Sciencemadness Discussion Board

Monitoring rxn temperature: which type of thermometer is best?

fusso - 9-3-2019 at 11:01

Never bought one for this specific purpose before.
Should I choose IR, electronic metal probe or others?
Please explain the pros & cons.
Thank you.

[Edited on 190309 by fusso]

XeonTheMGPony - 10-3-2019 at 04:23

the one that's accurate! I prefer Mercury ones as they are the oldest and well under stood tech, and from my experience pretty accurate, the alcohol ones can be faster responding in some cases.

So it really is a matter of personal preference, just get a quality one (Well buy several as they are easy to break)

IR ones are not very accurate, they are good for crude spot testing of a system (not very applicable for chemistry IMO)

Metal probe? I am wagering you're referring to thermal couple type, they tend to be fast and fairly accurate, but short of very expensive precision one the most you'll get is accurate to the degree or so. They do tend to be fast responding.

[Edited on 10-3-2019 by XeonTheMGPony]

Sulaiman - 10-3-2019 at 06:28

Mercury-in-glass thermometers are my favourite,
(probably just due to nostalgia, especially my two ground glass jointed ones)
but they are difficult to find and/or expensive,
they need great care when assembling or dis-assembling kit, washing, drying, storing ....
and there is the mercury toxicity to deal with - when I break one, or two.

Cheaper than mercury-in-glass thermometers are 'electronic' thermometers,
. cheap thermistor types - ok for casual use
. thermocouples - good enough for most chemistry purposes
. Pt100 platinum-resistance probes - most accurate

My personal choice is REX-C100 temperature controllers with type-K thermocouple and external SSR,
slightly more expensive than just a thermometer
ac-powered, not only a thermometer but also with the option of controlling heating power
e.g. https://www.ebay.co.uk/itm/Digital-REX-C100-AC-PID-Temperatu...
plus one or two different shaped thermocouples (e.g. bare wire, and long s/s sheathed)

IMO no matter what type of thermometer you buy (other than in-calibration-period certified)
you need to verify its accuracy.
Melting distilled-water-ice for 0oC
boiling distilled water for 100oC (with compensation for local amospheric pressure)
(I actually use tap water as I've found negligible difference in m.p. or b.p. compared to dH2O, but I do have nice tap water)

So far I have found that if the 0oC measurement is correct then the thermometer is within specification over the entire temperature range.
But I still do at least a two-point callibration check before I trust a thermometer..

Heptylene - 10-3-2019 at 06:49

If you want to go for precision, use a PT100 or PT1000 thermometer. You can reach AFAIK 0.1 °C precision with these. Personally I use a mercury thermometer for my reactions and distillations because it can fit in a thermometer adapter.

Ground glass joint thermometers are great for distilling nasty things like nitric acid. Otherwise you can get special fluorinated polymer sealing rings for a regular thermometer adapter.

You can make a glass-encapsulated thermometer from a thermocouple probe and a pasteur pipette whose tip has been melted shut. Quite handy, but usually not precise.

Whichever thermometer you decide to get, don't forget to calibrate it! Calibration is important if you want to measure melting points etc. otherwise you will draw the incorrect conclusions about your product. Ice-water mixture at equilibrium is 0 °C and boiling water is close to 100 °C (you can get the exact value for your altitude). Use more calibration points if possible!

fusso - 10-3-2019 at 09:38

For me, 0.1C is definitely enough. Do they tolerate down to -80 and up to... um, 300+ C? I want a wide range one for a wide range of experiments. Are these expensive?
For metal smelting/casting, I see many ppl on YT use IR. Is IR accurate at such temps?

Sulaiman - 10-3-2019 at 14:12

I am unable to measure to 0.1oC absolute accuracy.
(At work we have thermocouple simulators and meters with traceable callibration, but even then not to 0.1oC over the callibration period)

How will you achieve -80oC ?
(like buying a diving watch certified for 300m dives)

300oC maximum would be an unfortunate choice as sulphuric acid, many hydrocarbons and mercury boil at a slightly higher temperature.

For most distillations you do not really need an accurate thermometer,
just reliable and repeatable.

-10oC to +110oC is probably the most useful range due to water m.p. and b.p.,
sometimes I use my 50oC thermometer as it has a finer resolution,
electronic thermometers are MUCH easier to read.

Dr.Bob - 10-3-2019 at 17:46

Simple low temp analog glass therm will go down to -100 C and up to 20 or 50 C, I have lots of them for cheap, $15.

Basic non-Hg therm will do -10 to 260 C easily, within about 2 C accuracy. More precision than that is rarely needed for organic chemistry. They run about $20.

Hg ones can be found in most ranges and accuracies, but are a pain to ship or store, I have broken a few in my life, most often in hard to clean places. I sold one recently that was huge and was good to 0.05 C for a narrow range. But that is overkill for much.

Thermocouples are nice, but they can drift some in my experience, so unless you can calibrate them, they are not great for absolute numbers. The (yellow) K type work well at high temps, but the blue ones are better for most normal lab work. You have to have the right meter to read them, but easy to find those. But I use those as well, not ideal for immersion into many chemicals.

RedDwarf - 11-3-2019 at 07:15

IR thermometers have two main issues; firstly they measure the average temperature in an area which is bigger the further away you are from whatever you are measuring (think of a cone with the apex as the sensor) - the laser pointer is only a general indication to help you know approximately where you are pointing, secondly ir measurement is dependent on the colour and reflectivity of the target so at the extreme a shiny can and a lump of black charcoal at the same temperature will show different readings. You can use IR for smelting (it has the obvious advantage of being non contact) but cheap meters rarely go over 500C and still suffer from the inaccuracies mentioned. I'd use a thermocouple for smelting/forge work and a -10 to 110C thermometer for most lab work (I have both mercury and alcohol types). As others say you need to decide what level of accuracy you need - probably not as much as you think!

XeonTheMGPony - 11-3-2019 at 17:36

Quote: Originally posted by fusso  
For me, 0.1C is definitely enough. Do they tolerate down to -80 and up to... um, 300+ C? I want a wide range one for a wide range of experiments. Are these expensive?
For metal smelting/casting, I see many ppl on YT use IR. Is IR accurate at such temps?


Wider range = less accurate results in any one window.

I have -10c to 50c, 0 to 100c so on up to 300 even that is a wider range then I like when getting higher up.