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Brews & Views Bulletin Board Service * Brews and Views Archive 2003 * December 19, 2003 * Thermocouples < Previous Next >

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Joe Sandlin (66.207.81.26)
Posted on Monday, December 01, 2003 - 12:13 am:   Edit Post Delete Post View Post/Check IP    Ban Poster IP (Moderator/Admin only)

I'm shopping for a temperature controller on ebay. What's the deal on thermocouples? I've heard that they're easy to make, but can't find directions. What guage wire do I need to make one and connect it to a controller? Connections sound like a problem, but I can't find details. Which of the alphabet soup of thermocouples do I want for my HERMS?

Confused again....
 

Walt Fischer (24.221.196.114)
Posted on Monday, December 01, 2003 - 01:08 am:   Edit Post Delete Post View Post/Check IP    Ban Poster IP (Moderator/Admin only)

"Which of the alphabet soup of thermocouples do I want for my HERMS?"

You want the kind that the controller will take...

Mine takes a K-type probe...
A J-Type would not work on mine...

Walt
 

danny roy (142.166.234.108)
Posted on Monday, December 01, 2003 - 01:22 am:   Edit Post Delete Post View Post/Check IP    Ban Poster IP (Moderator/Admin only)

How do you control Temp with a thermocouple? They are usually used as a safety shutoff.When the flame goes out the couple shuts off the gas.
 

Farmer Ian (216.222.79.75)
Posted on Monday, December 01, 2003 - 01:56 am:   Edit Post Delete Post View Post/Check IP    Ban Poster IP (Moderator/Admin only)

While it may be fairly easy to construct a thermocouple, (welding two dissimillar wires together end to end, I believe) I would think that it would be a PITA to end up with one that would provide "calibrated" voltages to the controller. (or perhaps just luck, I dunno, I have never tried to make one.) While Physics/Chemistry dictates the theoretical performance of the thermocouple, I would think that the quality of the connection between the two wires would play a non-trival role. Or perhaps not, I suppose that I should just shut up and admit that I do not know for sure. :)

As Walt says above, the controller should state what type thermocouple it requires. Get that kind. :)

Ian
(I hear that Calhoun's is planning their first Pilsner! huzzah!)
 

Greg Harris (204.27.149.93)
Posted on Monday, December 01, 2003 - 02:00 am:   Edit Post Delete Post View Post/Check IP    Ban Poster IP (Moderator/Admin only)

Joe
Have you checked out the "Ranco controllers on Graingers" thread? dont know if this will help or not.
Greg
 

Joe Sandlin (66.207.81.26)
Posted on Monday, December 01, 2003 - 02:27 am:   Edit Post Delete Post View Post/Check IP    Ban Poster IP (Moderator/Admin only)

Many of the ebay controllers say that they take a zillion different kinds of thermocouples. One web page I looked at said to get a type K thermocouple unless there was a reason not to, since they're cheap and common. Make sense?

I have a couple of RANCO's. One on my fermenting fridge, and one on my dispensing fridge My current practice is to remove the dispensing fridge's controller and use it on brewday, but this is getting old.

But mainly, I'm not pleased with the sluggishness of the response of the RANCO probe inside my MLT's copper thermowell at mash temps. I'm hoping a thermocouple will respond quicker, and calm down my temp swings that I currently get. Will the thermocouple react quicker than a RANCO thermister?

(and Ian, I'll call Calhoun's tomorrow and confirm the pilsner rumour..) :)
 

Bill Aimonetti (12.72.105.63)
Posted on Monday, December 01, 2003 - 05:30 am:   Edit Post Delete Post View Post/Check IP    Ban Poster IP (Moderator/Admin only)

Thermocouple junctions are a junction of two dissimilar metals (different platinum / rhodium alloys) the wire type must match the alloy. They generate a voltage that is measured and scaled by the controller. The difference between r and j for example, is the number of micro-volts per degree generated (resolution). Also the size of the active range and limits differs with wire guage. If I remember correctly they are ~13 & 8 uV/ degree respectively. The junctions can be made easily but you need J type wire for example. Also the connector attaching the TC to the controller has to be the same type. The company Omega has books with tables of ranges and resolutions. Maybe try the Omega website for more info.
 

Bill Aimonetti (12.72.105.63)
Posted on Monday, December 01, 2003 - 05:51 am:   Edit Post Delete Post View Post/Check IP    Ban Poster IP (Moderator/Admin only)

Here is a link with more info.
http://www.omega.com/temperature/Z/zsection.asp
check the intro to TCs for starters.
 

