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Thermocouple vs Thermometer

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ken.rubin

Post Fri Mar 13, 2009 8:52 am

Thermocouple vs Thermometer

So thermometers are inaccurate. What about thermocouples?
Are they more accurate/dependable?
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redbeerman

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Post Fri Mar 13, 2009 9:00 am

Re: Thermocouple vs Thermometer

A thermocouple will be more accurate and more expensive. As I recall, a digital thermometer is just a glorified bimetal thermometer with a digital readout. I'm not going to go into thermocouple theory to much here, but a thermocouple works by generating a voltage at a juction of two dissimilar metals welded together. The range of the thermocouple depends on the metals used. If you google it, you will find more information on theory and pricing. Thermocouples are used in process control all the time. When I build my Brewmaster 3000, I will be using them in conjunction with temperature controllers for my HLT and BK heaters.
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blatz

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Post Fri Mar 13, 2009 9:02 am

Re: Thermocouple vs Thermometer

I use a thermocouple probe/reader and would NEVER switch back - one of the best investments I've made, but as was said - they are expensive.
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ken.rubin

Post Fri Mar 13, 2009 9:39 am

Re: Thermocouple vs Thermometer

Great, thanks.

Do thermocouples ever change their accuracy? Can you re-calibrate them?
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blatz

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Post Fri Mar 13, 2009 11:15 am

Re: Thermocouple vs Thermometer

I don't believe it would be possible on the amateur scale - I believe they can be recalibrated, but you need the proper equipment to do so.

if you ever felt it was straying - buy a new probe($10-$20) - but I've been using mine over 2 years now and never have felt the need to test it since my end product always turns out exactly as I want (with regard to mash temp at least).
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PaulHilgeman

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Post Fri Mar 13, 2009 11:50 am

Re: Thermocouple vs Thermometer

I think handheld digitals use thermistors. Unfortunately they are avilable in a wide range of tolerances, cheap ones are +/-5%, but really good ones can be +/- .1%, so unless you know what you are getting they can be good and bad, and the system as a whole would need to be calibrated as well. What tolerance means on a thermistor spec sheet is a little vague, probably how closely an measured resistance value relates to an actual temperature value.

The bottom line, find something that works well for you and use it exclusively. even if it reads 15 degrees off, but stays there, it is good. The precision is high on most temperature measuring devices that we use, however the accuracy is not so great for things that cant be calibrated on a set schedule.
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blatz

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Post Fri Mar 13, 2009 12:01 pm

Re: Thermocouple vs Thermometer

+1 to what paulhilgeman said.

here's what i use for my handset:

http://www.palmerwahl.com/product_home. ... 12&itm=604
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PaulHilgeman

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Post Fri Mar 13, 2009 1:07 pm

Re: Thermocouple vs Thermometer

This is what I use:

http://www.sweetmarias.com/Merchant2/me ... creen=PROD

I put some silicone on the tip and shoved it down a thin copper tube that I crimped the end shut on to do a makeshift thermowell.

Though I dont think water affects the reading (i tried)

It seems pretty good to me, it read 214 for boiling water about 6 months ago and still does today. So it is a little high, but I have adjusted it for my brewing techniques.

-Paul
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ken.rubin

Post Fri Mar 13, 2009 1:20 pm

Re: Thermocouple vs Thermometer

I have large face analog thermometers in my kettles. I also have a couple of thermocouples. The thermometer and the thermocouple almost never show the same temp.

However, I also have an infared thermometer. That seems to be more accurate than the thermometers stuck in the kettle. I was heating up water last night to 140 deg F (using the thermocouple to keep it there). The infrared thermometer matched the thermocouple exactly. That was surprising to me because I thought it was less accurate.
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nyakavt

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Post Fri Mar 13, 2009 3:36 pm

Re: Thermocouple vs Thermometer

PaulHilgeman wrote:This is what I use:

http://www.sweetmarias.com/Merchant2/me ... creen=PROD

I put some silicone on the tip and shoved it down a thin copper tube that I crimped the end shut on to do a makeshift thermowell.

Though I dont think water affects the reading (i tried)

It seems pretty good to me, it read 214 for boiling water about 6 months ago and still does today. So it is a little high, but I have adjusted it for my brewing techniques.

-Paul


Now that is by far the cheapest thermocouple reader I have ever seen. Looks like it uses a standard 2-pin thermocouple so you can replace the original if it loses calibration, those are pretty cheap. Does it have a decimal readout or just whole numbers? Either way, looks like a great deal.
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ken.rubin

Post Fri Mar 13, 2009 3:53 pm

Re: Thermocouple vs Thermometer

Cheap, perhaps, but not as cool as the infrared thermometer.
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nyakavt

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Post Fri Mar 13, 2009 4:01 pm

Re: Thermocouple vs Thermometer

ken.rubin wrote:Cheap, perhaps, but not as cool as the infrared thermometer.


So the infared thermometer reads fine in plain old water? That's pretty cool, didn't know they could do that. Where can you get a cheapo one?
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blatz

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Post Fri Mar 13, 2009 4:05 pm

Re: Thermocouple vs Thermometer

BTW - I highly recommend you guys getting thermocouple probes rather than using the wire ones - I've seen a lot of the wires get water logged and become useless very quickly. its much harder to ruin a probe.
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ken.rubin

Post Fri Mar 13, 2009 4:07 pm

Re: Thermocouple vs Thermometer

Not sure. I went for features and accuracy--not price.
I think BBB or NB sells a fairly inexpensive model.

What I really like about the model I have (I just tried to find a link to no avail) is that you can attach a k-type thermocouple to it (it comes with one) to calibrate and / or find out if the infrared mode is off. So its the best of both worlds.
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bluefoxicy

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Post Fri Mar 13, 2009 4:47 pm

Re: Thermocouple vs Thermometer

PaulHilgeman wrote:I think handheld digitals use thermistors. Unfortunately they are avilable in a wide range of tolerances, cheap ones are +/-5%, but really good ones can be +/- .1%


Kelvin or what?

Boiling +/- 0% K: 373.15K, 100C, 212F

Boiling +/- 1% K: +/-3.7K, +/-3.7C, +/-6.7F

Boiling +/- 1% C: +/-1K, +/-1C, +/-1.8F

Boiling +/- 1% F: +/-1.2K, +/- 1.2C, +/- 2.2F

So within 1% absolute temperature (i.e. Kelvin), 158F mash temp may be 151.82 - 164.18; if 1% on the Farenheit scale, then it's 156.42 - 159.58.

Mind you, it only really would make sense to retain 1% accuracy on an absolute scale, otherwise you have an arbitrary point where measurement becomes more accurate (should be 100% accurate at 0F or 0C, right?). Give that a thought for a moment.
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