Thermocouple Switch

I was wondering if anyone had an idea, or had used a relay to select the source thermocouple.

I am working on an application where I need to select between two thermocouples (type J). The switching would not be frequent, and this is a slow responding system.

I have 2-8Pt. thermocouple cards now and they are full-and I need one more input which is really the same as another input, hence the need to switch between the two.

Thanks.

Not a good idea unless you can find a relay with the proper alloys to match the J couple. There are manual switches for selecting multiple T/C's. Check at Omega.com or your local T/C supplier. The switch could be mounted on a panel for easy access. http://www.omega.com/ppt/pptsc.asp?ref=OSW_SW14&Nav=temg16

A standard relay would cause some error depending on the metals used for the terminals. Check into thermocouple theory, that will give you an idea of why this is important. That error will also be dependant on the temperature of the relay and socket. You don't want to try and compensate for it.

Sounds like its more hassle than its worth. If you have an open PLC slot then get another module. If you have an open analog input, get a thermocouple transmitter.

Thermocouples have been selected routinely with either a mechanical switch or a relay for decades.

The T/C will form an intermediate junction at the two switch/relay contacts, but the only error will be the difference in temperature between the contact points. The positive current at the intermediate junction is cancelled by the negative current on the other contact, as long as the currents are the same amplitude from the same contact temperature. If the switch/relay contact points remain at the same temperature then there is no error from an intermediate junction.

An intermediate junction would have to contribute to the gradient across the entire thermcouple hot end to cold end for an error to result. It's not just different metals that produce a Seebeck thermcouple effect, it's gradient.

You can prove this to yourself by breaking a thermocouple and inserting relay contacts, letting the assembly achieve its stable temperature (it won't be stable if you've been installing wires) and reading the temperature versus what the temperature is without the relay in the circuit.

What you have to look for in a relay is one that has contacts intended for very low voltage switching. Typically, low voltage contacts are gold (or otherwise) plated to offer the best contact low resistance when heavy currents are not available to 'wipe' the contacts clean.

The only other catch in switching thermcouples is that the input should not be read during contact bounce or during switching when there's a momentary open circuit. The open circuit will make the input want to indicate a burn-out condition.

Dan

National Instruments has a paper on switching low voltage signals at

http://zone.ni.com/devzone/cda/tut/p/id/3762#toc2

Dan

Thanks for the info DANW, that's what I thought.

Do you have any recomendations for a suitable relay?

I have a large oven that switches between which end it records every 15 minutes or so.

It uses a small relay from AB, (700-HC24A1) to switch between the two TC's at either end.

Unfortunately after about 6 months time, the relay starts to 'chatter' or not switch perfectly, and the chart records large temperature spikes (due to the TC being open for too long of a time). Replacing the relay fixes the issue.

I'm looking for a better relay to work with.

Tharon said:

I have a large oven that switches between which end it records every 15 minutes or so.

It uses a small relay from AB, (700-HC24A1) to switch between the two TC's at either end.

Unfortunately after about 6 months time, the relay starts to 'chatter' or not switch perfectly, and the chart records large temperature spikes (due to the TC being open for too long of a time). Replacing the relay fixes the issue.

I'm looking for a better relay to work with.

Click to expand...

Tharon,

Have you tried a 700-HC14A1 ? It is the same as the 700-HC24A1 but has gold plated contacts.

Bud

Use Idec RY22 or RY42 series with Bifurcated contacts.
Get it from Relay Specialties.
You'll also need the SY2S-05 or SY4S-05 socket.
Use thermocouple extension grade (type J) wire for all wiring.

keithkyll said:

Use Idec RY22 or RY42 series with Bifurcated contacts.
Get it from Relay Specialties.
You'll also need the SY2S-05 or SY4S-05 socket.
Use thermocouple extension grade (type J) wire for all wiring.

Click to expand...

I was sitting here trying to remember the word bifurcated, dealt with these type things before but could not get that one word.

For a signal switching relay, you need to look for a low current relay < 3A, typically with bifurcated contacts, gold flashed contacts, designed for dry contacts that carry little, if any current.
Note the specs for the Tyco, Potter & Brumfield
R10-T1P2-V700, available from NewarkOne as their p/n 94F1890.
data sheet at
http://relays.tycoelectronics.com/datasheets/R10_DS.pdf


Note the characteristics of the P version contacts

Dan,

Thanks for clearing up some common myths about thermocouples. There is a lot of mis-information about thermocouples floating around out there. Would you like to educate us some more?