Thermocouple Options

Hope everyone had a good Christmas. I have a question about
thermocouples. We have a kiln that have duplex and triplex
thermocouples. These individual thermocouple wires go to
Input Cards , High Limit Controllers,etc. I would like to go
with just 1 single thermocouple and distribute to each device.
Do they make a millivolt transmitter( or splitter)? I know we can go with a transmitter that as a thermocouple input with
a 4-20 ma output, but what about a millivolt input and a millivolt output equal to the input.
I just don't want the added expense of changing out the devices,to recieve the 4-20 input.

Thanks in advance,

Hello-
If the input impedances are high enough on all your devices, you might be able to get away with "splitting" the single thermocouple to 2 or more devices without using an expensive amplifier. Make sure you use the correct wire for your type thermocouple. They do make current amplifiers that would allow you to split milivolt thermocouple signals. Check out Omega OMNI-AMP IV. These are over $200, though, and you might find it cheaper to use multiple thermocouples.

Don't do it. Especially on your high limit. When you connect multiple devices to a single TC then the burn out detection circuits of the instruments can interfere with each other, causing you to loose the ability to detect a burned out thermocouple. That is why manufacturers make duplex and triplex TC elements in a single sheath to begin with.

If your product has any value at all then I would stay with multiple thermocouples. You wqant to especially keep the high limit.
Good kuck

The High Limit controller with a dedicated TC is mandatory. You will violate safety rules and certifications if you defeat it.

I'm guessing that if it's a kiln, you're running type R or S T/C's made of platinum elements and expensive at current noble metal prices?

The comments on high limits can not be ignored, especially for a high temp kiln, which can be a fire hazard.

The lack of T/C break (burnout protection) can be a serious issue for the control T/C, as well.

Do any of your devices have universal analog inputs that can be field configured for either T/C or 4-20mA (might have to add a shunt resistor for 4-20)?

The Omega OMNI-AMP IV cited above appears to be mV amplifier, not necessarily a mV follower. The Gotcha is the statement:


The spec sheet statement that the output is either 10mV or 1mV per degree F (depending on input type) is somewhat deceptive, because without linearization X mV/Deg is a merely an approximation.

The schematic in the user manual http://www.omega.com/manuals/manualpdf/M2074.pdf
confirms that, showing the CJC compensation, along with the amp, no linearization.

It isn't clear whether the amp can run at a gain of 1 which you'd want if you expected to make no other changes in your existing system. If run at higher gains, the lack of linearization puts the burden for linearization on the receiver device, which is what is already built into T/C inputs. To have to linearize a mV input to T/C curves is lot of work to set up.

With a spec sheet statement: 1mV per Deg F
a gain of 1 is questionable.

a 1mV difference for a Type R T/C at 2000°F is
131° difference (on the high side) 2,000 to 2,131
134° difference (on the low side) 1,866 to 2,000

1,866°F = 10.757mV
2,000°F = 11.759mV
2,131°F = 12.758mV

type R °F tables at http://www.pyromation.com/downloads/data/emfr_f.pdf

The calculation of the gain needed for a specific 'range' is rather cryptic, without a generic formula to follow.
Is the range 0 - 5.0 V below 32 to 532°F or 0 to 500°F?

You might want to figure out how you'd use it before you actually jump in the deep end.
Dan

If the expense of a type S, R, or B thermocouple is the issue, and your furnace atmosphere is inert then you might consider using a type C (also designated type W) W5%Rh/W26%Rh thermocouple. They are still expensive, but no where near as much as S/R/B.

Do the high limit and temperature controllers have the ability to retransmit the PV as a 4-20mA or 0-10V signal? We have several kilns that use single TCs. We use the comms on the temperature controllers or the retransmission features to get the data elsewhere when we need to.