Professional Member
oops! Was in response to the comment about the wet paper towel trick
Professional Member*
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.Originally Posted by andersonbe06
Keep in mind Airflow. You may have the correct input information but lack the proper airflow to utilize it.
Professional Member*
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?
Regular Guest
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.
Regular Guest
Oh its available at united refrigeration, and about $90 bucks.
Professional Member*
But they work great to get the grease right when cooking hot wings.
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Global Moderator
They do have their place in the right application.But they work great to get the grease right when cooking hot wings.
Professional Member
Got a Fieldpiece pocket digital psychrometer from my supplier today for $60!
Professional Member
I got mine for 52$.
Regular Guest
I seen that one too but counter Guy was a little skeptical on its performance.
Professional Member
The original posters question has distorted throughout the thread is a simple yes or no question as superheat is the temperature above the refrigerant saturation and subcooling is a temperature below refrigerants saturation at specific temperatures and pressures. So the answer is no you do not need a sling psychrometer to measure superheating or subcooling! As other posters have mentioned A gauge, temperature probe, and the pt chart is what is required to measure the units Superheat and subcooling. There are definitely some handy tips being offered and useful information,,
I know the guy who know's the "Chiller Whisperer"
Professional Member
Joe, the OP did not say measure, he said calculate. I (and others I imagine) think he is asking if the sling psychrometer (or other means of measuring WB) is needed to calculate the target SH or SC, not just to measure it. So, to measure, not required. To calculate the target, required. Agreed?
Professional Member
Maybe you got me on a technicality, maybe I've been out of the residential arena for too long! I guess I don't understand how it can be calculated as it is a real-time measurement of the here and now I don't know of an application where you project what the superheat will be, Anyway my bad if I misunderstood it, I just asked simple questions in the past only not to get answered But was offered a whole slew of other opinions not related to the question Cheers
I know the guy who know's the "Chiller Whisperer"
Professional Member*
Trane prefers the subcooling method for TXV's.
However if you use their supplied chart on some systems with TXV's- you must know the wet bulb.
I don't use the wet bulb chart much, so I'm not sure when they quit using them.
Professional Member*
It the calculation isn't of what the superheat will be, but of what it should be, which varies with differences in the wet bulb temperature of the air entering the evaporator coil, and the dry bulb temperature of the air entering the condenser coil.
Measuring the wet bulb temperature of the air entering the evaporator coil requires the use of a wet bulb thermometer, or a psychrometer.
If you don't take the measurements, and do the calculation to determine what the superheat should be, what good is taking the measurements to know what it is?
The chart you are speaking of is not for charging, it is for determining if the system is operating within its design parameters.
On TXV systems, the wet bulb temperature is not needed for either of the 2 charging procedures AS/Trane have used.
Regular Guest
Only one person has mentioned airflow so far though. There is no point in taking any of these measurements and using that formula (which doesn't have an air volume factor in it) if you can't determine the total amount of heat content in the air that you are conditioning. So how many cfm/ pounds of air is this formula based on? Would anyone even notice if they had a 15 degree SH and their designed SH was suppose to be 12 based on that formula? And would they add refrigerant to drop SH to design conditions rather than finding that the blower is moving a higher volume of air or a fresh air duct is present or ducts are not fully sealed in the attic? We are in a greatly technical field with constantly changing conditions, the only thing we tend to do is hang on to rule-of-thumbs and homemade formulas. In the end though that truly gets us close enough.
My Jerry Springer final thoughts speech
Professional Member*
That is why, whenever possible, I measure the return WB and DB temperature of the air as it actually enters the coil, and while I'm there, measure the supply air DB and do the TEET calculation.Only one person has mentioned airflow so far though. There is no point in taking any of these measurements and using that formula (which doesn't have an air volume factor in it) if you can't determine the total amount of heat content in the air that you are conditioning. So how many cfm/ pounds of air is this formula based on? Would anyone even notice if they had a 15 degree SH and their designed SH was suppose to be 12 based on that formula? And would they add refrigerant to drop SH to design conditions rather than finding that the blower is moving a higher volume of air or a fresh air duct is present or ducts are not fully sealed in the attic? We are in a greatly technical field with constantly changing conditions, the only thing we tend to do is hang on to rule-of-thumbs and homemade formulas. In the end though that truly gets us close enough.
My Jerry Springer final thoughts speech
Professional Member*
To measure superheat/subcooling you do not need a psycrometer. All you need is the existing pressures vs. the PT chart settings based on your low side temperature and high side temperature (heat pump sensitive) respectively. To determine if you have the correct superheat you will need a psycrometer as a minimum to determine indoor WB as well as the R/A temperature entering the coil (keep in mind you may have a certain level of heat gain if you take your reading from inside the conditioned space. The closer to the evaporator the better) and your OAT. There are other methods and formulas that can be used as listed in this thread, but this would be the bare bones approach to measuring superheat/subcooling and calculating the required superheat/subcooling. I hope this helps
Professional Member
the OP Originally asked a very simple question , Everybody is offering up opinions and charging procedures for equipment that he never asked for, All are valid and differ depending on the equipment On. Don't slag me for simply trying to answer his question and not offer opinions above and beyond
I know the guy who know's the "Chiller Whisperer"
Professional Member*
Would you have been happier if we had just simply said "yes," and left it at that, with no explanation as to why?
The OP's question wasn't particularly clear, as he asked if a psychrometer was required to accurately "calculate" superheat.
It wasn't a question that has a simple yes or no answer.
Technically the simplest accurate answer would have been "It depends."
Just the fact that the question was asked, and the unclear way it was asked, indicate that the OP is seeking a greater understanding of the subject.
To simply give a yes or no answer would have been a disservice to the OP.
You got on us a little for not just giving a simple answer...Don't slag me for simply trying to answer his question and not offer opinions above and beyond
Last edited by mark beiser; 05-21-2013 at 08:36 PM.
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