DIY_EFI Digest Sunday, 17 March 1996 Volume 01 : Number 079 In this issue: Re: thermocouples See the end of the digest for information on subscribing to the DIY_EFI or DIY_EFI-Digest mailing lists. ---------------------------------------------------------------------- From: Darrell Norquay Date: Sat, 16 Mar 96 13:05:05 MDT Subject: Re: thermocouples > I use the AD594 device to compensate and linearize thermocouples. > > Does anyone have experience multiplexing in front of one? > John: It's pretty difficult to multiplex the direct signals from thermocouples. Using an electronic approach, any type of analog multiplexer usually has a relatively high series resistance (compared to the t'couple) and introduces a lot of error. A mechanical approach (using relays) is better in some respects, but you still get a thermocouple effect from the metal in the contacts, as well as resistance, and it's slow. Mercury wetted reed relays may work, I've never tried this. In any case, you'd have to do the cold junction comp before the MUX anyway. A better approach would be to use some sort of a simple preamp circuit to bring the output of each thermocouple up to a more manageable level, and buffer the thermocouple output with a low impedence driver. You could then use a standard multiplexer chip and a final gain/offset stage to get a calibrated output. The Analog Devices AD594 has no linearization of the TC input. You can use some external components to improve the linearity, but a type K is reasonably linear anyway, (better than other TC types) and for this application you don't really care if the temp is 1294.8 deg or 1298.4 deg, a few degrees one way or the other isn't going to make a difference. Analog Devices has a relatively new chip, P/N AC1226, which does the cold junction comp, buffers the signal, and gives an output of 10 mV per degree C. It also has some rudimentary linearization built in, it's pretty general but does improve the linearity significantly. The device has a built in temperature sensor, and is extremely low power to minimze self heating, you would physically mount it right on your isothermal block where TC leads meets copper wire. I don't know the cost on these, but it should be reasonable. If you use one per TC, and then MUX these together into an A2D or display device of some kind, you should be able to come up with a relatively low cost system with good accuracy. If you don't feel up to building one, almost all manufacturers of PC A2D converter boards (National Instruments, Strawberry Tree, Burr Brown, etc.) have an optional multi-channel signal conditioner / terminal board that have cold junction comp and preamps built in. These usually run in the $150-$300 range, a bit dear but saves a lot of work, especially if you only need one. regards dn - -- - --------------------------------------------------------------------- Darrell A. Norquay Internet: dn@xxx.ca Datalog Technology Inc. Bang: calgary!debug!dlogtech!darrell Calgary, Alberta, Canada Voice: +1 (403) 243-2220 Fax: +1 (403) 243-2872 @ + < __/ "Absolutum Obsoletum" - If it works, it's obsolete -------------------------------------------------------------------- ------------------------------ End of DIY_EFI Digest V1 #79 **************************** To subscribe to DIY_EFI-Digest, send the command: subscribe diy_efi-digest in the body of a message to "Majordomo@xxx. A non-digest (direct mail) version of this list is also available; to subscribe to that instead, replace "diy_efi-digest" in the command above with "diy_efi".