Thermometers

[jdaw1]

Ages ago, I bought a pack of cheap digital thermometers on Amazon. Not good. Adjacent thermometers can differ by 2°C, and by 10% of humidity. I got what I paid for.

I recall a physicist remarking that thermometers are difficult to calibrate.

What do people use, and do they know it to be accurate? Are mercury thermometers the most likely to be stably accurate? Etc.

[Glenn E.]

I use a Thermapen in the kitchen. It is fast and reliable. I believe that mine is a Mk IV. The current Thermapen One claims to be accurate within 0.5 degrees F (0.3 degrees C).

Is it really that accurate? I don't know. When I'm smoking a brisket, the difference between 200 degrees and 200.5 degrees isn't important. Neither is the difference between 128 degrees and 128.5 degrees when I'm cooking a steak. (Fahrenheit, in my case.)

It does match my home's HVAC control thermometer, but that's probably not saying much. I would not be the least bit surprised if you were to tell me that those can also vary by a couple of degrees.

[M.Charlton]

[…]
What do people use, and do they know it to be accurate?
[…]

We used pre calibrated Brannan thermometers back in my day. They worked well.

[MigSU]

I use a Thermapen in the kitchen. It is fast and reliable. I believe that mine is a Mk IV. The current Thermapen One claims to be accurate within 0.5 degrees F (0.3 degrees C).

Is it really that accurate? I don't know. When I'm smoking a brisket, the difference between 200 degrees and 200.5 degrees isn't important. Neither is the difference between 128 degrees and 128.5 degrees when I'm cooking a steak. (Fahrenheit, in my case.)

It does match my home's HVAC control thermometer, but that's probably not saying much. I would not be the least bit surprised if you were to tell me that those can also vary by a couple of degrees.

Maybe this is just funny to me, but I believe JDAW was talking about thermometers to measure the temperature and humidity of a room, not a piece of meat that's being cooked

[flash_uk]

Ages ago, I bought a pack of cheap digital thermometers on Amazon. Not good. Adjacent thermometers can differ by 2°C, and by 10% of humidity. I got what I paid for.

I recall a physicist remarking that thermometers are difficult to calibrate.

What do people use, and do they know it to be accurate? Are mercury thermometers the most likely to be stably accurate? Etc.

My own experience confirms the view given by the physicist. I built my own digital thermometer, which uses a thermistor (something that sees differing electrical resistance at different temperatures. I think.)

Various formulae are available which can be used to calibrate the thermistor, and the calibration process requires a number of readings at different temperature points. Read here for a not bad explanation of this.

The greatest challenge in all of this, for me, was finding a way to be confident of the reference temperature being used for calibration. Fairly straightforward for 0°C, although even that requires more care than one might think. Much more difficult for a reference point of say 20°C.

The cheap digital thermometers you have purchased, I would expect are all programmed with the same equation constants from a single calibration effort. This will fail to account for small manufacturing variances in the thermistor in the product. Hence the stated accuracy tolerances. Which are likely poor tolerance measurements themselves.

All of the above is likely best taken care of by buying a pre-calibrated digital thermometer from the link that Michael gave above.

[PhilW]

Your choice of sensor depends on several factors; perhaps the most important are required accuracy, cost, connectivity (wired, wireless) and with what logging/display/alarm/control capabilities.

We've used Sensiron's SHT range for T/H sensors in several product design (typically around £20-25 per part depending on volume), which offer accuary of 0.1C and 2% RH (and I can confirm that multiple units containing these sensors show a very tight spread); however these are components with I2C interface wiring, not products with displays; I would highly recommend them for accuracy/quality if you were building your own unit with whatever control/comms interfaces (but not if you want off-the-shelf sensor units).

At home I use Klimalogg Pro wireless T/H sensors (TFA 30.3180, around £25/unit) with a central Klimalogg Pro logger (TFA 30.3039, around £80) which uploads (wirelessly, but not automatically) to my PC for log/graphing and storage of old history. This kit is quite old now (I have about 10yrs of logs) and operates in 868MHz band, but is only suitable if there aren't too many walls/obstructions/distance between sensor and logger. I suspect there may well be equivalent units created more recently with similar quality sensor measurements and with improved connectivity options, including to server/apps etc (if you don't mind paying for the service, where needed ofc).

