Digital Thermometer

[george graves]

I'm looking for a digital thermometer to test some NTC sensors.  One I'm going to be working with come with a crappy data sheet that I'd rather re-map it's curve myself, and other sensors I just want to test and make sure are in spec.  So a digital thermometer is what I need.  I have a fluke 87-5, but I'm going to be measuring fluids, from  freezing to about 300-400 degrees F.

Is there a probe that I can use with my fluke? 

Any other stand-alone unit recommendations?  I think I'd like to stick with digital (for quick reading) and under $100 - but I'd prefer to spend $50.

Thanks!

George Graves

[DaveW]

I bought one of these a while back, very good for the price (sure you can find similar in the US). Even with the supplied thermocouples will easily cover your temperature range and it's been accurate to within 1 percent when I've checked it against a fluke sensor
http://cgi.ebay.co.uk/2-K-Type-Digital-Thermometer-Thermocouple-Sensors-/220716879000?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item3363c04c98#ht_2253wt_1139
With these make sure that the unit itself will be kept inside a reasonable temperature range, as the ambient normally has a small range before it goes outside spec

[saturation]

Look at non-contact IR thermometers; they are very fast, safer to use and to -+ 2F or 1 C accurate.

http://www.amazon.com/Digital-Infrared-Thermometer-with-Laser/dp/B001A4E5JW

[alm]

Sounds like a standard type K thermocouple (there are also probes specifically designed for fluids) should work fine, and is supported by your DMM. If you want better accuracy, you can use other types of thermocouple or resistive probes, but these aren't supported by your DMM, so you need a separate thermometer (or one with integrated probe).

Not sure how well IR would work, you have to correct for emissivity, the specs usually state the emissivity it was calibrated for. Liquids would probably screw that up, you may have to measure it indirectly (measure something solid in contact with the liquid), which will increase the heat capacity of your 'probe'.

When measuring liquids, keep in mind that you either need to stir, keep it in an enclosed space with the same temperature for a while, or accept temperature gradients. Same with solids actually, except that they are usually harder to stir.

[george graves]

(I said I'd be measuting fluids - maybe I should have up that at the top? My bad)

BTW - even if a IR non-contact system would work, my sensors are going to be in fluid, so I'd really rather have the measurment sensor 10mm away from it in the same fluid and I gently heat it up and cool it down while I take measurements over period of time.

QUESTION:Can you actually put a probe like this into water/oil/ect?  That would surprise me greatly.

[Chasm]

Depends on the probe.
Those on your picture not really but there are many other forms for various environments.

[JohnS_AZ]

You might want to wander the offerings here ...

http://www.omega.com/temperature/tsc.html

Pricey at times, but they have just about everything under the sun.

[alm]

I believe some (most?) wire probes can be immersed in some liquids (low resistance might interfere), but thermocouple probes come in all shapes and forms (JohnS_AZ's link shows a bunch, Fluke sells some, and there are also cheaper alternatives). This includes those especially designed for surface, gas or liquid. Thermocouple does not imply wire probes, it just means the probes consists of a junction with two different metals.

[Mr J]

an isolated J or K type TC will work fine, the isolated ones are encased in thin layer ceramic and packaged in a stainless steel sheath. Rule of thumb TC are accurate within +/- 1 %. For something more accurate I would use and RTD (100 ohm) there a lot more accurate, about .1 %. Also when calibrating in fluid you should have a magnetic stirrer to heat the fluid at a consistent level.

Some standard calibration points that I use (you don't need to have a calibrated probe or thermometer but it helps)

32 Deg F (use ice, distilled water and a little bit of isopropyl alcohol, stir, measure)
134 Deg F Boiling Acetone (not nail polish remover, use 100% Acetone, no open flames on this one, be well ventilated)
181 Deg F Boiling isopropyl alcohol (no open flames on this one, be well ventilated)
212 Deg F Boiling water

Typically when we don't have a traceable standard we use this for TC, RTD, etc. anything higher than 212 Deg I use an oil bath with a heater / magnetic stirrer and an calibrated RTD with a NIST traceable calibrated meter. Also for safety use an oil with a high flash point. This might be over kill for you I'm not sure what accuracy you are trying to achieve.

[Chasm]

One caveat.
Whenever you are measuring anything be sure about what you are actually measuring.
Sound good and very vacant, eh?

Still you have to know if what you are asking for is actually what you get. (Here a certain temperature.)

