Using a Slip Ring for Type J Thermocouples-Any history of issues for any of you guys?

Baker in Virginia

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B
Join Date
Aug 2016
Location
Virginia
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Good Morning ,

We have a new packaging machine that have Slip Rings . We are using the Slip Rings for a rotary jaw assembly that has heaters installed. We don't have much problem with the heaters , because of the higher voltage and
current I imagine ,but we consistently have problems with our Type J Thermocouples.


- Have any of you had issues with Type J , or any other type of
Thermocouple not reading consistently , loosing contact , etc.?

- Would a RTD work better on a Slip Ring ?

- Would there be any better material for Slip Ring Bands or brushes more
suited for Thermocouples ?

- I imagine a gritty cookie dust would cause issues , in an open Slip Ring
also ? LOL

Thanks so much experience and advice.
 
I have used RTD's through a slip ring, with good success.



Where does your signal from your thermocouple terminate? PLC? Controller? I would suggest, if it works for your application, putting a signal converter on the thermocouple, to convert to 0 - 10V or 4-20ma signal thru the slip ring.
 
We have used them in the past. The ones we used were made by Mercotac. They are not the typical slip ring. They were sealed and the contacts wetted by Mercury. Contamination was not an issue. I don't think they have terminals made of thermocouple elements, but in our industry it did not cause negative results.
 
I don't have any experience using J type thermocouples on slip rings however given that a 0.5 mV change represents a 10 degree C temperature change I wouldn't do it. Even if you find a method that works now, things age and I can't imagine that with a very few exceptions whatever works today won't be a maintenance nightmare down the road. I'd convert the signal into something much more tolerant to slip rings and how they age. I'd even consider putting something in place that converts the signal into a communications protocol and run that across the slip rings (probably not feasible though)
 
I like all your ideas ……

- How small do wireless temperature transmitters come ? Do you have
any recommended brands or models ?

- drforsythe - " They were sealed and the contacts wetted by Mercury."
How do the wetted mercury method work ? What model
Mercotac are you using ?

Thanks for all your advice.
 
Another question

If I use a RTD in the rotating jaw assembly , do they make a conditioner / converter from RTD to Type J ?

If so , could you please send me a link ?

Thanks so much ,
 
Baker:

We don't have that equipment anymore, so I cannot get you a model number. The units look like a shaft with leads or terminals on each end and a bearing in the middle. The part that looks like a bearing has the slip rings that are lubricated and make connectivity using a small amount of mercury per channel. The sealed unit and mercury-wetting keep the contacts clean so oxidation does not become an issue. I did a quick check on their website and they still have offerings for thermocouples. Check this out:
http://mercotac.com/html/205.html

I hope this is helpful. We did not see any fluctuations in temperature and actual temp was always within 2 degrees F when compared to a separate contact temperature sensor.
 
Slip rings at that low voltage I have found to be a nightmare. They get dirty and all sorts of things. The best slip rings I ever found were the mercury bottle ones.
 
All sorts of red flags using slip rings with a TC. You don't give operating temp range but it sounds like lower temps so the signal from the J couple might be in the 10's of mv range? Dirty slip rings and dissimilar metals will cause errors for sure. How do equipment builders get away with stuff like this?
An RTD gives you a fighting chance but a converter to 4-20 ma would be best, if the converter can tolerate the mounting temperature.
 
We use Mercotac 430 on some packaging machines. Thermocouple passes through two 4A contacts. Heaters pass through two 30A contacts. They have been extremely reliable for years. They are mercury wetted.

As far as thermocouple goes, accuracy is fine for us. I figure the error induced from transition in and out offset each other for near zero change. We’re using type K.

http://mercotac.com/html/430.html
 

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