[akzy]

I recall a physicist remarking that thermometers are difficult to calibrate.

What do people use, and do they know it to be accurate? Are mercury thermometers the most likely to be stably accurate? Etc.

I do love a good deep dive.

Zak's top 5 4 Thermometers to buy your loved ones this Christmas that are common place in an quantum computing facility.

4. Room temperature and humidity with in built clock!

Pros:

• It's cheap

Cons:

• Low accuracy and precision
• Not going to impress your other half with it's clinical appearance

3. PT100
A thin wire of platinum which is 100 Ohms at 0 degC.
Pros:

• Simply measure resistance of a metal and compare to calibrated look up table* from -50 to 200degC.
• Small footprint for sensor
• Works as a small fishing rod should you need it.

Cons:

• Doesn't account for non-idealities of pure metals which means it's off by 25degC at -200degC! **

Switching units to Kelvin, 0 K = -273.15degC. Sorry Glenn, I can't bring myself to even entertain Fahrenheit.

2. Silicon diode thermocouple
Uses the magic of semiconductor physics to measure the temperature. Here's an example of it in action when I was testing some components. Look at all that precision!

Media.jpeg (60.91 KiB) Viewed 960 times

Pros:

• Voltage measurement and calibrated lookup table from 1 K to 400 K.
• Consistent performance and temperature response at low temperatures
• Simple friends will be impressed when you say that you have a silicon diode thermocouple

Cons:

• Thermal load of equipment ****

1. A single ion (take an atom, remove and electron, trap it in a pringled electric field. It's that simple)
Here's a picture of a trapped ion taken with a digital camera! Kinda nuts.

_MG_1200_compressed.jpg (208.64 KiB) Viewed 960 times

Pros:

• Extreme thermal isolation from environment
• Able to measure to 50uK (yes, 50 millionths of a degree above absolute zero)
• When you explain it to your parents they'll zone out and leave you alone

Cons:

• Once you've measured it, it heats up to ~1mK *****. But at least you know what is was
• >£250,000 brand new just to measure temperature - nothing else - second hand might be cheaper
• Not really measuring temperature, measuring energy ******

On a more serious note, what Martin suggested seems good.


* Calibration is a very long topic to cover. Taking Martin's example of calibration.

We used pre calibrated Brannan thermometers back in my day. They worked well.

This is calibrated to either one or two points. i.e. it's guaranteed (to some spec) to match at that temperature. But outside that temperature what happens?
When we consider a mercury thermometer, it uses the thermal expansion of mercury to give the temperature. When it heats up, its expands, raising the level on the thermometer. The problem is that this rate of expansion as a function of temperature is not constant i.e. at 0degC say my mercury thermometer rises 1mm for a 1 Celsius change in temperature. At 100degC that would only be 0.994mm rise. Tragic.
The look up table improves by provides the calibration across many temperatures which therefore accounts for the fact that this moves. A must have if your loved one truly cares about temperature.

** A true bain of my existence. Impurities. If you had a pure conductor, technically, when the temperature drops to 0 K (-273.15degC or coldest possible temperature ***) its resistance would also drop to zero. Because of impurities that doesn't happen. We measure this with a property called residual resistivity ratio (RRR) which is a ratio of the room temperature resistivity to that at steady-state low temperature (typically 4K). See the below example for copper.

Temperature-dependence-of-the-electrical-resistivity-of-copper-for-different-values-of_W640.jpg (53.67 KiB) Viewed 960 times

. As impurities are hard to control, this means it doesn't make a repeatable thermometer. Your father-in-law will therefore frown at you in disappointment.

***One for a future topic. You can technically get negative temperature.

**** Measurements are hard. One of the key principles is that if you're measuring something, the actual act of measurement shouldn't effect the result. Taking this thermocouples for example. These are connected with some wires to a multimeter at room temperature. This means they bring some heat load into the system and heat up what you're trying to measure. Embarrassing right?

***** Quantum measurements are even harder. The act of measuring the "temperature" destroys the that state so can't use that for future work. There's such things as quantum non-destructive measurements where you instead measure something else but that's another story.

