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Thread: psycrometer

  1. #14
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    Gulf Coast
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    Of course two weeks of recovery and recharging wouldnt be too awful, lots of h-talk time!

    Sent from my SCH-I500 using Tapatalk 2

  2. #15
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    Aug 2001
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    Mechanicsville, Virginia
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    Quote Originally Posted by Core_d View Post
    Anybody tried wrapping a little bit of a wet paper towel around a temperature probe? It was within 1 degree of actual WB.
    A sound principle, but not without it's problems. What I like best about your post is that it demonstrates "how it works". It's great to have expensive and accurate test equipment, software etc. but it's even better if you know what it's doing

    "If perfection is your goal, you may end up with good enough, what might you end up with when good enough is your goal?"
    Wayne Pendergast, efficientcomfort.net

  3. #16
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    Nov 2011
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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.

  4. #17
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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?"
    Wayne Pendergast, efficientcomfort.net

  5. #18
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    Nov 2011
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    St. Louis
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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.

  6. #19
    Join Date
    Aug 2007
    Location
    Edmond, Oklahoma
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    399

    Thumbs up

    Quote Originally Posted by penderway View Post
    FYI, Any chart that doesn't require a wet bulb input assumes 50% indoor relative humidity. Charts that specify pressures can be backward engineered to find the same data as the original superheat chart developed by carrier long ago. It applies to all fixed orifice air conditioners and heat pumps (cooling mode of course). The aforementioned formula was curve fitted from the same data and is very close to dead on for most conditions and many find it to be close enough for all conditions.

    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 you need a chart you can have this one
    Of course you're right about the 50% RH assumption.

    I wasn't aware of the formula, though, which seems much easier & more convenient than using a chart. Thanks!

  7. #20
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    Sep 2012
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    WV
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    That is exactly what I do. I figure the water evaporates the same way.

  8. #21
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    Sep 2012
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    WV
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    oops! Was in response to the comment about the wet paper towel trick

  9. #22
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    Dec 2011
    Location
    Grass Lake, MI
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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
    While I no longer use a Sling psychrometer, having a method of determining wet bulb is needed to accurately determine target superheat and subcooling. One you know your target, you can adjust the charge to match the target. +/-3 seems to be a usable SH or SC. Get yourself a slide chart or charging app for your phone and charge away.

    Keep in mind Airflow. You may have the correct input information but lack the proper airflow to utilize it.

  10. #23
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    Aug 2007
    Location
    Edmond, Oklahoma
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    Quote Originally Posted by mark beiser View Post

    If the RH is significantly higher or lower than 50%, or the space is not at or near normal temperature, the chart cannot be used.

    I'm still:[IMG]http://1.bp.********.com/_wAxDMfEGhoY/TQrlbN4gYXI/AAAAAAAAAXg/kmwztAAni44/s400/Not+Sure+if+serious.jpg[/IMG]
    My understanding is that SH measurements taken under a hot pull down condition (i.e., not near normal temp) should not be used to determine the correctness of the charge.

    Does that apply to the formula also?

  11. #24
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    Aug 2012
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    west burbs,il
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    I recently picked up on a nice digital psychrometer made by SUPCO #dsp1000. Very nice and accurate, give DB,wb, rh, and dew point. Highly recommend it.

  12. #25
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    Aug 2012
    Location
    west burbs,il
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    Oh its available at united refrigeration, and about $90 bucks.

  13. #26
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    Dec 2002
    Location
    Houston,Tx.
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    16,004
    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.
    __________________________________________________ _______________________
    “Experience is a hard teacher because she gives the test first, the lesson afterwards". - Vernon Law

    "Never let success go to your head, and never let failure go to your heart". - Unknown

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