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Sensor Accuracy

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woodshed
woodshed Member Posts: 36
in Radiant Heating
The outdoor sensor for the boiler is not accurate; using an old style mercury thermometer, the temperature reading from my truck and the temperature reading from the HO's car all show the same outdoor temperature reading. The outdoor sensor reading is 5* higher. The boiler manufacturer sent me a replacement sensor but it's 5* higher as well. So, isn't an accurate outdoor temperature sensor the key part of ODR? BTW- the outdoor sensor is a 12k not a 10k sensor. Any ideas on this or how to correct it?
Thanx.

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  • Leon82
    Leon82 Member Posts: 684
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    My Lochinvar sensor does the same thing.
  • bmwpowere36m3
    bmwpowere36m3 Member Posts: 512
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    If its consistent (5* high or whatever), then don't worry about it... just set the curve and adjust as needed.
    SWEI
  • woodshed
    woodshed Member Posts: 36
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    bmwpowere36m3 said:

    just set the curve and adjust as needed.

    How do you adjust the curve for a 5* higher/lower outdoor temperature?

    It would be helpful if the boiler board allowed input of a outdoor temperature difference or the actual outdoor temperature. If the sensors are going to be inaccurate, it seems it would be better if they erred on the minus side rather than the plus side.

  • bmwpowere36m3
    bmwpowere36m3 Member Posts: 512
    edited February 2016
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    Honestly I wouldn't worry about it. If it bothers you that much, then simply adjust your HI and LOW ODT points on the curve 5* over.

    This is assuming a ODR curve with 4 points: LO ODT, HI ODT, LO ODT water supply temp and HI ODT water supply temp.
  • Gordy
    Gordy Member Posts: 9,546
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    It would be a nice option to be able to calibrate a sensor on board. Some thermostats have this ability..

    Does the sensor meet the specified ohm readings outlined in the manual?
  • Jason_13
    Jason_13 Member Posts: 304
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    The Alpines have the calibration that applies to wired and wireless sensors. Ohm it from boiler on wires and than ohm the sensor. May be interference. Don't parallel line voltage wires, florescent lights etc. Or you can use shielded wires.
  • hot_rod
    hot_rod Member Posts: 22,158
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    woodshed said:

    The outdoor sensor for the boiler is not accurate; using an old style mercury thermometer, the temperature reading from my truck and the temperature reading from the HO's car all show the same outdoor temperature reading. The outdoor sensor reading is 5* higher. The boiler manufacturer sent me a replacement sensor but it's 5* higher as well. So, isn't an accurate outdoor temperature sensor the key part of ODR? BTW- the outdoor sensor is a 12k not a 10k sensor. Any ideas on this or how to correct it?
    Thanx.

    How long is the wire run to the sensor and what gauge wire? Sometimes there is a restriction in the installation regarding the wiring.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • woodshed
    woodshed Member Posts: 36
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    hot rod said:

    woodshed said:


    How long is the wire run to the sensor and what gauge wire? Sometimes there is a restriction in the installation regarding the wiring.

    HR - The wire run is 50 feet from sensor to boiler and using 18-2 wire.

  • Wellness
    Wellness Member Posts: 143
    edited February 2016
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    I'd be interested in a solution to this issue as well as I have discovered my outdoor reset is inaccurate. And unlike indirect DHW tank sensors, you can't really replace or calibrate outdoor reset sensors with 3rd party tools; and that's a shame.
  • hot_rod
    hot_rod Member Posts: 22,158
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    sound like all you need to do is "fit the coefficients of a third order polynominal in the log-resistenance values to best match the inverse temperature values" :)

    http://www.mstarlabs.com/sensors/thermistor-calibration.html

    Kurt would know, I think it can be corrected with the correct resistor. On our solar demo trainers we us 10K POTS and little rebel resistors to adjust the value wherever we need to "trip" the control functions.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Wellness
    Wellness Member Posts: 143
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    That sounds like a very looong weekend Hot Rod :wink:
  • woodshed
    woodshed Member Posts: 36
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    Gordy said:

    It would be a nice option to be able to calibrate a sensor on board. Some thermostats have this ability..

    Does the sensor meet the specified ohm readings outlined in the manual?

    Gordy -the sensor chart shows the reading is supposed to be 36,130 for 32* and I'm reading 37,000; however the outdoor temp is actually 27*

    I tried another 12k sensor and the spread is even larger.
  • hot_rod
    hot_rod Member Posts: 22,158
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    When you look under the hood to see what makes it tick..It's amazing how they can build them so small and accurate.

    I imagine there is an acceptable tolerance from the builder of that thermistor?
    It needs to work across a wide temperature range. For solar that temperature can be well below 0° in Alaska to over 500°F on a stagnant evac tube solar collector header.

    We test them with ice water, and boiling water to see how they match the resistance chart.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • SWEI
    SWEI Member Posts: 7,356
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    Either the thermistor is out of spec, or the ADC in the boiler controls is. As long as the error is even moderately consistent, I would probably just adjust the ODR points until things get comfortable.
  • woodshed
    woodshed Member Posts: 36
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    bmwpowere36m3 said:

    Honestly I wouldn't worry about it. If it bothers you that much, then simply adjust your HI and LOW ODT points on the curve 5* over.

    Wouldn't I adjust the HI and LOW ODT 5* lower?

  • bmwpowere36m3
    bmwpowere36m3 Member Posts: 512
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    It's reading 5 too high, right? No, move both points 5 degrees higher.
  • Leon82
    Leon82 Member Posts: 684
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    I think woodshed is correct.

    My sensor reads high and the math was saying I would need 145 degree water but mine was at 140 and was fine during the below zero cold spell.
  • Leon82
    Leon82 Member Posts: 684
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    Although you guys are talking about raising the outdoor temp instead of water temp so BMW may be correct.
  • Heat_n_CT
    Heat_n_CT Member Posts: 64
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    If it's actually 5* colder out than the sensor reads, you should raise your setting so you get higher SWT to compensate for the colder actual...like BMW states.

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