What Temp Is My Fridge (Measurement Question)
 

I'm trying to decide if my new refrigerator is working properly. The problem is, various thermometers are reporting different temperatures.

The dial thermometer on the fridge's upper faceplate says 45F. My digital probe fridge/freezer thermometers say 30-34F depending on where I place them in the box. If I place a probe in a bowl of water that has been sitting in the fridge for a day, it reads 30F. (Yet note the water is not frozen.). My digital meat thermometer in the same bowl of water reads 40F.

I am thinking the probe thermometer is faulty and the fridge is 40-45F, but it bothers me that I have to guess at which thermometer is accurate and which is not, and that I only have a rough range of temperature rather than a point.

Can you suggest a guaranteed reliable way to determine the temperature of the water in the bowl? That doesn't involve more than, say, $40 in new instruments.

Ha ha, my neighbor is a scientist at NIST. His job, literally, is measuring stuff. They have a piece of gear that's good for measuring temperature, guaranteed to be within something like a few thousandths of a degree. They can't move it without voiding the calibration, though, so you can't use one of those.

What you're discovering, BTW, is a joke that he likes to make. "Measure once, and you feel like you know the truth. Measure twice, and you realize you have no clue."

I have a couple of calibrated thermometers that work pretty well, but the good ones aren't cheap. The one I really like is a thermocouple connected to a Fluke multimeter. The Fluke was like $75 on eBay, and the TC and attachment were another $20. For as much as I've used it, it's a great price.

You can verify that a thermometer is "close enough" in this range, BTW, by putting it in a bowl of ice water. By definition, that water is at 32F. If the thermometer tells you differently, it's lying. Unfortunately, the magnitude of the error doesn't tell you anything useful: it could be either bias or gain, either of which will be indistinguishable for a single data point.

So to actually calibrate the thing, you'd need a second data point: boiling water. Tragically, a cheap thermometer may not be linear across that whole scale, so you still don't know anything.

Oh! And I should note that it seems pretty normal for a fridge to be non-uniform in temperature. So putting the thermometer in different places should produce different results.

Sorry, no help here, outside of "Get a real thermometer, dude." :)

Good luck.

Dan

Pretty obvious that the probe reading 30F in a bowl of liquid water is not working. And you will get different temps in the box if there is a lot of contents on the shelves. Empty should be a very close range when it's running or just after shut off. Don't put items on shelves in any way that they block the fan.

I like to turn mine down until the milk gets a tiny bit slushy in the coldest part (that would not be in the door racks where a lot of milk is stored), then go back a notch.

The boiling point of water, once corrected for barometric pressure , is a constant. An open pot of water, boiling on the stovetop could serve as a reference to tell you which thermometer is most accurate.
If you know your local barometric pressure, you can google "water altitude boiling point" and find a calculator for the correction.

measure only the bottom shelf, at the back. should be the coldest spot in the fridge. target "just a tad above freezing". i have a very accurate concrete thermometer stuck in a glass of water in the bottom 24/7. mine is at 34 or so. i bet my thermometer still has mercury in it...

let the rest of the fridge temps zone fall where they may.

for your appl. use a stirred ice bath to check or calibrate

the thermocouple connected to a Fluke multimeter will be a Type K - not the best for this range, but should be ok for your use. I use a type T Tcple for this range, but they are specialized.

one "error" you are getting is the fact that you are measuring the local T of any pool of air that the Tcple sits in - i.e. all the air will not be well mixed

set a pot of water in there and stick the Tcpl in that - check it the next day; make up a little graph above, at, and below the T you want to measure with at least 3 points on it - Fridge readout vs. the Tcple (you did check the Tcple in the ice bath, right?)

I wouldn't trust the fridge readout much

Your freezer temperature is best indicated by the consistency of tub of ice cream. If it is impossible to scoop then you are too cold. If it is soft and scoops too easily then you are too warm.

It really is that simple.

McMaster Carr sells calibrated thermometers for a reasonable price. That will resolve your fridge issue.

Okay, I'm measuring the temp of water glasses placed at various points in the fridge. I'll do my best to calibrate my meat thermometer in ice water.

If I'm not getting the desired 33-34F on the lower shelf, I will have to figure out how to adjust the thermostat. It is already turned to "coldest". It is a mechanical thermostat, with screw adjustments for the cut-in and cut-out temps.

Then I will see about the noise . . . although I'm already getting used to it, it will discourage guests from hanging out in the kitchen. Which is a good thing.

That's what you get for buying a
http://www.truemfg.com/images/TrueLogoBlue.png

What make and model do you own? The unit will regulate temp at the location of the thermistor or bulb thermostat. Local condition will depend on ambient temp, load, air flow restrictions and door gasket quality

True T-23 refrigerator. The thermistor is in the evaporator coils, right?