Need a reliable thermometer

[ConnieG]

I have gone through 2 Taylor digital thermometers with probes and ruined a couple of batches of cheese because of inaccurate readings.  What thermometer do you all recommend?  I really liked the probe feature but maybe that's not an option and still be reliable. 

[linuxboy]

I like analog thermometers that can be calibrated with the nut at the top. The small ones are really cheap, maybe $4. I calibrate them a few times a year and new ones before use. I am not a fan of digital ones; too much money spent over the years on new probes and broken thermometers. Same for hydrometers; I prefer analog ones.

[ConnieG]

Well there you go.  Last night I reached for the old analog one in the drawer so I could keep making cheese.  Darn!

[dthelmers]

I used to use a probe type when smoking my bacon; right up until the time it started reading 15 degrees too low. I now use the analog thermometers with the nut at the top for calibrating, and calibrate them each time before using. I liked the probe, but there was no way to calibrate it, and now I just don't trust them.

[ConnieG]

Yeah.  I got about 3 uses out of them and then I could tell the temperature more accurately with my finger than what the thermometer read. 

[Sailor Con Queso]

I'm sort of anal about temperature and like to be within 1 degree of desired temperature targets. I just can't read an analog accurately enough to get this close and on several occasions I have twisted the analog dial while it is on the pot and wiped out the calibration. So, I prefer a digital without an external plug in probe. Here's what I use.

Digital Thermometer

I occasionally test it in both frozen slush and boiling water to be sure it is "calibrated".

[steampwr8]

Can the CT-03 be calibrated?

[iratherfly]

I actually got that very thermometer (see photo below). I use it in my 20qt pot for Sous Vide cooking and for some cheese. (How I wish that the temp. alarm be adjustable to low sous vide / cheese temps)
My problem is that like many others here, I don't know what thermometer to believe anymore...

Analog thermometers should be the most accurate and reliable (even Extech tells you to calibrate their probe thermometers based in an analog thermometer).  That said, analog thermometers too have been plagued by inconsistent readings from one brand to another.

My assumption (for now) is that the most accurate readings are with analog thermometers that carry the NSF seal on them. These are the only ones approved for professional use - for a reason, I suppose.

What's wrong with this picture?
(Note in photo below, how the only thermometers that were in line with each other were the 2-week old digital oil/candy thermometer and the one analog thermometer that carries the NSF seal)

[linuxboy]

In the digital thermometer world, I only like the durable K/J probes, like the ones cooper atkins makes. But those are pricey.

I also calibrate regular dial thermometers against an alcohol lab thermometer. And I have a correction curve for the lab thermometer based on temp readings at boiling and ice water, corrected for atmospheric pressure. This lets me have a solid known standard that is authoritative.

[iratherfly]

Linuxboy, I think it takes it over the top for me... just need something dependable and accurate - and not too expensive either.  I am thinking that the Taylor brand professional food service thermometers are worth the $17-$25. They are all NSF and ANSI certified, some are ISO and HCAAP 9000 certified too and have accuracy as much as +/- 0.6F all the way from 0F to 230F. They can all be calibrated. Only question is to what - since I don't have a lab thermometer...

I am also planning on building a machine to do both sous vide and cheese. It will be made with industrial grade thermocouple connected to a PID unit that will be programmed to control a heating element based on the milk/water temp. Not sure how to calibrate that thermocouple either...  Has anyone here made such thing before? If so - advice will be helpful. If not, does anyone want to share the experiment? The parts should cost about $30-$60 only and it shouldn't take more than an afternoon to build.

[moodock]

They can all be calibrated. Only question is to what - since I don't have a lab thermometer...

Couldn't you just use the boiling and freezing points of water in absence of a lab thermometer?

[linuxboy]

Absolutely, you can use the boiling and freezing points. I like to use lab thermometers to help with calibration because they're cheap ($3) and once you determine the drift curve for that specific thermometer, it always works. It's a do once and re-use kind of approach, to have a single authoritative source. You can do the same thing for thermocouples and figure out the drift, sure. But with cheaper thermocouples, there's room for them to not perform well under all conditions. Like if you drop one and it experiences stress... it could throw off the reliability on cheaper ones. Thermocouples rely on an electrical signal and this can be degraded by physical damage of the cable, circuitry, or junction.

Let me know if that's not clear of if you need help with figuring out how to calculate the bias curve for a thermometer. There's probably some guide you can find online for it, too. I just learned the lab methods long ago, so I still follow the same ones.

Yoav, it shouldn't be too hard to create a PID-controlled unit. There are similar devices out there. One fellow has even created a converted rice cooker to act as an automated cheesemaking machine, like a bread machine.

[moodock]

I use lab thermometers for calibration as well. I was just suggesting the obvious if you do not have one available. My father was a chemist, so I have boxes and boxes of lab equipment and glassware (which comes in handy more often than you would think).

[iratherfly]

perhaps I don't understand what a lab thermometer is and how it's different from any other high quality thermometer. What is this mystery $3 thermometer that is so good that all those $25 thermometers should go by it? ..and why does it need all these extra recordings? Isn't there just a good reliable instrument out there?

Boiling and freezing points are inaccurate unless you calculate them at ISA (elevation 0 ft MSL and outside temp of 15C) or, you calculate the pressure altitude based on the changes to your air and elevation. Even then, impurities in the water can change their boiling/freezing points. You can use purified water, is the look of bubbles enough of an indication that you should be 100C? How hard should the ice be before you can declare it perfect 0C?  In other words, I hope this isn't getting to be as annoying as using pH meters. 

[linuxboy]

An alcohol lab thermometer is analog. And it functions based on the established properties of thermal expansion, which functions linearly based on temperature. Meaning that if you use RO/distilled water, and boil it for the high reading and put in ice water for the low, and record the barometric pressure and elevation (as you said), you can develop an exact bias curve for that specific thermometer. It will have that same bias curve from now on. You might need to make slight adjustments in the future based on the barometer, but those are minor.

It's so good because it's analog.

All thermometers need all the adjustments based on the bias curve of each one. But with calibrateable ones, the bias can be adjusted for. When you use a bi-therm (the metal dial ones), it suffers from the exact same situation as analog mercury/alcohol thermometers - the reading bias. But, unlike the lab thermometer, a bi-therm may develop different bias curves over time, especially as it is calibrated. So it needs to go through the entire effort or recalculating everything to be used as a standard, whereas with a lab thermometer, you just need to adjust for daily differences in the barometer.

With digital probes, they are actually more reliable, especially if of high quality and especially if the circuitry accounts for calibration fluctuations. But same sort of idea if used as a standard - the degree of drift may change, especially if a cord is kinked or it's dropped, or other some such. This rarely happens with good units and easy to spot by using two thermometers to compare.

Yes, there are good, reliable bi-therms, alcohol/mercury thermometers, and digital ones. Most of them are costly.