Comments on: Measuring Temperature: Thermocouples Made Easy and Cheap http://www.cookingissues.com/2009/06/19/easy-cheap-thermocouples/ The International Culinary Center's Tech 'N Stuff Blog Thu, 09 Jan 2014 18:17:16 +0000 hourly 1 http://wordpress.org/?v=4.0 By: Tom M http://www.cookingissues.com/2009/06/19/easy-cheap-thermocouples/#comment-16686 Fri, 11 Mar 2011 15:58:14 +0000 http://www.cookingissues.com/?p=1099#comment-16686 Hi Dave:

Sounds like a great experient, i.e. measuring the temperatures of ice baths with the BB at different ambient temperatures.

That being said there is at least one error in my previous post. I said that if the BB manual suggests calibrating the TC in an ice-water bath, it probably indicates no thermistor compensation.

I was wrong. TC’s (of the same type) have minor metallurgical variations and the system response itself (TC wires + BB) that introduce various errors.

Thus, even ambient T compensated BB’s should recommend calibration.

Woe am I! It’s time for my sack-cloth and ashes routine, again.

Your experiment, though, still sounds very interesting and relevant.

BTW, the ubiquitous digital read-outs giving T to 0.1 degree, when the system itself probably is accurate to only plus/minus 1C are misleading. Funny what amuses me ;-)

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By: davearnold http://www.cookingissues.com/2009/06/19/easy-cheap-thermocouples/#comment-16652 Fri, 11 Mar 2011 01:46:11 +0000 http://www.cookingissues.com/?p=1099#comment-16652 Wow.
I have to run an ice-batch test on mine and then change ambient, but I’ve never noticed a difference on temps (of ice baths) from room to room. I have, however, noticed a difference from probe to probe. I usually calibrate only when I put in a new probe. Each probe seems pretty consistent (precise at least, if not necessarily accurate). I haven’t checked the compensation on lower-end PID controllers. If it weren’t for the fact that thermocouple inputs were so ubiquitous, and thermocouples are easy to make (although few people make them), RTDs and other resistance based approaches are obviously much easier. It is rare we need the temperature highs and lows that a thermocouple can manage, and it is hard to interface a TC to a microcontroller without a dedicated amplifier (at least with an arduino).

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By: Tom M http://www.cookingissues.com/2009/06/19/easy-cheap-thermocouples/#comment-16621 Thu, 10 Mar 2011 17:29:19 +0000 http://www.cookingissues.com/?p=1099#comment-16621 Thanks Dave:

Of course you are right. The “Black Box” (BB) that the thermocouple (TC) is connected to, must translate mV to degrees temperature. The most straightforward way to do this would be to do a table look up. (Microchips are so cool!)

Let’s see how this would work. For a type K TC let’s use the National Institute of Standards and Technology (NIST) Tables in degrees Celsius available here:

http://srdata.nist.gov/its90/download/type_k.tab

For my first example, let’s assume the BB is really unsophisticated and that we’d need to have one TC junction in an ice-water bath (T=0C).

The other junction is in a liquid of unknown temperature. The BB sees that the thermoelectric voltage reading is 1.162 mV. Doing the table look up, the BB displays a temperature of exactly 40C.

So far so good!

Now, let’s assume the BB is a little more sophisticated and just needs to be calibrated by placing the probe in an ice water bath. (The other junction actually forms at the terminals of the BB and would be at room temperature, say, 21C.)

Now the BB notices that the voltage is -0.838 mV and stores this value as an offset, to be used when measuring unknown temperatures.

So now place the probe to measure an unknown temperature (I happen to know that the unknown temperature is actually 50C).

So now the “cold junction” (BB terminal) is at room temperature (21C) and the “hot junction” is at 50C.

Using the table look up again, we get

21C  0.838 mV
50C  2.023 mV
Δ  1.185

Thus, the BB sees an electric potential of 1.185 mV (2.023 – 0.838). But since we calibrated the probe the BB knows that whatever reading it gets, must be off-set by that value, so the BB adds 0.838 mV to the measured 1.185 mV and gets 2.023 mV. It does the table look up and displays the correct temperature of 50C.

So far, so good. Now, let’s assume the ambient temperature in the kitchen goes up by 4C (the ovens have heated it up) but the TC isn’t recalibrated. The thing whose temperature we’re measuring remains at 50C.

So now we have:

24C  0.960 mV
50C  2.023 mV
Δ  1.063 mV

The BB sees the 1.063 mV, but knows to add the offset previously determined (0.838 mV) and comes up with 1.901 mV. It now goes and does a table look up and displays the temperature as 47C.

So what exactly happened?? Ambient temperature went up 4C and we would have expected the reading to have gone down exactly 4C also. Instead, it went down only 3C.

The 1C difference is caused by the non-linearity of the temp vs. mV characteristic of the TC.

This leads us the next level of sophistication in the BB. It now has a thermistor which accurately measures the ambient temperature (i.e. temperature at the BB input terminal).

The thermistor correctly reports the room temperature as 24C. The BB does a table lookup and now knows to add, as the offset, 0.960 mV.

Thus it adds 0.960 mV to the 1.063 mV it sees (=2.023 mV) and again does a lookup and gets 50C, the correct temperature. Problem solved!

