Infrared thermometers question.

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I have been wanting an IR thermometer for a long time. For things like welding, measuring exhaust temps and such.But don't know much about them.http://www.maxtool.com/index/Digital...ermometers.aspI don't even know what temp range I would needI see there are a ton of the thermotech brand on ebay for 50 bucks or so.Is 900 degrees high enough for most preheat applications?Or do I need a higher range?Would this unit be worse than nothing.   I don't think I need it to be all that accurate.  I would like repeatabiltiy for tuning engines.Thanks Bernard
Reply:I worked quite a bit with infra red thermometers that measured the temperature of welded parts as they came through a forced cooling station on an automated production line.  Raytek is the brand I used.http://www.raytek.com/Raytek/en-r0/P...glethermometerTwo things to know about these things, first is focal length and spot size, second is surface finish and emmisivity.  The focal length and spot size tells you how big of a spot the device is measuring at a given distance.  The smallest spot may be at 12" or whatever, and as you go closer or further, the spot size increases.  Various models of IR thermometers have different spot size/distance capabilities.The surface finish of object you are measuring affects the temperature reading you get.  A flat black surface will have a emmisivity approaching 1.0 and will read higher than a polished chrome surface with an emmisivity of 0.5 or lower.  The better IR thermometers have an emmisivity adjustment, so you can calibrate the instrument by shooting a hot plate of known temperature (via regular contact thermometer), and adjust the IR thermometer emmisivity so it reads correctly for that particular surface.  If the IR emmisivity is not adjustable, you'll can still figure out how far out the readings are by reading a known temperature object.For my welded parts, I made a special heated sample part, with an resistance heating element, temperature controller, and digital readout.  The sample had the same surface finish as the production parts, so I could dial in the correct emmisivity on the sample part.
Reply:Thanks.   The models I am looking at have a fixed emissivity of .95So I guess I could calculate from that if I needed to be very accurate. Do you think 900 degrees is a high enough range?