Wet bulb temperature measurements
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  1. #1
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    Feb 2007
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    Wet bulb temperature measurements

    Asking the PROs to comment on my methods:

    I have a thermometer where the bulb is covered with a 'wick', and the wick is immersed in distilled water.

    In taking measurements in a room, I either wave the thermometer rapidly (simulating a sling psychrometer), or place the bulb portion in the air stream of a fan. I continue (either method) until the temperature reading is constant, and assume the reading to be correct.

    I would like to take WB measurements of the air stream in my plenum. I have a Fieldpiece SPK1 and propose to use a wet wick over the probe.

    Lacking an electronic instrument, could I expect to get a reasonably accurate measurement if the probe stabilized rapidly?

    Thanks for your input.

    Amp

  2. #2
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    Aug 2003
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    Get a decent hygrometer that measures relative humidity and an accurate thermometer to measure your dry bulb. You can then use a psychrometric chart (online psych calculators also exist, and there's a cool one for the iPhone/iPod Touch called PsychromeTree) to find your wet bulb temp. That way no need to fuss with a wet wick.

    Personally I like the Testo H2 Humidity Stick for measuring return and supply air conditions.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  3. #3
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    Quote Originally Posted by ampulman View Post
    Asking the PROs to comment on my methods:

    I have a thermometer where the bulb is covered with a 'wick', and the wick is immersed in distilled water.

    In taking measurements in a room, I either wave the thermometer rapidly (simulating a sling psychrometer), or place the bulb portion in the air stream of a fan. I continue (either method) until the temperature reading is constant, and assume the reading to be correct.

    I would like to take WB measurements of the air stream in my plenum. I have a Fieldpiece SPK1 and propose to use a wet wick over the probe.

    Lacking an electronic instrument, could I expect to get a reasonably accurate measurement if the probe stabilized rapidly?

    Thanks for your input.

    Amp
    You need 1,000 ft.per minute velocity on the air flow over a clean wick and distilled water to get an accurate WB. Also cooling a metal bowl slowly down with ice to the point of condensation indicates the dew point of the air. Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  4. #4
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    Since you wouldn't know the velocity of teh air in teh duct. You wouldn't know if your wetbulb temp was correct using a wet sock thermometer.
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  5. #5
    I have found that even just waving a thermometer in cotton wool wetted with tap water around gently gives me wet bulb readings very close to my digital thermo hygrometers, within a 10th of a C or a 20th of a F thereabouts.

    Wrapping cotton wool around a probe allows you to log the readings over time and even compare them to dry bulb. Or to take two wet bulb readings at the same time on a difference calculating twin probe thermometer allowing you to determine enthalpy with sufficient accuracy for practical purposes.

    Since both probes, air-on and air-off are going to be of similar inaccuracy it doesn't matter when you are working with enthalpy change because you are only concerned with the difference between them and not so much their specific reading.

  6. #6
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    Quote Originally Posted by Marc O'Brien View Post
    I have found that even just waving a thermometer in cotton wool wetted with tap water around gently gives me wet bulb readings very close to my digital thermo hygrometers, within a 10th of a C or a 20th of a F thereabouts.

    Wrapping cotton wool around a probe allows you to log the readings over time and even compare them to dry bulb. Or to take two wet bulb readings at the same time on a difference calculating twin probe thermometer allowing you to determine enthalpy with sufficient accuracy for practical purposes.

    Since both probes, air-on and air-off are going to be of similar inaccuracy it doesn't matter when you are working with enthalpy change because you are only concerned with the difference between them and not so much their specific reading.
    Very interesting, Marc.

    Since I'm a retired home owner with too much time on my hands, pin-point accuracy is not required. I did select my thermometers by comparing side-by-side readings, hoping to reduce one source of error.

    As I am now just getting into psychrometrics, I'm not into enthalpy, yet; just taking WB/DB readings and referring to a relative humidity & dew point chart to track my indoor humidity.

    Can you elaborate on the air-on and air-off probes vs. specific temeratures on the chart (if my assumption is correct)?

    Thanks.

    Amp

  7. #7
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    Clarification of a prior post in this field of Inquiry

    Welcome aboard UK Marc O'Brien & AMP!

    I need to clarify by showing humidity levels concerning what I wrote on a thread, that I cannot find to respond to, concerning room temperatures & wet bulb readings in relationship to humidity & sensible-ratio operating-capacities.

    A 3-Ton condenser, 4-Ton evaporator, 13-SEER, fixed orifice, 1434-CFM; 80-IDB, 67-IWB, 50%RH, Sensible 0.76, Total 37,000-BTUH. 28,120-BTUH Sensible, 8,880-Latent.