The only question outstanding is: do all BB’s, be they PIDs or whatever, have thermistor (or equivalent) compensation?

I would hazard a guess that the “high-end” ones do.

What about the other BB’s? Well, if their manuals say you must first calibrate the TC by using an ice bath, then this is surely an indication that thermistor compensation isn’t part of the BB and that both ambient temperature changes and TC non-linearity are problematic.

Sorry for being so long winded! I didn’t know how to make my point any better…

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By: davearnold http://www.cookingissues.com/2009/06/19/easy-cheap-thermocouples/#comment-16607 Thu, 10 Mar 2011 12:05:31 +0000 http://www.cookingissues.com/?p=1099#comment-16607 Hello Tom M,
As I understand it, cold junction compensation in a thermocouple thermometer is accomplished with a secondary temperature sensor, like a thermistor, at the “cold junction” instead of an ice bath. I also think most thermocouples these days are linearized with lookup tables, but I could be mistaken.

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By: Tom M http://www.cookingissues.com/2009/06/19/easy-cheap-thermocouples/#comment-16578 Thu, 10 Mar 2011 04:27:35 +0000 http://www.cookingissues.com/?p=1099#comment-16578 My previous comment, perhaps, wasn’t clear enough. Thermocouples measure differences in temperature between the “hot” and “cold” junctions — not absolute temperatures.

Thus, if the kitchen temperature changes by 4 degrees, a TC without having its cold junction in an ice bath, will show a 4 degree “error”; not what you want in sous-vide.

Also, TC’s are not linear… so “calibrating” by zeroing at 32F will introduce further error. TC’s nonlinearity is highest around 100F (in the range of 32F-212F).

Using RTD’s is much more stable, being independant of ambiant temperature.

BTW thanks for making me seek out my dusty Engineering notes from almost 4 decades ago. Graduating from McGill U in Montreal was something I had almost forgotten. Strange that I love cooking.

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By: Tom M http://www.cookingissues.com/2009/06/19/easy-cheap-thermocouples/#comment-16556 Thu, 10 Mar 2011 01:25:25 +0000 http://www.cookingissues.com/?p=1099#comment-16556 A hundred years ago, when I was an Engineering student, we used thermocouples, but always had the “cold junction” in a bath of heavily iced water. Unless the “cold junction” was at a precisely known temperature, the TC reading could not be calculated accurately.

Now-a-days, it seems like everyone uses TC’s without giving a thought to the cold junction… I guess they think that “ambient temperature” is constant enough. I can’t imagine using a TC in sous-vide unless there is a precisely known “cold junction” temperature, which usually is not the case.

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By: Dave A http://www.cookingissues.com/2009/06/19/easy-cheap-thermocouples/#comment-2016 Sat, 26 Dec 2009 16:46:49 +0000 http://www.cookingissues.com/?p=1099#comment-2016 Hey Beebe,
How did it work?

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By: beebe http://www.cookingissues.com/2009/06/19/easy-cheap-thermocouples/#comment-2015 Thu, 24 Dec 2009 23:49:19 +0000 http://www.cookingissues.com/?p=1099#comment-2015 Ok, I’ve looked into welding thermocouple wires a little more and I think a capacitive discharge welder can be made on the relative-cheap for those without welding equipment using parts available from Radio Shack. A high-current NPN transistor, ~2200 uF 30v cap (?), battery charger, and a push button switch would probably be the entire parts list. I’ll test and report back. I’m not sure shielding gas is needed for these hobby-grade junctions.

As an aside, I found that my TIG setup welds wire nicely. I set the current to the lowest setting, high frequency off, and DC Reverse polarity. I’ll be testing these junctions tomorrow on my Christmas ribeye roast!

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By: Dave A http://www.cookingissues.com/2009/06/19/easy-cheap-thermocouples/#comment-2014 Wed, 02 Dec 2009 02:14:15 +0000 http://www.cookingissues.com/?p=1099#comment-2014 Nice! We don’t have a real mig at the school (FCAW only) so I can’t reproduce. Anyone with a real Mig take note.

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By: beebe http://www.cookingissues.com/2009/06/19/easy-cheap-thermocouples/#comment-2013 Tue, 01 Dec 2009 19:28:54 +0000 http://www.cookingissues.com/?p=1099#comment-2013 I like the article!

I had good luck welding small therocouples with the following MIG method:

1. Sharpen a short piece of 1/16″ TIG tungsten (Pure Tungsten)

2. Drill out the MIG tip to 1/16″ and tap and drill for a set screw (I used 6-32) to hold the tungsten in the MIG tip.

3. Bend thin aluminum sheet to make “tweezers” out of aluminum to pinch the thermocouple wire in a vice. The melted thermocouple wire won’t stick to the aluminum.

4. Strip thermocouple wire and twist the two together and clamp it in the aluminum tweezers with the twisted part sticking out.

5. Clamp the MIG ground clamp to the vice/aluminum tweezers.

6. With all wire removed from the MIG gun, pull the trigger to activate the shielding gas (I set up with pure Argon) and touch the sharpened tungsten tip to the twisted wire sticking out of the “tweezers”.

7. The arc will melt the wire sticking out down into a tiny ball. Adjust the protruding length of wire to get the right ball size. The shielding gas will keep the wire from oxidizing as it melts.

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