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Old 05-10-2011, 05:51 PM   #1
acefaser
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Default K type thermocouple or RTD sensor

Just about to place the order for the PID and was wondering if anyone uses the K Type thermocouples. Do you find them to be not that accurate? Its hard to drop $65 for 2 RTDs when K type go for less then $10 on ebay.

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Old 05-10-2011, 07:23 PM   #2
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RTD !

I've used both and find RTD's to be more accurate. I also like to stick with the same source for both devices. I've found that the calibration is usually pretty close as delivered, whereas when I've bought other brands I have to work at it.

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Old 05-10-2011, 08:27 PM   #3
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You can get liquid-tight RTDs for under $10 on ebay, too:
http://cgi.ebay.com/Thermocouple-Tem...item3cb7542018

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Old 05-10-2011, 09:07 PM   #4
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THANKS! Dont know how I missed those searching this morning. Now if they could only shorten that probe up a little. I think I can make it work.

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Old 05-10-2011, 09:51 PM   #5
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Quote:
Originally Posted by acefaser View Post
THANKS! Dont know how I missed those searching this morning.
Probably because they mistakenly labeled them as "thermocouples" and don't mentioned "RTD" in the listing.

I actually searched for "pt100", which is a common type of RTD, and that's what showed up.
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Old 05-11-2011, 06:58 AM   #6
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Thermocouples can be very accurate. We use them in the lab all the time, in fact i can't think of a temp probe that isn't a thermocouple that we use on a daily basis.

I think that in our context here (homebrewing) the problem is that many of us need to extend the leads on our temperature sensors or put them through a wall of some sort (control panel side, etc) using a disconnect. In these cases you have other issues to contend with such as matching wires and proper connectors with thermocouples. These aren't problems for RTD sensors.

As Walker said, though, you can get RTD's for about the same price point as thermocouples. You sacrifice some temperature range going with RTD's, but again in this context they really have no significant limitations and a host of benefits.

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Old 05-11-2011, 02:48 PM   #7
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excerpted from here

Selecting sensors for reliability

For temperatures ranging from cryogenic to about 800°F (427°C), a resistance temperature detector (RTD) is better than a thermocouple. Though RTDs were once more expensive, today the cost difference between the two is negligible, and an RTD provides greater accuracy, stability, and life span. Over its useable range, a standard-grade RTD provides more than twice the accuracy of a thermocouple.

To understand why, consider the materials of construction. A thermocouple is comprised of two dissimilar metals welded together; the resulting junction begins to deteriorate immediately. The deterioration is faster at higher temperatures and can result in drift of several degrees per year. On the other hand, an RTD consists of stable, highly purified platinum.

RTDs drift just fractions of a degree per year and are accurate for many years when used at temperatures below 800°F (427°C). Thermocouples, however, are not and require frequent replacement. This tips the scale for installation costs in favor of the RTD.

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Old 05-11-2011, 03:10 PM   #8
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Quote:
Originally Posted by samc View Post
excerpted from here

Selecting sensors for reliability

For temperatures ranging from cryogenic to about 800°F (427°C), a resistance temperature detector (RTD) is better than a thermocouple. Though RTDs were once more expensive, today the cost difference between the two is negligible, and an RTD provides greater accuracy, stability, and life span. Over its useable range, a standard-grade RTD provides more than twice the accuracy of a thermocouple.

To understand why, consider the materials of construction. A thermocouple is comprised of two dissimilar metals welded together; the resulting junction begins to deteriorate immediately. The deterioration is faster at higher temperatures and can result in drift of several degrees per year. On the other hand, an RTD consists of stable, highly purified platinum.

RTDs drift just fractions of a degree per year and are accurate for many years when used at temperatures below 800°F (427°C). Thermocouples, however, are not and require frequent replacement. This tips the scale for installation costs in favor of the RTD.
Well, I can agree with this in theory but I can also tell you in practice, in any lab setting I have ever been in and I have been in allot, thermocouples can last a very long time and because they have a very predictable response curve they are very easy to calibrate initially and when they do drift out of cal over time.

That being said, trigger is right about the potential issues we may see in homebrewing and if the RTD will work better for your setup then you should certainly go that rout!
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Old 05-11-2011, 08:15 PM   #9
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I think I am going with the RTDs for the following reasons:
1. I'm anal and strive for perfection and the RTDs are more accurate.
2. The Auber Instrument RTDs have the nice quick disconnect to remove the wire. (even though they are charging me $18 to ship something that would easily fit in a $10 USPS box).

I plan on using 1 PID and having the quick disconnect I could swap the wires from the HLT RTD to the RTD on the boil pot.

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Old 05-11-2011, 08:45 PM   #10
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ace, you can use a compression fitting to mount the probe. something like http://www.homedepot.com/h_d1/N-5yc1...atalogId=10053

PS - I have a couple Fluke temp meters. Thermocouple based. I have 1 of them re-certified to the NIST every 6 months. Part of the re-cert is a statment if adjustment was needed. In the 30 year range with no adjustments.

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