    70-IDB, 59-IWB, 52%RH, Sensible 0.80, Total 32,700-BTUH. 26,160-BTUH-Sensible & 6,540-Latent at the lower 70-F room temps.

    If you operate at room temperatures much below 75-F, or 70 or below a 3-Ton’s going to deliver close to a 2.5-Ton when figuring equipment sizing.

    If there are long run-times due to proper sizing & good air circulation, I see scant reason to size below 75-F & 50% or less humidity; but the customer is always right. – Darrell
    Last edited by udarrell; 05-27-2009 at 02:41 PM. Reason: spelling

  8. #8
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    LOL... I like 72 at 48%RH
    Sometime 70 at 48.

    75 at 48, just doesn't feel cool enough to me.
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  9. #9
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    Quote Originally Posted by beenthere View Post
    LOL... I like 72 at 48%RH
    Sometime 70 at 48.
    75 at 48, just doesn't feel cool enough to me.
    What temps & humidity do you use in the winter?

    I would be way too cold in those summer-time temp-conditions, with good air movement, I'm very comfortable around 75-F 50% RH.

    That's why, for sizing, we always need to know if they keep it set at 68 or 65-F in the summer; many assume 75-F & under 50%RH.

    Gets mighty cold here in the north country.
    My metabolism must be lower; we turn into hibernating bears up here in WI.

    Due to the cost of #2 furnace fuel oil, I keep it set real cool in the winter.
    The furnace would even kick on now, if I didn't have the t stat set so low.
    - Darrell

  10. #10
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    Well, when oils high, I keep it around 68.
    When oils almost reasonable, 70.

    Seldom get it above 30%RH in winter.

    I too dog gone cheap to install a good humidifier.


    PS: I put a small electric heater in the bathroom in the winter. I like it warm when I get out of the shower.
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  11. #11
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    One advantage of weight loss; I cool off faster and stay comfortable at a higher temperature than when I was carrying extra pounds around.

    When I'm not active, I find 75 degrees dry bulb and between 45-50% RH comfortable, at times with added air movement, at others without. I also think, even at rest, there's some variability in our metabolic rate that affects how comfortable we feel. This is in addiiton to other factors cited in my signature line, such as mean radiant temperature of the surrounding room and objects, etc.

    ASHRAE's book for buildings in hot, humid climates states that the best percentage level of occupant satisfaction with the designed indoor conditions is at best 80%, meaning if you have 100 occupants in a building, the best you can hope for pertaining to folks satisfied with thieir comfort is 80. For my wife and I in our own dwelling, it's 50/50! Either she's comfortable, or I am, but not always at the same time!
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  12. #12
    I don't have time right now to talk more about psychrometrics but to add to shophound's points regarding human comfort I paste below the opening terms in our mini split quote template:

    1. Purpose
    1.1. The air conditioning equipment proposed by the contractor has been selected for the purpose of comfort cooling which is to achieve 22C 2K during ambient temperatures of 28C when thereafter the indoor temperature is expected to rise in sympathy with rising ambient temperatures.
    1.2. The secondary purpose is to achieve humidity’s between 30% and 70%.
    1.3. Some examples of other human thermal comfort parameters known but not considered controllable by the contractor in the selection of mini-split and VRV systems are:
    1.3.1. Vertical air temperature differences between ankle and neck should be less than 3C.
    1.3.2. Radiant Temperature asymmetries should be less than 10C.
    1.3.3. Surface temperatures of floors should be between 19C and 29C.
    1.3.4. Air velocities should be less than 0.25m/s and turbulent free.
    1.4. The customer must note that while the contractor has confidence of methods and experience the thermal loads derived from a free site survey are estimates only and as such the contractor cannot guarantee the air conditioning systems selected will achieve precisely the temperatures targeted.
    1.5. In order for the contractor to guarantee achieving any specific temperature the customer must first commission the contractor for a price to carry out a full thermal properties and properties exposure audit of the building’s fabric and contents associated with those areas being air conditioned. Ordinarily the best source of such information would be the building architects.

  13. #13
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    Quote Originally Posted by teddy bear View Post
    You need 1,000 ft.per minute velocity on the air flow over a clean wick and distilled water to get an accurate WB. Also cooling a metal bowl slowly down with ice to the point of condensation indicates the dew point of the air. Regards TB
    I think you are fine once you are over 500 FPM, and given that the A coils are sized for 500 FPM, I think most main trunklines the air is moving plenty fast

    You are trying to measure the lowest temperature water will evaporate at. If the water is not distilled you run the risk of reading high.

    I would trust what wet toilet paper on a thermocopule bead gave me over what most of the less expensive electronic hygrometers would read, in particular for air right off of a cooling coil, at a high level of RH
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

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