Accuracy vs. Resolution - and both of them vs. the circuit you use.

Resolution does not mean accuracy.
Yet the higher they get the more impact have are other, external influences. You have to know what you can archive with "stock" parts and solutions and when it gets necessary to put a lot of thought into the whole measurement system.

Using an 8 bit ADC is pretty straightforward.
Using a 24 bit ADC the same way will not result in measurements that have 3 times the resolution or accuracy, often not even twice as much. That should be pretty obvious and known to anyone in electronics.

But what is "simple" or "high end" when it comes to other things?
We can measure distances with extreme accuracy, IIRC it's what we can measure the best.

Measuring temperatures with 0.1°C accuracy is pretty high end for industry use.
If you mess up your PTD-100 circuit (4 wire measurement of course!) you are actually getting the temperature of a resistor in that circuit. - At least as far as that .1 digit is concerned. Which pretty much obliterates the the whole effort.


So if if "a bit more" accuracy in commercial systems costs suddenly much more you maybe should take a second look.


Oh, "no open flames on this one, be well ventilated" is no joke, that stuff burns with a light blue and nearly invisible but really hot flame.

[tyblu]

Using an 8 bit ADC is pretty straightforward.
Using a 24 bit ADC the same way will not result in measurements that have 3 times the resolution or accuracy, often not even twice as much. That should be pretty obvious and known to anyone in electronics.

This depends entirely on the environment and implementation. A 24-bit ADC may well give 24 valid bits ... an an anechoic chamber, super-cooled, and <1 LSB (1/2^24 or ~60ppm) ripple supplies.

Never say never!

[Chasm]

Didn't say so.  But then I don't have such a chamber in my part bin either.

ADC wise just a couple Atmel and those do not really count.

[george graves]

Wow - lot to think about.

I did some shopping and it seems to be three levels of stuff

Things you stick in your butt <$10
Food grade stuff(very narrow range) <$25
Lab grade that (claims 1 degree C accuracy) <$50
Crazy industrial stuff >$200

I went with a lab grade unit that will measure to ~400 F.  That should give me the data I need.  More importantly, I was just looking for something that I can trust.  There's nothing worst than a crappy instrument, know what I mean?

[saturation]

Yes, if the container is colored black, life is very good.  Like a trade off, the speed and response of the IR thermometer can be offset by the reduced accuracy caused by emissivity. The $100 varieties can make measurements from many meters away, localized at the laser point, adding to safety, and convenience.  When I use them, you can also sample the object at various spots making a 'thermograph' of the heat distribution.  Unlike an immersion probe, which gets the average temperature of a body, a IR probe can measure small differences [ i.e., precision] within the object itself.

Sounds like a standard type K thermocouple (there are also probes specifically designed for fluids) should work fine, and is supported by your DMM. If you want better accuracy, you can use other types of thermocouple or resistive probes, but these aren't supported by your DMM, so you need a separate thermometer (or one with integrated probe).

Not sure how well IR would work, you have to correct for emissivity, the specs usually state the emissivity it was calibrated for. Liquids would probably screw that up, you may have to measure it indirectly (measure something solid in contact with the liquid), which will increase the heat capacity of your 'probe'.

When measuring liquids, keep in mind that you either need to stir, keep it in an enclosed space with the same temperature for a while, or accept temperature gradients. Same with solids actually, except that they are usually harder to stir.

[apex]

You could use something like this one here:
http://www.pollin.de/shop/dt/Nzc5OTYxOTk-/Messtechnik_Uhren/Messtechnik/Temperaturmessgeraete/Temperatur_Tastkopf_Temp_1000.html

Translated:
http://translate.google.de/translate?js=n&prev=_t&hl=de&ie=UTF-8&layout=2&eotf=1&sl=de&tl=en&u=http%3A%2F%2Fwww.pollin.de%2Fshop%2Fdt%2FNzc5OTYxOTk-%2FMesstechnik_Uhren%2FMesstechnik%2FTemperaturmessgeraete%2FTemperatur_Tastkopf_Temp_1000.html&act=url

apex

[Jon Chandler]

One thing to keep in mind with IR measurements....

The field the IR probe "sees" is cone shaped, anywhere from 8 degrees wide to as much as 30 degrees.  The temperature reading on the probe is the average of what it sees.  It is not the temperature of just the area covered by the laser dot!  This assumption can lead to some big errors.

