ReplyI'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.
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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
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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.
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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.Originally Posted by andersonbe06
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.
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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
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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.
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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...
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?
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I use this formula. (Thanks HVACMedic)
(3 x Entering Wet Bulb - 80 - Outdoor Temp) / 2 = Target Superheat.
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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.
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Anybody tried wrapping a little bit of a wet paper towel around a temperature probe? It was within 1 degree of actual WB.
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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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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
Originally Posted by adam_s05
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
adam_s05 is correct "It not required". Whether he deserves it or not, nobody's gonna sue him for malpractice.
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Yeap, they assume 50% RH, and that the space is at, or very near, the normal temperature it is kept.
Rheem/Ruud, and a few others have charts based on just the outdoor DB and suction pressure, or outdoor and indoor DB.
Those charts are merely a dumbed down compromise by manufacturers that understand the sad reality of how most "technicians" operate, and came up with a simple charging procedure that will usually not result in to many dead compressors if followed.
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 pretty much ignore those kinds of charts on split systems, but will go by whatever off the wall weirdness for a charge checking procedure I find on a sticker on a package unit.
My personal favorite screwy charging procedure is Lennox's subcooling method for fixed orifice systems.
I'm still:[IMG]http://1.bp.********.com/_wAxDMfEGhoY/TQrlbN4gYXI/AAAAAAAAAXg/kmwztAAni44/s400/Not+Sure+if+serious.jpg[/IMG]
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The package unit comment took me back a few hours today when tge boss told me to figure how much freon is in the equipment at one of our accounts.... Would be nice if they were packs, but since tgey are all 5-30 ton splits with varying line sizes... I may be figuring for a while! Ill go gst rough lineset distances tomorrow and calculate what ammount is supposed to be there...Yeap, they assume 50% RH, and that the space is at, or very near, the normal temperature it is kept.
Rheem/Ruud, and a few others have charts based on just the outdoor DB and suction pressure, or outdoor and indoor DB.
Those charts are merely a dumbed down compromise by manufacturers that understand the sad reality of how most "technicians" operate, and came up with a simple charging procedure that will usually not result in to many dead compressors if followed.
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 pretty much ignore those kinds of charts on split systems, but will go by whatever off the wall weirdness for a charge checking procedure I find on a sticker on a package unit.
My personal favorite screwy charging procedure is Lennox's subcooling method for fixed orifice systems.
I'm still:[IMG]http://1.bp.********.com/_wAxDMfEGhoY/TQrlbN4gYXI/AAAAAAAAAXg/kmwztAAni44/s400/Not+Sure+if+serious.jpg[/IMG]
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Of course two weeks of recovery and recharging wouldnt be too awful, lots of h-talk time!
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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
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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.
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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.
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Now that's what I'm talkin' about...
Thank you.
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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!
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That is exactly what I do. I figure the water evaporates the same way.
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