I am noticing a large variance in what my temp gauge is showing (205 degrees) vs. what my infrared thermometer is reading (178 degrees) at the thermostat housing. Reading at the radiator cap is 140 degrees. Sending unit is stock (340 LA)connected through new factory wiring harness to an aftermarket (Autometer Pro Comp) temperature gauge. I am thinking gauge needs to be calibrated or a new sending unit. Any suggestions would be greatly appreciated.

Emissivity is the measure of an object's ability to emit infrared energy. Depending on the color and/or reflectivity of the material you are measuring the temperature of, the temperature gun needs to be calibrated to that material or you will get a false reading. Emissivity is generally as low as zero for an extremely shiny surface to as high as 1.0 for a surface painted matte black.

Unless the temperature gun has been calibrated for the emissivity of the material you are measuring the temperature of, the reading will be incorrect.

I understand some of the cheap IR temperature guns don't even have the ability to calibrate to the emissivity of different materials which IMHO renders them next to useless.

Add in reflectivity and the IR readings can be pretty loosey goosey.

My AutoMeter pro comp temp gauge came with a new sender. Yours didn`t?

I agree...should have used the new autometer sensor..that will eliminate a few 'maybe' iassues.

70Duster, good info! I had no idea & it makes me question how accurate my HF gun is going to be (still in pkg/ain't used it yet)

Although i do a lot of temperature measurement,
I held off buying an IR gun until i saw the
Craftsman Pro 1400 unit
That includes a plug and type K thermocouple probe
for fine tuning the emissivity reading

http://www.searsoutlet.com/1400-Degree-N...amp;mode=seeAll

I think the Fluke IR gun also has this feature.

I have now seen $19 IR guns,
and along with "free" Harbor Freight multimeters,
i would not argue that these very inexpensive units have their place in nearly any tool bag that all vehicles ought to have

You can't get a fully accurate temperature reading checking different surfaces with the temperature gun because the different materials absorb heat differently.

When I was trying to test my temperature gauge I boiled a pot of hot water and checked that and it was pretty accurate, then stuck the sender in the pot and compared what the gauge said to what the gun said and it was pretty close.

Thanks to all. Good info. My IR gun I purchased from the HVAC supply house I work at is made by Fieldpiece which at my cost was around $80. However it does not have emissivity fine tuning. I will look at the Craftsman Pro 1400. Thanks again.

That sounds like the right kind of setup. But I question the need for the IR gun if you get good temp readings with the type K probe.

I have a type K thermocouple probe that plugs into my multi-meter, I believe it's a GreenLee Meter. It has proven to be extremely accurate and I've never needed an IR gun because I've always been able to accurately measure temperature with the type K thermocouple probe plugged into the multi-meter.

Thanks for you advice. I went out an purchased an infrared thermometer with an adjustable emissivity feature. I then contacted the manufacturer of my aluminum radiator to find the grade of aluminum that their radiator was constructed with. They replied that it was 4043 rough aluminum. At that point I began to Google emissivity ratings for the above mentioned. Not knowing enough about what this would translate to I am seeing ratings for oxidized aluminum, polished, plain sheet etc., but nothing that I can understand would pertain to my radiator. Any suggestions on where else I might look to for a more definite answer? Thanks again for the advice, I learned quite a bit about the importance of knowing what I am pointing that device at.

You really can't depend on bare aluminum for good measurements. Tape 2 pieces side by side of flat black electrical tape (or shiny black and scotch brite it real well) and put it on the bare aluminum. Aim your temp gun a couple of inches away from the tape and you will be good to go.

You guys are good!

You need the proper sending unit for your gauge and then verify it's reading accurate with a good, accurate, heating and air conditioning pocket thermometer
I had a high dollar racing 2 5/8 mechanical Auto Meter gauge that red 140 F when the circulating coolant in the radiator was body tempature I bought a good pocket thermometer and it read 98F when that stupid Auto Meter racing gauge read 140 F Same thing on two new Stewart Warner mechanical industrial temp. gauges
Always trust and then verify when in doubt

I recently installed a matching Autometer sending unit and a Mr.Gasket radiator cap that has a built in temperature display. There is about a 20 degree difference in what the cap displays (190) vs. what the dash gauge shows (210). I will take all the suggestions posted and apply them. In conclusion I will take the cap and put it in a pan of hot water and take the temperature of the water with my radiator thermometer and compare it to the cap for accuracy. Thanks to everyone for your suggestions. Good stuff!

if you want an accurate reading you will need an 1/8" thermocouple probe and meter. I put the probe in as close to the sending unit as possible making sure that it is in the coolant but not touching anything. Thermocouples are extremely accurate and respond very quickly. Using a probe extension one can put the meter in the car, drive it around and compare #'s.
My experiences show that if the readings are off, they typically go further out of calibration the warmer the temp. if it's an electrical gauge this can be due to grounds, other electrical issues, the sending unit, or the gauge itself.

Inexpensive meters and probes are available on ebay. Do be aware their are different types of probes with J or K being the most common. There are also different connector styles so heads up on that as well