K Type Thermocouple vs RTD temperature sensor

I am about to pull the trigger on some PID temp controllers from Auber Instraments. I can't decide on the type of temp sensors to go with K Type Thermocouple vs RTD temperature sensor. RTD is a little more expensive but not enough to really matter.

Does anyone have any advice? Pros or cons of each?

Thanks

This link gives a nice comparison of the two: http://www.smartsensors.com/spec.htm

I've seen a lot of posts recommending RTDs for their higher accuracy and linear response. However, it's splitting hairs, IMHO. Both sensors are capable of less than +/-1% accuracy (effectively, less than +/-1.5 degrees at mash temps). For brewing applications, a decently calibrated thermocouple is just fine. I chose thermocouples for my rig, mostly due to the faster response time.

In our application I don't think you'll see a lot of difference. Just make sure to match your controller to the type of temperature sensor you are going to get. Here's a chart that tells some of the differences
http://www.smartsensors.com/spec.htm

Edit:
darnit, beat by a couple minutes

Nice link,

Thanks guys.

one column made my decision for me...Ruggedness. other than that for the temps we are talking about both look like they would be fine.

Thermocouples are just a junction between two dissimilar metals where a RTD is simply a resistor that changes value with temperature. As the previous posters pointed out, for our purposes, it's a non issue. I went with a thermocouple because I found one, for not too much money that had a 1/2" NPT thread on it so I could screw it right into my keggle.

The tech at Auberins told me that the RTD coupler is a more accurate sensor. The price difference is not much. What I like about mine is that the actual 1/4" NPT probe is able to be removed from the probe via a quick disconnect. I did not plan on this, just discovered it after I ordered it (to be honest with you I had no idea what I was ordering) after talking to the Auberins tech....who I pissed off with all of my elementary questions.

:rockin:

RTD is my vote. They offer more repeatable readings over time, and thus more robust in my opinion. I use 3wire RTDs that compensate for the resistance of the wires, which is important if you want to use long runs like I do. 4wire will compensate for different resistances between the wires.

Doorbell wire is a cheap source for wire... get some quick connectors from omega they are only a couple of bucks. Mine have 1/8mpt which I thread into 1/8fpt - 1/8straight thread. A silicone oring and a nut are used to make watertight connections. That's how I modded my stainless conicals to monitor temperature of the beer. I use a dial probe on the kettle... only used to gauge approach to boiling. Good luck!

Any thoughts on the type of thermocouple to choose? J vs K vs T ect?

Also, I'm thinking about ordering from omega since they seem to have better quality than the Auberins ones I've gotten. The question is the configuration. I can get a connector on the end of the probe, braided line with a connector on the end of the line as well. Does anyone have the connector at the end of the probe? If so, do you like it?

StMarcos said:

RTD is my vote. They offer more repeatable readings over time, and thus more robust in my opinion.

Click to expand...

Not true. You can't change Physics. A junction between between two dissimilar metals will always induce an EMF and unless the two metals change their chemical composition over time the EMF will stay constant.

It is true. There are more sources of drift in a thermocouple than in an RTD. They are also more accurate.

StMarcos said:

It is true. There are more sources of drift in a thermocouple than in an RTD. They are also more accurate.

Click to expand...

Ok, explain, I am all ears. I agree that the RTD can be more accurate. I am not sure I agree that a thermocouple can drift but I am not above learning something new.

StMarcos said:

It is true. There are more sources of drift in a thermocouple than in an RTD. They are also more accurate.

Click to expand...

Sawdustguy said:

Ok, explain, I am all ears. I agree that the RTD can be more accurate. I am not sure I agree that a thermocouple can drift but I am not above learning something new.

Click to expand...

Nevermind, I agree with you now. I was curious and did some reading. I now understand why metallurgically they can drift. I stand corrected.

"Thermocouples are based on the effect that the junction between two different metals produces a voltage which increases with temperature. Compared with resistance thermometers they offer the clear advantage of a higher upper temperature limit, up to several thousand degrees Celsius. Their long-term stability is somewhat worse (a few degrees after one year), the measuring accuracy is slightly poorer (on the average +0.75% of the measurement range). They are frequently used in areas with temperatures above about 250C."

Sawdustguy said:

Nevermind, I agree with you now. I was curious and did some reading. I now understand why metallurgically they can drift. I stand corrected.

"Thermocouples are based on the effect that the junction between two different metals produces a voltage which increases with temperature. Compared with resistance thermometers they offer the clear advantage of a higher upper temperature limit, up to several thousand degrees Celsius. Their long-term stability is somewhat worse (a few degrees after one year), the measuring accuracy is slightly poorer (on the average +0.75% of the measurement range). They are frequently used in areas with temperatures above about 250C."

Click to expand...

You're a good man for being humble, and this is a great thread. Thanks for the info, guys!

Go RTD or thermistor.

Thermocouple meters require a cold junction compensation reference very near the thermocouple connector. This compensation device is in the PID and you can actually see it through the vents (Auber uses a small leaded diode). If your PID gets hot, your reading will become inaccurate, up to several degrees. I know this from experience