****** Quantum comes from the concept of quantisation. Long as short of it is, we view lots of things as continuous. Take a pendulum for example swinging backwards are forwards. You could always imagine that you could keep swinging it a smaller and smaller amount. Problem is that at very low energies, that is not true and you realise that it is quantised i.e. there is one finite level of swinging and the next finite level with nothing in between. This means so far we've just measured energy. Temperature however is a statistical quantity so you'd need a ensemble. We can cheat however and approximate temperature to energy. A much better option would be to use a Bose Einstein Condensate. Nope. Not covering it.


And finally, I did originally include a thermal camera for the list. It was horrifically obtuse to use and there isn't a pit deep enough to throw it down.

[MigSU]

I like that what started as an informal chat about thermometers has devolved (evolved?) into a detailed post about the Planck length.

[jdaw1]

I can't bring myself to even entertain Fahrenheit

Of course not: °R is the way forward.

• Extreme thermal isolation from environment

Hopefully this is thermal isolation of the back end of the thermometer, as we want the front end connected to the reality being measured.

• >£250,000 brand new just to measure temperature - nothing else - second hand might be cheaper

For that much money I’d want to know the humidity as well. And maybe have records of the recent max and min, of both temperature and humidity.

As impurities are hard to control

Ahh, impurities.

we view lots of things as continuous.

But this audience counts bottles rather than measuring litres.

there is one finite level of swinging and the next finite level with nothing in between.

President Calvin Coolidge was known to be a man of few words. He came home from church one Sunday. His wife had been unable to attend, so she asked what the pastor had spoken about in the service. Coolidge responded, “sin”. “What did he say about it?” “I think he was against it.”

A much better option would be to use a Bose Einstein Condensate.

Presumably this is the quantum-mechanical version of Crusted Port, except not quite as good.

and there isn't a pit deep enough to throw it down.

Surely a SMBH would be deep enough, but I acknowledge that getting to one would require climbing a mountain likely too tall for your research budget.

[Alex Bridgeman]

Zak, you’ve solved Julian’s problem! He just needs to run his cellar at 0°K and he won’t need to take temperature readings.

[MigSU]

I think Heisenberg will rise from the grave and stop him if he tries!

[jdaw1]

He just needs to run his cellar at 0°K and he won’t need to take temperature readings.

Indeed, wouldn’t be allowed in the cellar at all, because doing so would add energy to the cellar, and also perhaps slightly for my own comfort (southener).

The disadvantage, OK perhaps a disadvantage, of running a cellar at 0°K is that the Port wouldn’t age.

[Alex Bridgeman]

I think Heisenberg will rise from the grave and stop him if he tries!

Surely you can't be certain of that?

[PhilW]

and also perhaps slightly for my own comfort (southener).

Loving the throwaway disdain.

[mcoulson]

What an excellent thread .... thank you everybody for your input especially Zak as always with a painfully interesting deep tech post !!!

Meanwhile ....

I have a room the size of a garage - it's got rack down either side and more Celotex than I care to imagine so it is thermally very stable - takes ages to move temperature, it's got some aircon for the summer and two small oil filled radiators for the winter - these two bit's of kit ensure that it's basically always between 12 and 14.

I have Tapo temperature sensors and Tapo mains plug controllers but I leave the aircon all on it's own and that's sensing the return air temperature.

"Celler desk" is a meter off the floor in the middle of the room
"Cellar Left floor" is at the back 6" off the floor on the left
"Cellar Left Front" is a meter up from the floor at the front of the racking
"Cellar Left Rear" is a meter up from the floor at the rear of the racking

I leave you all to work out where the right hand ones are ...

.

temp small.jpg (20.32 KiB) Viewed 838 times

.

As you can see from the Tapo interface left front and left rear are reading pretty much the same and they do that almost all the time.

Since the floor readings for left and right are agreeing I think it's hotter in the upper half of the room

I use the Cellar Desk reading to turn on and off the radiators - they come on at anything below 12 deg and then off at anything above 12.3 deg

The heaters tend to come on once and stop the room dropping temp wise and then stay off for a couple of weeks even with me going into the room every so often 

The Tapo kit is very cheap and has a sensible interface and a bunch of other home automation things that I have.

