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Most IR gun type thermometers with laser beam projectors do not use a beam splitter for both IR and Laser to be on the same axis. You may have to move the laser pattern a specific amount of offset to get the resistor in the IR sensor's field of view for very close work. My Raytek Raynger ST appears to have an offset of about 13mm.

Chuck

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Some guns will give correction factors for a given emmisivity. Usually there is a correction table for aluminum, steel, plastics, etc... I have successfully used it to check temps of mosfets, transistors, with TO220 cases.

I was prompted by a discussion in another thread on this forum to buy one of those contactless IR thermometer guns. It is an IR4890 by IWH.

I tried to measure the temperature of a surface mount resistor. The thermometer shows about 25 to 26C but the resistor is hot enough to cause me to immediately remove my finger from it.

The thermometer works on some other objects but has trouble with the PCB solder mask and surface mount resistors. Can anybody explain this?

Thanks, John

Look at the beam size. A laser indicator does NOT mean that the sensing size is the same size. it'll typically be several degrees. It will not read typical surface mount resistors accurately - they are too small.

The laser has a center dot and then 12 dots around the central dot forming a circle. They say it samples the area of the circle. I put the gun so close to the resistor that all the dots merged into one (as far as I could tell). Also, the resistor is a size 2010.

Thanks, John

The manual doesn't address much of anything. It appears that you are right, though, about the laser and accuracy up close. That was quite a revelation to me.

Still can't get the resistor to read much over room temp. The PCB, however, is saying up to 94C in spots. Another revelation.

Thanks for your input.

John

You are correct. Thanks for pointing that out.

I still can't seem to get the resistor to show much above ambient, however.

Thanks, John

Aha! I had not been paying attention to the gun's IR input area. By carefully placing the gun such that the resistor is on the sensing hole axis, it seems to be working.

Call it pilot error.

Thanks to all for pointing this out.

John

The thermopile does not make a peak measurement. It integrates the radiation from the beam angle, however wide this beam is. You only get a good reading from a target that covers the whole angle and is equal in temperature over this area.

Rene

Furthermore it ASSuMEs a fixed radiation coefficent (field emissitivity of 0.98), which is patently false when sweeping from a rough colored surface to a reflective (solder) surface.