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Pipe thermometer

28W Member Posts: 141
I got one of those pipe thermometers that strap onto the pipe with a metal spring. To test it, I attached it to my boiler's outlet pipe, where it exits the boiler (before it goes into the low loss header). The thermostat reads about 20 degrees lower than the boiler's temperature display (Vitodens 100). I assume the boiler's display is showing the water temperature in the heat exchanger. Is it possible that the twater emperature drops 20 degrees by the time the water has left the boiler and traveled 6 inches, or is the thermometer just very inaccurate?


  • icesailor
    icesailor Member Posts: 7,265
    Pipe Temperatures:

    The fluid inside a pipe is hotter in the middle than in the outside. A strap on thermometer, unless heavily insulated, is subject to the surrounding air temperature.

    The thermometer with an internal probe is the more accurate thermometer.

  • Plumdog_2
    Plumdog_2 Member Posts: 873
    Maybe its out of calibration

    Some of those have a screw in the end where the probe is, and some have a nut to hold while you turn the face to align the pointer with the correct temp. They aren't usually dialed right out of the box. And pretty crude and slow to respond as well.
  • 28W
    28W Member Posts: 141
    Yes, this one has a screw.

    I guess I will adjust it to match the temperature on the boiler's display. That should compensate for the issue icesailor raised also.

    All I'm trying to do is get an accurate assessment of how the return water temps changes in response to different circulator speeds.
  • icesailor
    icesailor Member Posts: 7,265
    Pipe temperature readings:

    I've taken two digital pocket thermometers and wrapped them in insulation and rags around the outlet and return with the display available to look at. I also once had a digital thermostat with three remote probes that I did the same with. That instrument went South years ago. But, once running, you can watch the rise and fall of the two pipes.

    It worked for me.

    You can always check the calibration of a thermometer by first putting it in ice water at sea level. It MUST read 32 degrees. Then, put it in a pot of boiling water. It MUST read 212 degrees. If it does, the thermometer is calibrated. Pure water and ice cubes will be 32 degrees because if it is any colder than 32 degrees, it will be ice. There is a way that liquid water can be lower than 32 degrees. If you bring that up, you will get the argument. Water at sea level, that is boiling, is 212 degrees. Any higher than that and it is water vapor. At higher elevations, the boiling point is less than 212 degrees. Below sea level (Death Valley, CA or the Dead Sea, it will be higher.
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