Whether you want to spend more depends largely on what you want to use it for. There are three things you get for more money:
1). Wider detection range. There are some models that can accurately measure from ā20F (about ā30C) to 2200F (1250C) in a single range.
2). Adjustable emissivity. The accuracy of the measurement depends to a certain extent on the emissivity of the material you have the device pointed at. For most materials you can approximate it, and you wonāt be far off. IIRC, the usually approximation is about .9. the more accurate the emissivity setting, the more accurate the temperature measurement.
3). Measurement spot size. On the lower end thermometers the measuring spot will be about 1/10 the distance from the object. So if you are ten feet away, the measuring spot will be about 1 foot in diameter. For more money you get a small spot, I have seen them down to 1/120. Obviously if you are looking at relatively small objects from a healthy distance, you want a much smaller spot, at 1/120 the spot is 1 foot in diameter at 120 feet.
The top end devices with the wide temp range, emissivity adjustment and very narrow spot will cost upwards of $1000. As a hobbyist it is doubtful you need that capability, and for most hobbyist applications, a low priced unit will be more than adequate.
Thanks, that was very helpful. I guess for looking at flashlight temperatures the cheaper ones will be fine. Donāt think I will ever have 30Ā° in my applications. Also great info on the spot size, but you are right pointing at a flashlight LED or driver will be fine with almost any of these devices.
M4D M4X had a good point re: range, for my automotive uses a higher range might be useful.
Thatās fine if youāre checking to see if you should take an aspirin, thatās not what I was talking about. I know Wikipedia is not the best source, but I think this quote sums up what Iāve always been taught as an EMT, so Iāll just leave you with this.
Iām wondering what thermometer to buyā¦.GM320 or GM300E.
The latter costs double, but has adjustable emissivity (and larger range which means little to me).
Do you think itās worth to pay extra?
If you want to do measurements on a shiny titanium light, I can imagine you want to adjust emissivity. Comparing titanium to aluminium in terms of thermal properties is an interesting aspect of the hobby.
This is something you can take into consideration.
Emissivity also matters on a silver aluminum light versus a black one. Shiny is tough to measure as the shiny surface bounces back the light. It gets complicated, like most things, the further you check into it.
I seeā¦for many surfaces GM320 will be OK, but for some - it will be way off.
I could write a calculator to convert values back and forth, but itās not worth the effortā¦
So wait, maybe Iām not getting something. How does the expensive one work? You set emissivity value manually, and then the thermometer gives you a reading? Because then you can indeed do the conversion manually. But it also means youāre the one who determines emissivity value must be altered.
It would be nice if youāre thermometer can read/scan emissivity value of the surface, and then gives you this value along with the temperature, but Iām starting to think this is not the case.
Thermometer canāt tell what is the surface emissivity. User has to estimate it by themselves and tell the thermometer, so it can do the calculations internally. Iām not sure how does this particular thermometerās configuration work - f.e. Flukes have High/Medium/Low.
Note: sometimes instead of adjusting thermometer for surface emissivity you can adjust the emissivity itself by f.e. adding a piece of black tape on the measured element. For various reasons it may not be possible thoughā¦
I looked at both, and found out theyāre both cheapos; Iāve got the GS320 btw. So youāre probably not going to get accurate results anyway. But maybe with the more expensive one with the adjustable emissivity you can perform like a calibration; you adjust it so the readout equals the value that you know is true.
So the emissivity function is a bit of a gimmick I think, but you could nevertheless use it for your convenience. Itās just a few $ extra anyway.
I tried a calculator that converts between apparent and actual temperatures for various emissivity values:
The results are way off for low-emissivity materials.
Letās take apparent temperature of 150 Ā°C, thatās what IR thermometers measure directly.
GM320 will show the real temperature of 157 Ā°C and it will be correct for 95% emissive body.
But if the body is 60% emissive (not very low really), itās actually 230 Ā°C - way outside of expected error even for a cheap thermometer.
What you said makes sense: according to Banggood site, the fixed value of emissivity is 0.95, and the adjustable value is 0.10-100. This means there is little adjusting to higher values, and there is more adjusting to lower values. There is adjusting on linear scale, from 0 to 1, but emissivity is I suspect just like many natural phenomena something occurring in the logarithmic scale. So this means large deviations, as youāve illustrated, comes from stuff thatās shiny. But how shiny is shiny? Eventually you need to know emissivity, or look it up. The GM300E has a ābuilt inā calculator, and thatās it. If you do a lot measurements it could be useful, but if youāre only doing measurements sporadically you may not need it and perform calculations manually. But then again, like I said, itās only a few $ extra.
Normally you just look it up.
For example from https://www.thermoworks.com/emissivity_table
So āshinyā is 0.03-0.1. I donāt think at this end of the range any of the thermometers above will really work, thatās why I used modest 0.6 in the calculation above.
Yes, all the calculations can be done manually. I just find the extra user friendliness worth the $6.
The table shows that basically most materials that are sort of āroughā have a value of 0.9~0.95. But polished aluminium has a value of 0.05ā¦ So this means there are lot of surface finishes of aluminium (and anodisation) that will be in between that will be hard to look up.
So what we could do is to take a bunch of battery tubes, put them in the oven with fixed temperature, letās say 50C or 100C, then open the lid and measure all battery tube temperature values. This should give us insight how emissivity values can vary.
Weāre moving about topics far outside of my knowledgeā¦but quickly seeking I found 2 effects:
Surfaces also reflect some IR light from their surroundings, in this test it would be noise. Maybe measuring mirror would tell us something about the noise floor? Anyway, moving up in temperature would certainly reduce the effect of reflection.
Also I see that radiant power is proportional to the fourth power of temperature, so even modest increase would bring the results further apart. Not sure if it matters though.