Slip Ring for a "J" thermocouple.

[Kbradio]

Looking for a slip ring "J" TC Iron & Constantine for getting a TC signal from a TC mounted through a shaft on a mixer. Omega discontinued SR-2. See link below... Thanks for any help sourcing these!

http://www.omega.com/pptst/SR-2.html

 

[waross]

This was my first though, reinforced by the note on the Omega page:
Products containing MERCURY are NO LONGER AVAILABLE.
Please consult Temperature Engineering for possible substitues for your application.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[roydm]

In theory you should be able to use just about any slipring.
Because of the "Law of Intermediate Metals" changes in material should cancel out'
Google "Law of Intermediate Metals"

Roy

 

[danw2]

Google thermocouple slip ring
Lots of hits.

Roy,

If the hot junction touches the rotating part so that the slip ring is part of the hot junction measurement, then yes, the Law of Intermediate Metals applies; like direct hot junction metal contact, solder or brazing on the hot junction.

But, if the hot junction is located wherever and the extension lead wires from that hot junction are connected to the slip ring for connection through the slip ring and the slip rings are not the same metals as the thermocouple metals, then the slip ring location is the T/C's cold junction and the temperature of the cold junction has to be taken into account.

The way I read the statements below, I think the vendor (below)
a) is stating the issue about cold junction at the slip rings, but
b) is offering slip rings made from thermocouple materials
c) assuming copper wire connection at the slip rings so that cold junction measurement and temperature conversion is done there because of the final statement: T/C signals read as mV, and correction for rotating ref temp must be done.

It isn't clear to me why one wouldn't use thermocouple wire or extension wire from the slip ring to a thermocouple input and let the T/C input do the CJ and temp conversion like those things normally do.

--------------
Sensor Considerations

Slip Rings can transmit virtually any type of sensors. No signal transformation or change occurs to the signal inside the slip ring. However, some considerations need to be given to various sensors for accurate signal transmission.

Thermocouples

Thermocouples are commonly used on rotating hardware due to their simplicity and robustness. Any type of thermocouple can be transmitted through a slip ring. However, the user must be aware that the thermocouple alloy material does not continue in the slip ring and that the rotating reference junction is created where the thermocouple alloys are terminated to the slip ring input PCB.

Because thermocouples only measure the difference between the hot junction and rotating reference junction, the temperature at the reference junction must also be measured. This is commonly done using a rotating RTD or thermistor located on the input PCB. Aerodyn PCBs are designed to accommodate these sensors.

These sensors must be considered in the selection of the overall channel count for the application. Inside the slip ring, Aerodyn uses the same alloy for ring and brush contacts so that no thermoelectric potential is created at this location.

Thermocouple signals are then read as millivolts at the output of the slip ring, and correction for rotating reference temperature must be performed.

http://aerodyneng.com/our-products/slip-ring-manufacturing-1/sensor-considerations-and-noise

 

[waross]

Thermo-couples are typically wired back to the instrument with thermo-couple extension wire. The cold junction is in the instrument and the instrument does the cold junction temperature compensation.
Why would you not use thermo-couple extension wire to wire from the rotating connection to the instrument?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[danw2]

Exactly. So why does the vendor say, "Thermocouple signals are then read as millivolts at the output of the slip ring, and correction for rotating reference temperature must be performed.

Why suggest all that effort and work when some T-C wire back to the T-C input is such a clean solution?

 

[ScottyUK]

Are the rings and brushes both of the same material? Not impossible but unusual in my experience.

 

[waross]

The only issue will be if there is a temperature gradient across the slip ring/ rotating connection assembly.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[danw2]

>Are the rings and brushes both of the same material? Not impossible but unusual in my experience.

It depends on the what word 'same' means. Sames as the thermocouple materials ? ? ?
Aerodyn uses the same alloy for ring and brush contacts so that no thermoelectric potential is created at this location.

 

[ScottyUK]

Thanks Dan, I missed that. I generally deal with power sliprings, not signal-level stuff. My little bit of learning for the day.

 

[solocam92]

Kbradio- Did you find anything that works?

I have been using a Mercotac slip ring connected to either a J-type thermocouple or 5k ohm thermistor, but with poor measurement accuracy.

 

[SceneryDriver]

Late to the party, but:

Is there a reason you can't place a T/C-to-4-20ma loop powered transmitter on the rotating part of the machine, and transmit 4-20ma through the slip ring? It would do away with all the issues of dissimilar metals in the slip rings, and the current signal is arguably much more immune to noise than the T/C signal.



SceneryDriver