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From: pierson@ggone.enet.dec.com (Dave Pierson)
Newsgroups: comp.sys.ibm.pc.misc,sci.electronics
Subject: Re: Peltier effect device
Summary: two junctions
Keywords: Thermocouples
Message-ID: <18522@shlump.nac.dec.com>
Date: 28 Dec 90 22:19:38 GMT
Sender: newsdaemon@shlump.nac.dec.com
Followup-To: comp.sys.ibm.pc.misc
Organization: Digital Equipment Corporation
Lines: 40

In article <6792@bgsuvax.UUCP>, valdes@bgsuvax.UUCP (oscar Valdes) writes,
in part:

>	Check again.  You don't need two junctions to make a thermocouple.
>Heating a single junction will produce a voltage that depends on the
>temperature of the junction.
	There are always two junctions.  One may be the juction of the
	TC leads with the measuring instrument terminals.  TC rigs always
	measure the difference between the two junction temperatures.  To get
	an "absolute" reading, the temperature of one junction is measured
	by independent means.  In "lab" TCs, this is typically an RTD type
	detector, mounted on a comon thermal mass with the screw terminals.
	The terminal block temp is then "added" either analog fashionm, or
	digitally.  TCs are non linear over large delta t's, but reasonably
	linear over small ones.

	TCs are perfectly happy as either voltage or current output devices.
	Current (8)>>) practice treats them as voltage sources, due to the
	availability of good, hiZ amps.  In the past, the were commonly used
	with microammeters.  (Hi precision work used manually or automatically
	balanced bridges, which is voltage source mode.)

	(attempted graphics, Cu/Con couple...)
                 _______Cu_____________o_(Cu)____>Amp hi
		/                      |
	       /measuring junction     |<-Ref temp
	       \                       |	
		\______Con_____________o__(Cu)___>Amp Lo
				       ^-the "other" junction
	
	(As i write this, i sit next to a logger with 16 TCs on it...)

thanks
dave pierson			|the facts, as accurately as i can manage,
Digital Equipment Corporation	|the opinions, my own.
600 Nickerson Rd
Marlboro, Mass
01752				pierson@cimnet.enet.dec.com

"He has read everything, and, to his credit, written nothing."  A J Raffles