Posted by joew on February 05, 19100 at 01:31:46:
In Reply to: Building a Pyrometer posted by Pat on February 04, 19100 at 21:48:25:
No idea about the precision you need, or if its for an oven, or what the intended use is, so there is no good way to answer the question. To build a crude, rough estimate of a commercial pyrometer, do the following:
Get a cheap analog (or expensive digital, if you like nice things) multimeter at Radio Shack/flea market/wherever. Go get a thermocouple (ceramics suppliers have them, for instance, maybe $20).
Type K thermocouples deliver about 54 ma. (milliamps) at most, at a temperature of around 2400 F.
Set the dial on the meter to read 50 miliamps DC amperage (full scale). If there is no 50, but there is 100 ma, put it there. Best to find a meter that has a setting that reads 50 ma. full scale. Settings other than 50 ma will hurt accuracy (only half of the tiny meter face is useful if set at 100 ma, for instance.)
Put one of the thermocouple wires into the + connector and the other into the - connector of the meter, and heat the thermocouple with a match. If the needle goes to the right, you have the connections correct. If needle tries to go the left, reverse the wires. If you can find a milliamp-to-temperature chart [ http://www.pyromation.com/] for whatever type of thermocouple you have, be it a type J or a type K or whatever, you are done.
If no chart, you can calibrate the pyrometer.
Find an aluminum roofing nail, heat the oven(?), stick the business end of the thermocouple near the nail, observe when the nail melts, and mark the needle on the meter-face where it melts. (probably around 25 ma. ... write 1200 F at that point)
Find some more or less pure lead, copper, silver, or whatever, look up their melting points, melt them and mark where the needle is. Put masking tape on the meter face for a place to put your pencil markings. Then fill in with marks every 50 or 100 degrees by estimation.
The bigger the meter face, the more accurate you can be. Any meter can be adapted if you know some electronics basics.
All meters react to millamps, even if it says 'volts' or whatever on the face of the meter. A 'multimeter' has internal components that transform an input to millamps, telling you several things, like ohms, volts, along with several scales of milliamperage.
Pyrometers dont need anything but the meter part.. you are already feeding milliamps into the meter. A bare meter can be easily burned out unless you know what you are doing, so the commercial multimeter is the way to go, as it has fuses to protect it.
A thermocouple's amperage (amount of electricity) output rises with an increase in 'temperature difference' between the cold end (wire ends plugged into the meter) and the hot end (the end stuck in the oven).