[saturation]

Yes, its part of the device's spec, called distance-to-spot ratio (D:S).  A typical one  is 12:1, point it about 12 inches away for a 1  inch diameter spread; 12 feet away, 1 foot spread etc., some short distance ones have lens so the laser spreads proportionate to the D:S ratio, a nice convenience.



Pricey Fluke $700+ one's are 50-60:1, the tighter the beam the more expensive it is.

I use a $6 handheld one everyday, its rated 1:1, for general use 12:1 types are $24.

http://www.fluke.com/fluke/usen/thermometers/infrared-thermometers/fluke-570-series.htm?PID=56091

One thing to keep in mind with IR measurements....

The field the IR probe "sees" is cone shaped, anywhere from 8 degrees wide to as much as 30 degrees.  The temperature reading on the probe is the average of what it sees.  It is not the temperature of just the area covered by the laser dot!  This assumption can lead to some big errors.

[tyblu]

I have this one: http://www.dealextreme.com/details.dx/sku.26611

12:1 D:S ratio.. no clue about actual accuracy, but only need within 5-10C.

[Wim_L]

Indeed. Some models have not one, but several dots to better approximate the field of view of the sensor.

Regarding emissivity corrections, note that at very low emissivity (mostly shiny metal surfaces) it becomes almost impossible to do a proper correction for emissivity due to the effects from the environment. For example, if emissivity is 0.40, there will be a reflectivity of 0.60. A large amount of the signal measured by the detector will then not be coming from the sample, but will be heat emitted by the environment and reflected off the sample. The best measurements with a simple detector with emissivity correction will be when the object being measured is much hotter than the environment, so that the contribution from reflections is minimal.

There exist IR thermometers that are designed to take readings from very close distances only. Those can correct for this problem in a better way than long distance sensors, because the reflection will mostly becoming from the detector itself, whose thermal properties are known.

The best situation to do such measurements is when you can modify the sample to have a matte black finish with near-unity emissivity.

[saturation]

You can calibrate IR thermometers just like a thermocouple, I use extremes like an ice cube and boiling water as per Mr, J [ we also have these lying about easily].   When you get used to using a IR thermometer look for dark/black objects as much as possible, without it yes, errs creep in.  A simple way for liquids in white or stainless steel metal pots is to have a dark or black thin colored plate inserted into the liquid, wait a minute to equilibrate then measure the temp of the plate.  It also helps when you tend to select dark colored cylinders that carry fluid.  The smaller and thinner the plate the better.

Make sure optics are cleaned when used, just wipe it with an clean rag before use.

[george graves]

You can calibrate IR thermometers just like a thermocouple, I use extremes like an ice cube and boiling water

Just for the record, an ice cube can be below freezing.

[JohnS_AZ]

   Was thinking the same thing.
That's why you use an ice bath, not ice cubes alone.

[saturation]

Yes, a water bath is best, but you need to get temeprature equilibration with the cubes and for 100C, set to boiling.  An IR thermometer focuses on a single spot on a cube, and roughly with read within 3-5F, and easily the same with any boiling/hot water; hot water dispensers often come at a fixed temp 170-180F, and it can confirm the IR readings quickly.  The issue is speed versus the inaccuracy you can tolerate.

[saturation]

My old IR thermometer spontaneously died and I ran into this one.  If anyone is looking for well made  IR thermometer but at an unheard of price, this is one.  Its very confusing as there are so many look alike and brand less ones and the quality control on the brandless versions are very varied.  The clones seem to use the model number DT 380, whereas Cheerman model numbers are mostly DT 8380, for digital thermometer, distance to spot ratio, then upper temp limit.

Currently its selling for $12, $16 with S&H from Amazon. 



http://www.amazon.com/HDE-Temperature-Infrared-Thermometer-Laser/dp/B002YE3FS4/ref=cm_cr_pr_product_top

Although listed as 'brandless' I tracked the manufacturer to Cheerman:

http://www.cheerman.cn/

IR thermometers and other variants seems to be all they do, and they did a good job on these models.  Finish, fit, material and workmanship is top quality, and for $16 delivered, its very good.  Although rated to 380^oC, I tested it from 0-500^oC and its accurate, compared against a thermocouple.

Online references and manuals, its the same that's in the device packaging:

http://www.cheerman.cn/EnDownload.asp?Title=Download

I haven't been able to disassemble it to see the PCB as its partially glued together.