These are the units I use for the racking either side and the desk

And these are the ones on the floor as I can't actually see them

And this is the smart plug

They all exist as multi packs

I've basically concluded that as long as the temperature and humidity sensing is kinda in the right place than I'm happy especially consider how much I have managed to not spend on it - I do think that the small fortune I spent of Celotex is what's given me a space that's so stable.   

[akzy]

Hopefully this is thermal isolation of the back end of the thermometer, as we want the front end connected to the reality being measured.

You actually want to measure things? Pfft. No one told me that!

For that much money I’d want to know the humidity as well. And maybe have records of the recent max and min, of both temperature and humidity.

It does have a humidity high sensor in that too high with cause a rather catastrophic "bang".

we view lots of things as continuous.

But this audience counts bottles rather than measuring litres.

Well about that...

My vintage port collectively gains an extra bottle year every 1 hour, 26 minutes and 42 seconds.

Surely a SMBH would be deep enough, but I acknowledge that getting to one would require climbing a mountain likely too tall for your research budget.

One of the technicians got it working after I sent this post. I now look the fool.

[MigSU]

As a science master and a physics enthusiast, this is one of the best topics on TPF.

On topic: I have a cheap temperature/humidity sensor in my cellar that has no wireless or wired capabilities, it just keeps local records of high temps and humidity that I occasionally consult. I have an Arduino and a Raspberry Pi that I'm using for other purposes but I've been considering repurposing one of them for a more bespoke solution, but I haven't taken the plunge yet.

[jdaw1]

My previous electronic device didn’t work with rechargeable batteries and so, just to prove to my wife that I am capable of learning (exceptions apply, within limits, past performance is not a guarantee that future results won’t be even worse, etc), I don’t want a battery device.

Mains devices use power: i.e., leak heat. Me no like.

Mercury is long-term reliable and unchanging. As a thermometer, it works. So I’ve just ordered two of the THERMcert, pre-calibrated 215mm workplace thermometer, which is calibrated to be correct at 16°C, and hence at 13°C negligibly wrong (for my purposes). I doff hat to:

We used pre calibrated Brannan thermometers back in my day. They worked well.

But be cautious with mercury as it is nasty toxic (extremely nasty toxic, a video not for those of delicate disposition). Hence, always, I double-affix a mercury thermometer, such that one attachment failing cannot cause spillage.

[Glenn E.]

Maybe this is just funny to me, but I believe JDAW was talking about thermometers to measure the temperature and humidity of a room, not a piece of meat that's being cooked

Ha! Given the rest of the discussion, I think you are correct!

My thermapen does read air temperature reasonably accurately, too, but probably not accurately enough for whatever Julian needs.

I like that what started as an informal chat about thermometers has devolved (evolved?) into a detailed post about the Planck length.

Yes they are way over my head now!

[jdaw1]

Yes they are way over my head now!

If you know accurately how far over your head, then you must be very ignorant about where the conversation is going.

[Glenn E.]

Yes they are way over my head now!

If you know accurately how far over your head, then you must be very ignorant about where the conversation is going.

I'm not capable of accurately measuring how far things go over my head.

[M.Charlton]

Mercury is long-term reliable and unchanging. As a thermometer, it works.
[…]
But be cautious with mercury as it is nasty toxic (extremely nasty toxic, a video not for those of delicate disposition). Hence, always, I double-affix a mercury thermometer, such that one attachment failing cannot cause spillage.

We used mercury (horrible stuff) for a few different applications in my old lab, but we always had ready access to flowers of sulphur to account for any spillages. I suspect such a product is probably not an under-the-sink clean up agent for the home Port enthusiast - might be worth having some on hand just in case.

[Glenn E.]

We used to play with mercury in high school chemistry class. Not for long, and we probably weren't supposed to play with it, but we did.

We weren't allowed to play with the lithium, but our teacher did in order to demonstrate for us why we weren't allowed to have access to it.

We did a lot of things in the late 70s/early 80s that I look back on now and am amazed that we turned out okay.