best ( most accurate ) way to check the temperature, of a sealed up tank...

[Magnum Man]

so, my RO storage tank is pressure sealed to an over flow, going to the drain... I'm trying to figure out how to most accurately check the temperature... we have a laser thermometer, but even pointing it at an aquarium, that is within except-able error, the outside of the tank reads 73 degrees, when the water is supposed to be 78-79... I expect the error being from the room temp acting on the outer surface of the glass.. my holding tank is plastic, likely a better insulator, than the glass aquarium... the RO filter is filtering pretty cold well water, but I've been filling it 24 - 48 hours. before I use it, the tank is 100 gallons... room temp, is 70 ish... there is currently no unused thermo well, or outlet that I could insert a probe... I plan on putting a pad heater on the side, but it would be nice to have an approximation of the temperature... the most practical thing I can think of now, is fill a cup, and test that... any other suggestions??? if I stuck an adjustable stick on thermometer on the outside of the tank, and occasionally tested with a cup, and adjusted the stick on, to match the cup readings, seems like my best bet??? there are seasonal variations in room temp, so I could test it seasonally, and adjust as needed, or since the room doesn't vary, more that 4-5 degrees, just call a cup test and adjustment "close enough" thoughts???

 

[Magnum Man]

well, with the morning caffeine flowing, I think I could modify the stainless "T", I threaded into the cap, to accept some sort of probe or thermometer stem... it would have to be water tight, so the over flow can go to the drain, rather than leak... some more thought or searching would be involved, but in that case, the actual temperature could be measured, rather that trying to guess by the outside tank temp.

 

[Magnum Man]

this is best so far, if I drill a hole in the Center of the "T" to where the thermometer stem will go down, into the tank, and then tap that hole to 1/2inch NTP threads, I can screw this thermometer into the cap, with teflon tape, and get a water tight seal, and be able to get the actual temperature of the water, if the tank is at least close to full

https://www.amazon.com/gp/aw/d/B00960RPY4/ref=ox_sc_act_title_1?smid=AG3HM2L81RV9O&psc=1

 

[GaryE]

Honestly, in search of simplicity, I'd pour to a cup and measure that.

 

[MaloK]

Before drilling a hole somewhere. I would try to stick the thermometer stem on the outside of the tank, the lowest possible without being too close to the floor. Using a good thick piece of duct mastic to insure closest contact and insulation. Then adjust the temp to match what's inside.

And check from time to time if the readings concur and are reliable, from there, I would consider it good.