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anyone know how to hook up old pyrometer

I bought an old pyrometer off ebay. its a sim-ply-trol. it can be seen here:

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It says "iron-constantan" and "ext. res 10 ohm" on the front. So I guess that means Type J. But what does the ext. res 10 ohm mean?

Also, on the back, there are two screw terminals coming out of the meter, but one of them has a little plastic dohickey screwed to it that has some kind of wire wrapped around it which is attached to another, floating screw terminal, which is shorted to the OTHER panel mount terminal with a piece of wire. Is this so you can measure meter temperature without a thermocouple? And do I connect a real thermocouple directly to the screw terminals, or do I go through the dohickey?

And how does a mechanical device like this compensate for the cold junction?

Reply to
acannell
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Yes.

The thermocouple should be 10 ohms (total loop resistance) for an accurate reading. The error if you're not at 10 ohms will depend on the coil resistance (umm.. if you try to measure it, use a digital and start at a high ohm range so as not to bend the needle accidentally).

The meter has to measure the temperature at the terminals in order to read a thermocouple. Also the coil resistance is going to vary with temperature so there's probably another device inside to help compensate for that.

I canna see it, but most likely the screw terminals.

Something like a bimetal typically. Some don't bother, you just adjust the 'zero' screw so it reads 70°F or whatever at room temperature and hope for the best.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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Reply to
Spehro Pefhany

I measured the thermocouple resistance, its 6.4 ohms (DMM across thermocouple leads).

I measured the meters resistance (DMM across direct screw terminals)

25 ohms

How do I figure out what the error should be?

Thanks,

Asa

Reply to
acannell

Put a few ohms metal film in series with the T/C (keep the leads short-- bend them into a 'U') and don't worry about it. Those things are VERY crude at the best of times.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

Typically, it indicates that calibration will meet specs if the external resistance of the thermocouple wire is 10 ohms or less.

It's intended for hot furnaces (type J is good for 1600F) so ten degrees one way or the other is not important. Probably no cold junction compensation.

Reply to
whit3rd

10 ohms or less? Not exactly 10 ohms? So if my thermocouple measures 6.4 ohms I am good to go? Also, this meter goes from 0 to 300C, does that mean its probably meant to be more accurate than a 1600F one?
Reply to
acannell

Exactly 10 ohms. Modern electronic indicators have a maximum resistance (usually at least 100 ohms).

Maybe.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

Try it! Connect it the way that looks obvious, and get a thing of ice water and a thing of boiling water and see what readings you get.

Good Luck! Rich

Reply to
Rich Grise

I did. It seems to work okay for ice water, but boiling water went way beyond 125C on the meter. I measured the water with another known good K type thermocouple and digital meter and it was 99C.

I guess this meter is F-ed. But what goes bad in a mechanical meter like this? Its all metal right? Looks like I wasted $50

Reply to
acannell

I wouldn't say it's fried - it deflects, right? But the different reading might be what the little unknown thingie on the back is - a calibration thing or something.

It's worth further investigation. :-)

Cheers! Rich

Reply to
Rich Grise

- maybe it's connected wrong and that thing has to be in series

- I'd expect an error of only about +11% with the resistance the way you have it (about 10'C on 80'C above room temperature) if the correct total resistance is 35 ohms.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

maybe it needs all of the 10 ohms resistance (and you currently have 6?) try adding some resistance in series with the cold thermocouple.

Bye. Jasen

Reply to
Jasen

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