIDA!!!

haha, well, what I have come to realize is that what I thought was 72 degree water, according to my cheap and apparently very inaccurate pool thermometer, was actually closer to 79 or 80 degrees.

So apparently I am not some sort of super Floridian capable of swimming in cold water. I need it to be 80 degrees like everyone else.
 
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