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Old 01-14-13, 04:51 PM   #1
Bill Sanford
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Default How are you measuring water and refrigerant line temps?

Hi Guys,

I recently bought a cheap but seemingly accurate dual channel digital thermometer on EBay and I have a question for you all.

What are you guys all using to measure temps and how do you actually connect the sensors to various points to get accurate readings? This is obviously a critical piece in determining system performance.

With the thermometer I bought, I'm having issues getting repeatable and reliable temp measurements. My problem seems to be attaching the thermocouples to the copper lines in such a way that it reads accurate temps. This unit just has thermocouples on the end of the leads that run to the meter. I assume that clamp-on type thermocouples would fix my problem, but I don't want to spend a bunch of money for as little as I'll use this.

Suggestions?

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Old 01-14-13, 05:20 PM   #2
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Bill,

Good question! The key is to insulate the "back side" of the thermocouple or thermistor, so the dominant temperature reading is from the line you are trying to get the temperature reading from. Just a bit of foam or other insulating material will work.

Always measure at exactly the samer spot and make sure that there is physical contact of the temperature reading transducer (thermocouple or thermistor) to the actual pipe.

Does this help?

Steve
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Old 01-14-13, 10:58 PM   #3
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Originally Posted by stevehull View Post
Bill,

Good question! The key is to insulate the "back side" of the thermocouple or thermistor, so the dominant temperature reading is from the line you are trying to get the temperature reading from. Just a bit of foam or other insulating material will work.

Always measure at exactly the same spot and make sure that there is physical contact of the temperature reading transducer (thermocouple or thermistor) to the actual pipe.

Does this help?

Steve
Thanks Steve.

I did use a piece of copper tubing insulating foam, but I still had inconsistent results. I think maybe I didn't have the thermocouple pressed up against the tubing enough. The thermocouples are very cheap. They seem to measure ambient air temps very well, but I'm fighting with them to do the same on water and refrigerant lines. Maybe a small piece of aluminum foil around the thermocouples might help. I'll keep playing with them.

The HVAC guy that came out to do a repair for me had a Fluke clamp-on thermocouple temp sensor the plugged into his Fluke meter. It seemed to work VERY well and was super fast to reach the measured temp once it was clamped on to a pipe. The only issue is that it cost $150. My dual channel thermometer with two type "K" thermocouples wasn't too much more than $20!

I guess you get what you pay for! :-)
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Old 01-15-13, 02:08 AM   #4
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You could also use a little bit of thermal compount as used for thermal contact between a CPU and the heatsink in computers, if you happen to have that lying around. The simplest white goop would be good enough.
Alternatively you could use a drop of vegetable oil, it is not as good in transfering heat but this too will close the airgap.

Then use a little tape to clamp the sensor to the tubing so the sensor doesn't move. Covering it with a piece of tubing insulation foam is a good idea.
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Old 01-15-13, 10:11 AM   #5
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You could also use a little bit of thermal compount as used for thermal contact between a CPU and the heatsink in computers, if you happen to have that lying around. The simplest white goop would be good enough.
Alternatively you could use a drop of vegetable oil, it is not as good in transfering heat but this too will close the airgap.

Then use a little tape to clamp the sensor to the tubing so the sensor doesn't move. Covering it with a piece of tubing insulation foam is a good idea.
I actually tried thermal heat sink compound. The white pasty stuff. I still had readings that seemed to vary.

I wonder if the leads coming off my thermocouples are the issue. The actual thermocouple is a tiny dot with two very fine wires coming from it. Just the thermocouple and a bit of the leads are exposed and the rest of the end of the cable is covered by a piece of heat shrink tubing.

I wonder if the two fine wires aren't insulated well and I'm having issues because of it. I'll try a few things and see what I can figure out.

Bill
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Old 01-15-13, 01:35 PM   #6
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Quote:
Originally Posted by Bill Sanford View Post
What are you guys all using to measure temps and how do you actually connect the sensors to various points to get accurate readings?...
I have used the dual probe cheapo thermometers and except for the fact that they all had a different reading for the same temperature, they were pretty reliable as far as change in temperature goes.

So you might want to strap all of your sensors to the same tube, close together, and look at the readings for all of them and label each of them uniquely. Then decide what the average is, you could actually calculate it or you could look at the high and the low and figure that average is half way in between. Then note how far each sensor is from the 'average' and change the sign of that number. For instance if sensor 'B' is 1.4 over, assign -1.4 compensation to that sensor, and when you read the temp from that sensor, add the compensation factor.

The next part is the attachment part that you originally asked about. I used the sticky-backed foam tape and made sure it was under tension when I wrapped it around the sensor & tube.

That should do it.

BTW, I don't know if you are geekishly inclined, but a 1-wire sensor coupled to an Arduino reads quite accurately, and is fairly linear.

Best,

-AC
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Old 09-28-13, 11:45 PM   #7
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Quote:
Originally Posted by stevehull View Post
Bill,

Good question! The key is to insulate the "back side" of the thermocouple or thermistor, so the dominant temperature reading is from the line you are trying to get the temperature reading from. Just a bit of foam or other insulating material will work.

Always measure at exactly the samer spot and make sure that there is physical contact of the temperature reading transducer (thermocouple or thermistor) to the actual pipe.

Does this help?

Steve

Velcro strapping ( used instead of cable ties ) ?

God bless
Wyr
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Old 09-29-13, 12:16 AM   #8
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Most accurate I have found are digital temperature sensors like the LM92s. Solder a bundle of wires to it (don't forget the decoupling cap!) and encapsulate it in hot glue and heatshrink. Then use foil tape to attach it to the line and cover with foam insulation tape.
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Old 09-29-13, 10:40 AM   #9
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Electronically, there are two different ways of measuring temperature with a contact probe. One is with thermistors and the other is with thermocouples.

Thermistors have a VERY rapid response as they are tiny. They are a device that changes resistance (non linear) with temperature. In the absence of a thermal mass around them, their response time is in the msec. The electronics to display the non-linear resistance change is a bit tricky, so their accuracy is low compared with their inherent precision.

Thermocouples are larger and have a much longer time constant, but in this application, a few msec compared to a couple hundred msec is irrelevant. These have characteristics of both high precision and high accuracy. Any degradation of the signal reflect the electronics and not the probe.

Thermocouples put out a voltage linear with temperature change and the electronics to display this is easy.

I suspect that cheap meters have cheap electronics that have offset voltages that degrade both precision and accuracy in both cases.

A microprocessor is not necessarily more "accurate" as the A/D converter (even "double precision") still relies on some electronic analog formatting - and that is where degradation typically occurs.

I think AC's suggestion of getting multiple measurements on an identical surface is a good one. Years ago, I would do this by starting with nearly frozen water in a beaker on a stir plate so that the entire volume of water was at an identical temp. Probes were placed in the water right next to one another and I plotted the curves as the water warmed up.

Didn't have to worry about contact issues as the probe was submerged.

It is frustrating to buy a temperature meter and not have it display the right temp . . . .

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Old 09-29-13, 11:17 AM   #10
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There are also thermal diodes, which are more accurate than most thermistors. Despite their name, they're actually transistors with the base and collector connected to each other since that is more "ideal" than a diode for that application.

Digital temperatures may be thermal diodes built onto the same chip as the ADC, but some use two oscillators whose frequency varies in a predictable manner over temperature.

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