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Old June 13th, 2010, 01:55 PM   #1
mer
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thermocouple relay is their such????

Hello again i've started another project and i have limited inputs and i need to read several kilns with one reader so my question is,,is their a relay made that can or could read thermocouple (j or K )wire through the relay or if anyone knows some sort of way to read several points with one reader im game to any idea thanks in advance..Mer..
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Old June 13th, 2010, 02:10 PM   #2
Ron Beaufort
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something along these lines? ...

http://www.omega.com/Temperature/pdf/OSW_SW14.pdf
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Old June 13th, 2010, 02:10 PM   #3
Phill77
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I have been trying to find some locking thermocouple plugs, still had no luck with that.

However, one supplier I spoke to told me to use any connector I wanted to, as so long as there is no temperature differential across the connector it won't act as a cold junction and affect the measurement significantly in my application.

Whether this is true for all temperature ranges I don't know, but I would be tempted to set up a trial using any old relay you like to see if it does actually affect the measurement.
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Old June 13th, 2010, 02:15 PM   #4
Mickey
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More links

http://www.omega.com/toc_asp/subsect...perature&all=1

http://www.newportus.com/products/acccalib/thmowire.htm
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Old June 13th, 2010, 02:33 PM   #5
mer
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Didoo!!!!! Thanks

Quote:
Originally Posted by ron beaufort View Post
something along these lines? ...

http://www.omega.com/temperature/pdf/osw_sw14.pdf
thanks dat be it !!!
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Old June 13th, 2010, 02:35 PM   #6
mer
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I,ll try but !

Quote:
Originally Posted by phill77 View Post
i have been trying to find some locking thermocouple plugs, still had no luck with that.

However, one supplier i spoke to told me to use any connector i wanted to, as so long as there is no temperature differential across the connector it won't act as a cold junction and affect the measurement significantly in my application.

Whether this is true for all temperature ranges i don't know, but i would be tempted to set up a trial using any old relay you like to see if it does actually affect the measurement.
i have connected thermocouple wire with a butt connector and has threw the reading off by 40deg...
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Old June 13th, 2010, 04:05 PM   #7
danw
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Quote:
Originally Posted by mer View Post
i have connected thermocouple wire with a butt connector and has threw the reading off by 40deg...
What the supplier said is true. If there is no temperature difference between the connection points, there will be no error. Getting connection points to be isothermal (no temperature difference between them) can be problematic.

The scheme is functionally a de-multiplexor, many inputs to one output. There has to be no temperature difference between (+) & (-) terminals at both input and output.

You are practically guaranteed a temporary error if you are handling the connectors and the wire while crimping a connector terminal, you finish, and connect it. Your body heat has transferred to the wire/connector.

Drafts or circulating air in a panel can heat/cool some terminals and not others. Any temperature difference between the connection terminals is a direct error at the cold end of the thermocouple. Shielding the connection points from any draft of air, whether cooling or convection heat is a wise idea.

The polarity of the thermocouple wires has to match.
+ has to switch to +
- has to switch to -
You can be big errors by swapping polarity.

The type thermocouple wire has to be identical on both input and output. K to K, J to J, etc.
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Old June 14th, 2010, 04:47 PM   #8
keithkyll
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Get connectors at Omega too. Two styles, both need a good crimper tool.
MTC
D'Sub
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Old June 15th, 2010, 01:47 AM   #9
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We use a similar situation here, multiple temp. probes into one analogue input. OEM used a multiplexer card, and this works very well in our situation
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