Cheap thermometer calibration technique?

I know this subject of "cheap" and "calibrate" used in the same sentence may well be anathema to some of you but I need to verify that either my IR temp gun is accurate or my DMM/thermocouple is, or neither. Accuracy to 2 or 3 degrees F is fine.

I'm looking for suggestions for a simple way to provide some kind of common temperature "standard" (I use the term loosely, here) I can compare these against.

Thanks,

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DaveC 
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DaveC
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The freezing/melting point of ice is quite exact. Boiling water is good if you correct for your local air pressure.

Reply to
MooseFET

Freezing and boiling water are obvious points.

Tapwater stirred with a lot of crushed ice will get within 10-15 mK of

0 deg C. Boiling water, corrected for barometric pressure, will get within your error budget.

You can buy a thinfilm platinum RTD, for a few dollars, that is accurate to a fraction of a deg C, if you measure it with a good ohmmeter. Some of the semiconductor temp sensors are better than 1 deg C.

Fever thermometers are very good close to body temp. That and ice point is a combination that's hard to beat.

John

Reply to
John Larkin

I don't think i have boiled any water or iced a thermometer lately. I try to check out an unknown by a known in the shop. Sometimes I may have to use a third or fourth device to be sure. That is a problem to contend with, its very difficult to place more than one probe in one spot. I try to test using a beaker of water being stirred very rapidly, otherwise temperature differential will kill you. Measuring IR can be trickey, as I found out at home measuring aluminum foil. The heat of the reflection is what you will measure. Since I have a few thermometers on the wall at home. viewing the IR probe vs the readout gives a feel for the accuracies involved.

greg

Reply to
GregS

Don't forget that with your IR gun, the emissivity of the source is important in calibration. I suspect water and ice are not friendly calibration sources. Maybe a black anodized heat sink that has been heated to some calibrated temperature would be a good source. Does your gun have an emissivity adjustment?

Chuck

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Reply to
Chuck

difficult to place more

very rapidly,

as I found out

measure.

If you IR measure shiny copper or brass, what you're measuring is your own reflection.

I dab things with black whiteboard marker to improve the emissivity. Apparently Scotch Magic tape has high emissivity, too.

John

Reply to
John Larkin

Ice mixed with distilled (or for your purposes, tap will be fine) water and allowed to rest for a few minutes will give you 32°F within millidegrees. Be sure to stir it every half minute or so to get the thermoclines well distributed.

Boiling water at sea level will give you 212°F fairly accurately, less 1°F (for your stated accuracy) for every thousand feet of altitude you are above sea level up to a few thousand feet.

Jim

Reply to
RST Engineering (jw)

Not sure what you mean by "cheap", but you can get a YSI thermilinear network, which consists of a composite of three thermistors and some precision resistors for about $27. The one that goes from 0-100C claims +/- 0.15C accuracy (+/- 0.27F). That might be expensive for one calibration, but you might use it in the future. Then, you could get a black anodized heat sink with one side smooth, and drill little holes in the side, close to the flat surface, and insert the YSI thermistor and your thermocouple, maybe with some heat sink glop. Then point the IR detector at it. Let it go through some temperature changes slow enough that the whole thing has time to equlibrate, and collect some calibration data for both devices.

-- John

Reply to
John O'Flaherty

At what temperature? You have to consider emissivity with IR measurements. If your DMM/thermocouple reads okay with the input shorted (terminal block temperature) chances are it's accurate enough at typical heat sink temperatures. Chances are your IR is not.

Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

~130F, max. I am frequently asked to measure the temp of a stack of press output (big 6-color sheet-fed presses) to determine if the IR dryer lamps are functioning properly. Sometimes using an IR gun is fine, but some press operators prefer the traditional method of a probe inserted halfway down the stack. It would be nice if my tools' readings corresponded.

Re. the ice "calibration": as someone pointed out, ice & h2o isn't very IR-emissive, it it? Would it help to mix in a little black ink? (Lots lying around a print shop...)

Thanks,

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DaveC
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DaveC

It's possible to get a couple of degrees F error (high) from boiling water, particularly if you use a smooth vessel, such as a glass beaker, to boil the water. Maybe put something like an airstone in the bottom of the vessel.

Best regards, Spehro Pefhany

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Spehro Pefhany

third

difficult to place more

stirred very rapidly,

as I found out

measure.

I once used cigarette ashes and spit on a piece of aluminum. :-)

Cheers! Rich

Reply to
Rich Grise

Can you take the probe and the IR out to the line simultaneously and check a pile of paper with both of them simultaneously? You'd need the cooperation of the foreman and the workers, of course.

Good Luck! Rich

Reply to
Rich Grise

Measurement is taken after printing is done and lamps are cooled (they aren't in direct "sight" of the delivery stack on the press, anyway).

We've got lots of (large format) inkjet printers, too! Would h2o-soluble ink in the ice water increase the emissivity?

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DaveC
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Reply to
DaveC

So where's the "standard" in this experiment?

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DaveC
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Reply to
DaveC

All that IR from the lamps might be reflecting around and giving you problems too.

It won't mix in (it's oily), that's kinda the point with offset printing. ;-)

Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

If you use a stainless thermos or Starbucks coffee mug thing, and fill it with stirred crushed ice and water, and fire an ir sensor down into that, it should be pretty good.

I've got an ice cube on a paper plate on my desk, just starting to melt. Our FLIR thermal imager shows it at about +0.5C, and a cheap Extech IR thermometer claims -2C.

The emissivity of the ice cube seems close to 1.0. As I image it and wave my hand around nearby, there's no hint of thermal reflections or temperature change, so it's almost totally black in the thermal IR.

Oh my red plastic Presidential Water Cup is on my desk, too, with about an inch of water in the bottom. It reads 6.8C with a thermocouple, 7.0 on the FLIR, and 4C on the Extech. Again, no hint of reflections. So water also looks black at thermal wavelengths.

John

Reply to
John Larkin

Yes, but I don't know how you're going to get a matt finish on the water. More importantly perhaps, the ink might alter the boiling point.

Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

Is your FLIR imager one of the cryo-cooled ones, or do you have a newer one (less than a decade old) with the uncooled detector?

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Tim Wescott
Wescott Design Services
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Reply to
Tim Wescott

Talk about measuring with a micrometer, marking with chalk, and cutting with an axe!!!

The fellow is looking for a couple of °F accuracy.

Jim

Reply to
RST Engineering (jw)

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