I'm curious (PAH)

Dave Yates (PAH)

are familiar with the use of psychrometers?

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Unknown

Very little AC now but,

I do use a "sling psycrometer" You wet one thermometer- (the other is dry) and then sling it around for a minute or so then it slides back togeather and gives you the relative humidity on the outside of the device. Relatively cheap too;-) It's made by Bacharach. You use a psycometric chart to plot where the "comfort zone" is. RH seem to be less important with radiant heat.

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don_49

Inserach

of enthalpy.I use the amprobe thwd-2 digital sling psychrometer.Along with the chart its a great service tool.

With some math here in there you can plot the capacity of the coil.

David_5

wet bulb

I spent about 1 month on phsychrometric chart in school. It helps to understand what is going on when sensible and latent heat are removed from air.

David

jim lockard

I like to

Play around with the heat pump and the cooling condenser.
when confronted with a humidty problem, I have been known to slide a thermoerter into a wet shoe lace a check the relitive humidty.

Pete Groenewold

Psychrometers

I designed swimming pool and industrial dehumidifiers for over a decade, so knowing how to use a psych chart and related instruments was a pretty big deal.

I've used the digitals and like their speed and convenience, but when I'm commissioning or troubleshooting A/C systems I find myself falling back on my Bacharach sling psychrometer. Maybe my concern is baseless, but in my experience even expensive humidity sensors had to be recalibrated annually. So it's hard for me to accept that the $100 pocket-sized electronic "RH pens" could remain accurate over the long term.

BTW, if any of you have experience that contradicts this, I'd love to hear more.

Regards,

PG

jerry scharf_2

accuracy

Pete,

How accurate do you need the readings to be?

I would trust the cheap RH units to about +/- 4% over 5 years, based on current automotive RH sensor technology. The sensor demands of the automotive industry is just amazing, and that people figure out how to build things for $5 that meet them is even more so.

I had a great talk with a person at AHR who built these. His RH unit could be dropped in 110C oil, then -10C brine alternating many times, then bench tested within specs! Yikes.

jerry

Jeff Lawrence_24

I do

I use both a digital sling and a 'manual' one to properly charge air conditioners here in the south.



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David_5

I have both also

A short attention span and swinging mercury thermometers don't go together well. I use the digital most of the time.

David

Pete Groenewold_2

Humidity-Sensing Epiphany

Jerry, I appreciate you sharing your AHR experience. I had no idea that the new RH-sensing elements were so durable. As it turned out, the glass thermometer in my sling psychrometer shattered.

In part because of your comments, I picked up a new digital unit so I guess I'm ready to join the 21st century! Thanks again for the recommendation.

Regards,

Pete

Robert O'Connor_7

I think..

I've had a few used on me. Thats how I fooled them all and got an early release!!!..Robert O'Connor/NJ

Dave Yates (PAH)

A blast from the past!

Thought this had long since fallen off The Wall. I asked this question to determine how many wet heads use or understand the chart. My reason for asking was the prep I was doing for my ISH presentations. Debunking the hype surrounding "free A/C" requires utilizing this chart. Separating fact from fiction.

For those who attended ISH-NA and my seminar on Open vs. Closed systems on Saturday morning, you saw first-hand that the chart clearly demonstrated dew points will be met and exceeded if cold water is sent coursing through staple-up tubing.

This, of course, sets up the perfect mold breeding scenario whereby moisture (condensation) levels within the insulated (on one side only) joist bay will exceed 50% RH with many food sources for mold & bacteria.

Free A/C? Stay tuned(G).

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Glenn Harrison_2

One of my co-workers...

Sent in his Amprobe THWD-2 earlier this summer for calibration, and it was less than 1% off after 3 years (I think) of ownership.

Pete Groenewold_2

Blast from the Past

Dave, I resurrected this thread because a previous poster had downplayed my concerns about sensor drift as electronic psychrometers begin to age. In part because of his recommendations, I recently replaced my broken sling with a digital.

As far as using a psych chart, I would be truly surprised if most HVAC/R contractors used (or even knew how and why to use) them. I say that not out of intellectual snobbery, but based on prior experience. In a previous career designing and manufacturing dehumidication equipment, many of the contractors I dealt with, and even some of the firm's reps and sales personnel, were not fully comfortable using psych charts.

In fairness to them, even as a newly-minted BSME, I wasn't comfortable with the things! Psych charts were just a couple pages of scrawl in my Thermo textbook. Unless you had a compelling reason to use them, why bother?

Dave, I'm not familiar with "free A/C," but it sounds like it involves circulating cool water through existing radiant heating loops during summer operation. I'm not sure where the cool water comes from, but I do know that I'd be concerned about condensation in any residential or light commercial application simply because you can't guarantee that the space will stay at a constant temperature and humidity (with occupants opening windows, fiddling with thermostats, etc.).

Finally, if any of you are looking for a good brochure that explains relative humidity and the psych chart in layman's terms, check out http://www.desert-aire.com/pdf/TB/Tb3.pdf .

Thanks again for all your posts. This site has been a great source of information, and constantly shows me how much I still have to learn.

Dave Yates (PAH)

exactly!

I can't explain why, but I always liked the psychrometric chart. Maybe because it lifted a fog from in front of my eyes?

You're exactly correct in your surmise & that's done with no regard for the relationship between RH and temps. Radiant cooling can work very well, but dew points and RH must be carefully controlled to avoid mold & bacteria growth.

At ISH-NA, I worked in a whole house RH scenario in conjunction with the P-chart and ended up in the basement. To be more than fair, I kept the dew point at 62F in each of the varying RH/temp conditions when, in fact, dew points in basements average somewhat higher. Toss in some cooler than the dew piont circulation in a joist bay where just the lower side is insulated & BAM.

Then there's the really limited Btu net gain, which Tim Doran of Wirsbo helped me work out so we could see just how limited "free A/C" really is. Once again, to be more than fair, we did not take anything away for flooring - sub or finished materials. The numbers were worse than even I thought they'd be and I was already a skeptic. The delta-T was only 3 F, so no wonder it can't work well.

Then we examined the costs associated with "free A/C" and detailed what they were too.

Evicerated is the term that comes to mind.

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