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How to Test and Calibrate Aquarium Heater


Mark303
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I purchased two 150W Eheim Jager heaters for a 75 gallon tank. Before putting in my display tank I thought I would test the calibration in an empty tank to not risk harming my fish. However the only empty spare tank I have is a 10 gallon. Both heaters were wildly inaccurate. Testing individually I set at 75 degrees and tank reached 85degrees, which maxed out the calibration adjustment ring. Since it happened with both heaters I was wondering if this is due to the small size of the tank and overpowering of the heaters for that size. 

Any suggestion of how to accurately test and calibrate?

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On 4/4/2023 at 1:39 PM, Mark303 said:

I purchased two 150W Eheim Jager heaters for a 75 gallon tank. Before putting in my display tank I thought I would test the calibration in an empty tank to not risk harming my fish. However the only empty spare tank I have is a 10 gallon. Both heaters were wildly inaccurate. Testing individually I set at 75 degrees and tank reached 85degrees, which maxed out the calibration adjustment ring. Since it happened with both heaters I was wondering if this is due to the small size of the tank and overpowering of the heaters for that size. 

Any suggestion of how to accurately test and calibrate?

Yes, I had a pretty similar experience.  Those things being +/- 2 degrees is pretty normal.  That being said, I had one peg on and I highly recommend using a controller if you can find one for that specific heater in question.

Alright, as far as what's going on, let me break things down for you.
1.  you need to have good circulation to keep them from overheating and damaging themselves.  Place it near flow and try to keep it as vertical as you can.  In my 75 I tend to run 2 HoBs and 2 heaters.  Heater goes right next to the input and right under the HoB itself.

2.  Calibrating a high wattage heater in a lower wattage volume won't work as well as you thing, but it's a good idea.  For the heater required on a 75G volume I would imagine you're in the 200W+ range and I wouldn't put that in something smaller than a 40B tank.

3.  Thankfully the heaters are a breeze to calibrate, but you do need to follow a few steps and keep an eye on it from cooking the tank.
A.  Turn the heater on to the desired temp in the tank you want to use it in.
B.  Place it so the flow works to the advantage of the heater
C.  Test the temperature of the water every 30 minutes with a thermometer, preferably 2 different ones.  This gives you a value to base your heater on.
D.  Once the heat has "settled" and is no longer trying to heat the tank, record that temperature.
E.  Set the offset on the heater so that you can adjust the internal thermistor up / down the degrees you need to.  This is a red sliding dial on the top of the heater itself.
 

 

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My experience with every brand of adjustable heater is that it might be kind of close for a bit, but as soon as you move it, all that's out the window and you can't get it back.  I have everything cranked to max and use InkBirds.  It's the way to go.  They can be had for $25 at the moment on EBay and occasionally they'll dip down under $20.

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I do have an inkbird that I haven't set up or really learned yet. I guess my thought process was to test the heaters for any issues and have them somewhat close to calibrated to the desired temp before putting in the display tank and using the inkbird as a failsafe rather than relying on as the temp controller. I expected the heaters to be off a couple degrees but was confused as to the 10 degree variance that is greater than the calibration ring allows to adjust that much. But sounds like the small tank is the issue. 

really appreciate everyone's input!

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