Fredrik (213.114.44.229)
Posted on Monday, December 01, 2003 - 06:41 am:   Edit Post Delete Post View Post/Check IP    Ban Poster IP (Moderator/Admin only)

One thing with thermocouples is that the voltage vs temperature is not linear. This is the reason you can't put another type in there and just do a linear recalibration (offset and scale). The voltage vs temperature are usually tabled as polynomial series.

If you are doing your own linearization (in the PC) though it would be managable either way. Also any junctions with other metals means you have another effective "thermocouple" in serial. Apart from the formulas needed you also need to know the temperature of each junction in order to perform the correction. A good thermocouple amplifier contains a CJC sensor to measure the temperature of the junction.

/Fredrik
 

Walt Fischer (24.221.196.114)
Posted on Monday, December 01, 2003 - 07:09 am:   Edit Post Delete Post View Post/Check IP    Ban Poster IP (Moderator/Admin only)

Hey Joe...
Just get a K-type ss probe then.. thats what i use, 6 inch, with a minijack, and it cost around 20 bucks...Plugs right into my Omron PID.
My PID reacts very quickly to changes...within less then 1 degree..Plus its gets 'smarter' over time with the changes needed to maintain the temp...
Its prob overkill... but i got it pretty cheap on ebay, and ive been very happy with it...

BrewON!
Walt
Lama Brewery
----
 

TexanBrewer (63.174.45.1)
Posted on Monday, December 01, 2003 - 02:35 pm:   Edit Post Delete Post View Post/Check IP    Ban Poster IP (Moderator/Admin only)

My setup is similar to Walt's; however, I use bulk thermocouple wire.

If you do not want to buy pre-made thermocouples, get some appropriate type wire from Omega (or ebay) and strip about an inch of insulation from the two wires. Twist these together and ta-da, thermocouple junction.

I've done this and tested at freezing a boiling with little calibration problems. Be aware that wort does terrible things to the junction. You must attempt to thoroughly clean the twisted junction after each use or you're forced to cut a couple of inches off and make a new junction.

---
Scott
http://texanbrew.com
 

Tom Gardner (66.82.182.25)
Posted on Monday, December 01, 2003 - 05:34 pm:   Edit Post Delete Post View Post/Check IP    Ban Poster IP (Moderator/Admin only)

Does anyone use "thermal grease" in their thermowells?
 

Marlon Lang (68.18.122.22)
Posted on Tuesday, December 02, 2003 - 01:31 am:   Edit Post Delete Post View Post/Check IP    Ban Poster IP (Moderator/Admin only)

Hooray for TexasBrewer. You are absolutely correct. As for the alphabet soup, the best choice is ISA Type "K", Chromel-Alumel. Both leads are nickel alloys and thus in the stainless family. Over the normal range for type "K" (-200/+1800 deg F) the non-linearity/inaccuracy is in the range of 9 degrees. Over the 150 deg F range we use for brewing, the non-linearity/inaccuracy would be less than a degree. Perhaps there is a correlation between thermocouple non-linearity and bubbles-per-minute Fredrik, but I'm not about to worry about a possible error of less than one degree. RANCO uses a thermistor probe, not a thermocouple.
 

Fredrik (213.114.44.229)
Posted on Tuesday, December 02, 2003 - 07:47 am:   Edit Post Delete Post View Post/Check IP    Ban Poster IP (Moderator/Admin only)

I'm not using any thermocouples for brewing myself, but you're right Marlon - sorry, it indeed seems in the brewing range the non-linearity error isn't really significant.

/Fredrik
 

Joe Sandlin (66.207.81.26)
Posted on Tuesday, December 02, 2003 - 11:00 am:   Edit Post Delete Post View Post/Check IP    Ban Poster IP (Moderator/Admin only)

If I get bulk "K" type thermocouple wire and make my own, can I just slide it into my copper thermowell, or will it "short out" on the copper? Would a little heat shrink tubing be necessary? It sounds like this would be the way to go...
 

Brandon Dachel (63.238.222.190)
Posted on Tuesday, December 02, 2003 - 01:20 pm:   Edit Post Delete Post View Post/Check IP    Ban Poster IP (Moderator/Admin only)

> or will it "short out" on the copper?
Yes.

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