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  1. #1
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    psycrometer

    Is a sling psycrometer required to acuaratly calculate superheat and subcooling as well as a temperature probe. All we have at work is a laser\infered thermometer . Thanks guys and gals

  2. #2
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    I'll give the short answer and let the more experienced techs give explanations...

    Yes... Get rid of the infrared. Get one.. And some temp probes.

    Sent from my GSIII on Tapatalk

  3. #3
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    Quote Originally Posted by andersonbe06 View Post
    Is a sling psycrometer required to acuaratly calculate superheat and subcooling as well as a temperature probe. All we have at work is a laser\infered thermometer . Thanks guys and gals
    An indoor wet bulb temperature is needed to determine the required superheat on a unit using a fixed metering device. As wet bulb temperature increases, more superheat is required.

    On a system using a Thermal Expansion Valve (TXV), the required superheat is maintained by the TXV. That is why on these systems you need to charge by subcooling.

    I too have never been a fan of infrared thermometers for measuring liquid and suction line temperatures.
    Instead of learning the tricks of the trade, learn the trade.

  4. #4
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    Quote Originally Posted by rundawg View Post
    I too have never been a fan of infrared thermometers for measuring liquid and suction line temperatures.
    But they work great to get the grease right when cooking hot wings.
    __________________________________________________ _______________________
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  5. #5
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    No. It it not required. In fact i see no use for a sling psycrometer when checking superheat or subcooling. A temperature probe, a gauge and a pt chart is what you need

  6. #6
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    As others have stated, on fixed metered systems you need to know the wet bulb temperature of the air entering the indoor coil to know the required superheat.

    Rather than a sling psychrometer for measuring the indoor wet bulb temperature, I highly recommend a psychrometer that has a probe that can be inserted through a test hole to measure the wet bulb and dry bulb temperatures where they are entering the indoor coil.
    My current favorite is the Fieldpiece SDP2,

    Most people seem to be unaware that the old standard sling psychrometers are inherently less accurate, rated for +/-5% RH, than most decent digital psychrometers, typically rated at +/-2 or 2.5%RH.

    Retire the IR gun to looking for hot electrical connections/breakers, and big temperature differences in wall sections.

    Many people have come up with, and/or been taught methods that they "never had a problem" with, but in every study that has been done, anywhere from 54% to over 70% of the systems checked during the study were significantly over or under charged by >10%, so clearly there is a big frickin problem...

    Quote Originally Posted by adam_s05 View Post
    No. It it not required. In fact i see no use for a sling psycrometer when checking superheat or subcooling. A temperature probe, a gauge and a pt chart is what you need
    On fixed metered systems, without the wet bulb temperature of the air entering the indoor coil, how do you know what your superheat is supposed to be?
    If more government is the answer, then it's a really stupid question.

  7. #7
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    Quote Originally Posted by andersonbe06 View Post
    Is a sling psycrometer required to acuaratly calculate (target ) superheat and subcooling as well as a temperature probe. All we have at work is a laser\infered thermometer . Thanks guys and gals
    the answer is yes or a wet bulb thermometer and a convoluted mathematical formula.
    It`s better to be silent and thought the fool; than speak and remove all doubt.

  8. #8
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    I use this formula. (Thanks HVACMedic)

    (3 x Entering Wet Bulb - 80 - Outdoor Temp) / 2 = Target Superheat.

  9. #9
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    Quote Originally Posted by Brent Ridley View Post
    I use this formula. (Thanks HVACMedic)

    (3 x Entering Wet Bulb - 80 - Outdoor Temp) / 2 = Target Superheat.
    Cool. Threw it in excel, entered various numbers against a Carrier charging chart and it proved accurate to .5*.

    Piggy-backing on Rundawg's statement " As wet buld increases, more superheat is required," is there a reasonably concise explanation as to science behind this formula?

    I feel that, even though I can competently follow direction, charging charts, Lennox's approach method, Tranes charging curves etc; I'm still at a loss as to explain or reason it all the way through.

  10. #10
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    Quote Originally Posted by hurtinhvac View Post
    Cool. Threw it in excel, entered various numbers against a Carrier charging chart and it proved accurate to .5*.

    Piggy-backing on Rundawg's statement " As wet buld increases, more superheat is required," is there a reasonably concise explanation as to science behind this formula?

    I feel that, even though I can competently follow direction, charging charts, Lennox's approach method, Tranes charging curves etc; I'm still at a loss as to explain or reason it all the way through.
    hurtinhavac, Let me know if this explanation is helpful.

    With a fixed orifice, the amount of refrigerant that flows through "it" is dependant on the pressure difference across "it".

    The pressures are dependant on airflow, humidity and temperature. The airflow is not supposed to change but temperature and humidity of course will.

    The condenser pressure is only affected by changes in sensible heat (outdoor dry bulb) but the evaporator is affected by changes in both sensible and latent heat (indoor dry and wet bulb) of which the latent is the driving force.


    If the superheat is accurately set for the current conditions, future conditions will vary but never be too high to prevent compressor cooling or too low to cause flooding.


    "If perfection is your goal, you may end up with good enough, what might you end up with when good enough is your goal?"
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  11. #11
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    Quote Originally Posted by penderway View Post
    hurtinhavac, Let me know if this explanation is helpful.

    With a fixed orifice, the amount of refrigerant that flows through "it" is dependant on the pressure difference across "it".

    The pressures are dependant on airflow, humidity and temperature. The airflow is not supposed to change but temperature and humidity of course will.

    The condenser pressure is only affected by changes in sensible heat (outdoor dry bulb) but the evaporator is affected by changes in both sensible and latent heat (indoor dry and wet bulb) of which the latent is the driving force.


    If the superheat is accurately set for the current conditions, future conditions will vary but never be too high to prevent compressor cooling or too low to cause flooding.

    Now that's what I'm talkin' about...

    Thank you.

  12. #12
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    Quote Originally Posted by andersonbe06 View Post
    Is a sling psycrometer required to acuaratly calculate superheat and subcooling as well as a temperature probe. All we have at work is a laser\infered thermometer . Thanks guys and gals
    All that is required to measure SH / SC is pressure gauges, temperature probes, and P/T chart.

    WB may or may not be required to determine target SH / SC, depending on the unit's charging chart.

  13. #13
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    Quote Originally Posted by bobster View Post
    All that is required to measure SH / SC is pressure gauges, temperature probes, and P/T chart.

    WB may or may not be required to determine target SH / SC, depending on the unit's charging chart.
    majority if not all SH charts have wet bulb that i remember looking at lately. And with the DB out and WB inside, superheat can be anywhere from 30 to 6 (or something like that)

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