What The Fluke ?

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BulldogAcres

BulldogAcres

Member
Feb 12, 2010
61
Northeast Washington State (Colville)
I just purchased a Fluke 561 IR Thermometer with a direct reading probe.
I have found some interesting differences in the temperature reading with the Direct Reading Probe. If I use the probe and have it directly in the stream of hot water coming from my faucet it reads 122 degrees F. If I hold the probe against the faucet pipe it reads 118 degrees F. Could there be a 4 degree difference between the hot water stream and the faucet pipe. Would the pipe insulate the hot water by 4 degrees. Maybe some one out there could explain the differences.
 
My answer would be yes. Think about it. Water starts all at the same temp, but the water right next to the interior of the pipe is losing heat through the pipe to the air, while the water in the center of the pipe is staying hot. If you could "turbulate" the flow inside the pipe so that it all loses heat evenly, then the same reading. The same thing happens inside boiler hx tubes, which is one reason users get turbulators, and the same thing happens inside the flue/stack of your boiler chimney. My stack probe reading may be 400F, while a stack surface reading might be 250F.
 
You're killing me. Not really but I had to think back more than 30 years from my thermodynamics classes. I would say google 'heat transfer through a pipe' and take a look at the math involved. Pipes of different materials have different thermal conductivity ratings so in a sense anything is an insulator. Copper just happens to transfer heat very well, stainless a number of times less and pex even less so.
 
Now I'm musing. My gut tells me the water molecules next to the surface of the pipe are sticking a bit to the pipe by friction, and if you could look at the molecules, you might see a somewhat circular flow of molecules right next to the surface, and that these are not mixing much with the molecules in the interior of the flow. That also means that the cooler molecules next to the surface of the pipe in the circular flow are continuing to cool even more, which may explain the very large difference between interior of stack and surface of stack temps.
 
BulldogAcres said:
I just purchased a Fluke 561 IR Thermometer with a direct reading probe.
I have found some interesting differences in the temperature reading with the Direct Reading Probe. If I use the probe and have it directly in the stream of hot water coming from my faucet it reads 122 degrees F. If I hold the probe against the faucet pipe it reads 118 degrees F. Could there be a 4 degree difference between the hot water stream and the faucet pipe. Would the pipe insulate the hot water by 4 degrees. Maybe some one out there could explain the differences.
Click to expand...

A surface mounted thermometer will always read lower than the fluid inside the pipe. Depending on the device doing the measuring, sometimes by as much as 10*.

Surface mount vs immersion well type aquastats are a good example.
 
We have a Fluke 66 IR gun here on the ship . I went and tested it on the faucet here and I put a piece of masking tape on the chrome faucet . I let the tap run for five minutes and It was only 3°C lower than the well mounted temp gauge in the top of the HW heater which was 25'away. Then aimed it at the faucet beside the tape and it was 10 °C less. The chrome gives off a reflection and so does the water coming out of the faucet. In the manual it says to use flat black paint or masking tape at your temp reading locations. I went and checked at a bunch of other locations and it is within +1 or -1 degrees of the well mounted gauges.

Huff
 
Accurate temperature measurement can sometimes be difficult. I spent many years measuring and analyzing heat exchanger performance where accurate temperature measurement of both fluid streams is crucial to get good results. For us accounting for heat to within 2-3% was the best that could be achieved. The measurement of the water stream will obviously give the best measurement. But even within the stream the temperature can vary. As some have previously stated you will have some heat loss from the fluid through the pipe to the ambient. As a result you will have a temperature gradient across the pipe. However a 4 degree drop across across a steel pipe would require a significant heat loss. Likely your measurement error is attributed to a thermal contact resistance between the surface and the sensor. This is analogous to electrical resistance and in this case you will have a temperature difference between the pipe and the sensor. Did you try wrapping insulation around the probe and pipe to see if the difference decreases?
Concerning the IR measurement, this too will have an inaccuracy. Material, surface roughness, and angle will impact his measurement. The IR sensor assumes the surface act as a blackbody which is the reason the Fluke suggests using a black tape. A black roughed surface will give the best measurement where a shiny round surface will not. Do you have the ability to adjust the IR sensor to match the measurement of the direct probe?
 
heat4steve said:
Accurate temperature measurement can sometimes be difficult. I spent many years measuring and analyzing heat exchanger performance where accurate temperature measurement of both fluid streams is crucial to get good results. For us accounting for heat to within 2-3% was the best that could be achieved. The measurement of the water stream will obviously give the best measurement. But even within the stream the temperature can vary. As some have previously stated you will have some heat loss from the fluid through the pipe to the ambient. As a result you will have a temperature gradient across the pipe. However a 4 degree drop across across a steel pipe would require a significant heat loss. Likely your measurement error is attributed to a thermal contact resistance between the surface and the sensor. This is analogous to electrical resistance and in this case you will have a temperature difference between the pipe and the sensor. Did you try wrapping insulation around the probe and pipe to see if the difference decreases?
Concerning the IR measurement, this too will have an inaccuracy. Material, surface roughness, and angle will impact his measurement. The IR sensor assumes the surface act as a blackbody which is the reason the Fluke suggests using a black tape. A black roughed surface will give the best measurement where a shiny round surface will not. Do you have the ability to adjust the IR sensor to match the measurement of the direct probe?
Click to expand...

I have no way of adjusting the probe or the IR sensor.
Thanks
 
I look at it different, the faucet is acting like a heat sink just pulls the heat out so no wonder it is less then the water itself. My internal flue temp is almost exactly 115 degrees hotter then the outside surface temp.

Steve
 
It's maybe from stratification in the hot water tank. The hottest water comes out first and less hot water follows.
Variations in temp seems logical. 4 degrees difference is a perfect measurement result I would say
 
The pipe is insulating the hot water, and the pipe is also losing heat, therefor it cannot get up to the exact temp of the water.
 
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