How to measure wet bulb temp?


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Old 08-22-08, 10:18 AM
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How to measure wet bulb temp?

I have a newly installed AC system, and I have been trying to measure its performance and compare to specs, more out of curiosity than out concern, as it is cooling very well.

The instructions for performance measurement provide charts for superheat, subcool, CFM, pressures, indoor dry bulb temp and "entering wet bulb temperature".

I know how to measure all of these parameters except for the "entering wet bulb teperature". I assume this is the temp of the air entering the coil? How do I measure its "wet bulb" temp - must I have a RH meter, or can I wrap a wet gauze or something around a probe thermometer and put it into the airstream?

Thanks in advance.
 
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Old 08-22-08, 12:44 PM
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You could use relative humidity and dry bulb temp to calc wet bulb.

Hopefully you have an understanding of dry bulb vs wet bulb. Pretty much 100% of residential hvac works around dry bulb, so I wouldn't get overly concerned with wet bulb figures.

Really so long as your dry bulb values are where they should be, your wet bulb values CAN'T be that far off.
 
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Old 08-22-08, 01:09 PM
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Thanks! I realize that I can convert dry bulb to wet bulb if I had a RH meter, but I don't have one. From what I found online so far, it looks like wet bulb can be measured by wrapping a thermometer bulb in some wet cotton and sticking it into the return airstream. I realize that sometimes dry bulb and wet bulb are not far apart, but performance parameters can change significantly with just a few degrees, and in dry environments like we have here in CA, the difference can be >10 deg. So I'd like to be able to measure the actual wet bulb temp somehow.
 
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Old 08-22-08, 02:27 PM
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I think you want to know the RH inside your house, not what's doing outside. Therefore, converting OD RH to WB (inside) is questionable. Also, humidity inside can vary (damp, humid basement).

What I have done: I went to my local dollar store and looked through there thermometers (of the same make) to find 2 that were reading the same.

Even if they are off by a couple of degrees, as long as they read the same, you will be able to determine the RH (and therefore, be able to evaluate your unit's performance). You need to find a thermometer where the bulb is exposed enough to slip the wick onto it.

To make your wet bulb thermometer, try to locate a small class bottle (vial, such as Rx vial will do). You then need to make a wick. You will have to experiment with this.

What I did, was to take a length of white shoelace. Some actually have a hollow core and will slip onto the bulb. Otherwise, you can carefully sew 2 lenghts together.

I use a rubber band to 'mount' the vial onto the body of the thermometer.

On your computer, Google "US Department of Commerce Relative Humidity and Dew Point Table".

When I want to take a reading, I hang the WB thermometer on a room fan for about a minute. This is the same as swinging a sling psychrometer. Try to avoid holding the thermometer so that your body heat will affect the reading.

Good luck.

Delta

P.S., my next project: making an inclined u-tube manometer to measure static pressure of my system.

P.S. II, If you're into measuring superheat and sub-cooling, I'm surprised your not familiar with WB thermometers.
 
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Old 08-22-08, 03:09 PM
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Thanks for the suggestions, they make sense.

I agree that what I need to measure is the wet bulb temp of the air entering the evaporator (indoor) coil, not the outside air.

I was planning on making a wet bulb thermometer as you desrcibed (sticking it into a saturated/wet shoe lace or similar wrapper).

Since I am measuring the wet bulb temp in the airstream though, I don't think I need to use any additional fans etc, just place it in the flow of the blower - right?

As far as not being familiar with wet bulb thermometers, I am just starting to read up on the topic and figured I'd post a question here first .
 
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Old 08-23-08, 06:23 AM
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Originally Posted by lyonkster
Thanks for the suggestions, they make sense.

Since I am measuring the wet bulb temp in the airstream though, I don't think I need to use any additional fans etc, just place it in the flow of the blower - right?

As far as not being familiar with wet bulb thermometers, I am just starting to read up on the topic and figured I'd post a question here first .
Yes, that is correct if that's all you want. Sometimes, it's nice to know what's going on in various locations within your house.

When I first started tweaking my system, I found that I had a rather large return leak (in my very humid basement). So the system was pulling wet air in, but because my system has very ample latent capacity, it was able to pull the moisture out of the air. I found that there was no direct correlation between basement and household RH, but after the system was on for a couple of days (no open windows), I was able to reduce RH below 40% on one occasion.

As (you state) that you're just starting out, it will be interesting to know how you plan to determine superheat or subcooling, as it requires refrigerant pressures to be measured (use of guages and licensing to do same). Plus, the thermometers used tend to be somewhat expensive.

Delta
 
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Old 08-23-08, 11:33 AM
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Originally Posted by delta l
As (you state) that you're just starting out, it will be interesting to know how you plan to determine superheat or subcooling, as it requires refrigerant pressures to be measured (use of guages and licensing to do same). Plus, the thermometers used tend to be somewhat expensive.
Delta
Sure thing, good question.

I did the install of the system, including all the Manual J and D sizing calcs. It's been done with a permit, with the inspector saying it's the best install he's seen in a long time.

I hired a guy to come in and charge it, which was a mistake. The guy charged it without measuring SH or SC, or how much R410A he put in there. He went by pressures only, which does not provide the full info.

As a result, I am now doing what I did not want to do, i.e. checking up on the "professional's" work. So I have a manifold/gauge set to check pressures, I have a multimeter with K thermocouple to check line temps, I know the cfm of the blower, I have checked delta T across coil, and ESP across blower. The numbers are actually not too bad (about 9F SC, 13F SH, 16F across coil, 0.8 in across blower). But for completeness, I'd like to verify the complete performance numbers as stated in instructions, as Goodman recommends, which is where the wet bulb temp comes in. Does this help?

I've found out after the fact that no license was required for handling R410A, and I could have done all the charging myself, without the "professional". That way it would have been done right, using SC and SH measurements, not "winging it" like he did. Oh well, next time.

I also agree that it may be good to make some more measurements to observe RH etc; I'll probably do that after I make sure the system is dialed in properly .
 
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Old 08-24-08, 04:45 PM
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Agree with bad charge. Install Lit spells out how final charging should be done.

Rather than a long post, here is a simple way to measure wet buld. Wife have any cotton balls?

http://www.engineeringtoolbox.com/hu...ent-d_561.html

Save that link and look around, lot of good info there.
 
 

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