Pressures are Meaningless

[Gary]

Our industry seems to have a pressure addiction. It's kinda like being "huked on fonix". It works... sorta... sometimes... maybe.

But what do pressures (or temperatures for that matter) tell us? Nothing.

Every bit of useful information about any system comes from comparing temperatures. The temperatures/pressures by themselves are meaningless.

For example, consider head pressure. On a hot day we would expect the head pressure to be higher. On a cold day we would expect the head pressure to be lower. Right? Whether we realize it or not, we are comparing two temperatures. Without also knowing the outdoor temperature, head pressure doesn't tell us anything.

Similarly, in order to judge the suction pressure, we also need to know about the heat load. A freezer full of warm products (heavy heat load) is going to raise the suction pressure. Light load (fan out, filter dirty, coil dirty or frozen, etc.) is going to bring down the suction pressure. Whether we realize it or not again we are making a comparison. Suction pressure by itself tells us nothing.

The examples go on and on... delta-T, TD, subcooling, superheat, approach...

The point is: Every bit of useful information comes from comparing one temperature to another temperature.

[dsp]

Gary,
Good point. It shows why when i was learning refrigeration my tradesmen, above me always felt by his hand tempuratures. (suction, discharge, head etc.) before putting on a gauge. Temp. does tell you a lot about how a system reacts under diffferent circumstances.
dsp

[Gary]

Personally, I prefer real temperature measurements.

Actually, human skin is a poor judge of temperature although it can sense rate of heat transfer. Wrapping your hand around a suction line, you might sense a surge of liquid, because the heat transfers from your hand to the pipe much more rapidly.

And you might identify extreme problems by the fact that a line is much too cold or much too warm. But then you really don't know what is causing the problem, just that there is a problem... and perhaps the general area of the problem. It won't identify borderline problems and you certainly can't fine tune a system that way.

In order to trouble shoot you need to compare various temperatures. And for that you need something a little more accurate than your calibrated fingertips. I use a clamp on probe that is every bit as fast as my hand and a lot more accurate.

[Gary]

Gary,
Good point. It shows why when i was learning refrigeration my tradesmen, above me always felt by his hand tempuratures. (suction, discharge, head etc.) before putting on a gauge. Temp. does tell you a lot about how a system reacts under diffferent circumstances.
dsp

Using your calibrated fingertips is not a valid shortcut, and temperatures by themselves are as meaningless as pressures. If you wrap your fingers around a suction line and it is warm, there is a long list of possible problems. The suction line being warm doesn't tell you what you need to know. The same thing if the suction line is too cold. It just says you need to look further. The temperature by itself doesn't tell you what you need to know. Everything useful (delta-T, TD, subcooling, superheat and approach) is a comparison of two temperatures.

You want a real shortcut... here it is:

Every day, on every job, no matter what the nature of the service call is, always start with a quick visual inspection of the system. Here is what you are looking for:

1. Proper airflow through the evaporator (fan running properly and airflow clear).

2. Proper airflow through the condenser (fan running properly and airflow clear).

3. Compressor running.

This just takes a few minutes and once you have eliminated airflow problems, your trouble shooting job gets much, much easier.

If it isn't a control problem and it isn't an airflow problem, then it has to be a refrigerant flow problem.

[Refrigerologist]

The greatest tool in our industry is adequate training, both technical and practical. Then comes experience you can't teach it and you can't buy it. You just have to learn it the hard way. How many faults have you guys' found that had never been shown to you. I bet there are a good few. I know I have had some very strange ones. Lets say you have a tandem compressor set up that you have never seen before and one of the compressors is tripping on overheat. I can guarantee a lot of inexperienced guys would condemn the one that is tripping out. But it could just as easily be the second compressor valves that are shot and it may be that hot gas is being pushed through the defective compressor straight back into the common suction line. Thess type of faults are fairly rare and so can't always be shown to the apprentice. It might be years before he comes across this type of fault. But if he has been taught to fault find correctly then he/she should have an enquiring mind and look for the cause before jumping to a conclusion. Plus we all make mistakes some of us admit them others bull****!

[Refrigerologist]

Gary,
Good point. It shows why when i was learning refrigeration my tradesmen, above me always felt by his hand tempuratures. (suction, discharge, head etc.) before putting on a gauge. Temp. does tell you a lot about how a system reacts under diffferent circumstances.
dsp

I totally agree with you mate! Your hands are in fact very sensitive at finding subtle differences in temperature. So easy to find a partially restricted drier with your hands! Fairly easy to determine that the compressor windings are too hot. Obviously they are not accurate enough to take temperature readings for pressure /temp relationships, but good enough to provide a guide of where to look for the fault. I also just attended the update of our brazing and safe handling of refrigerant certificates and the guy who did the training there also believed the hands are a useful tool. In fact the KOTZA training CDs also recommend feeling your way around the system as a first indicator of problems. A warm suction line feels warm. A hot liquid line feels hot! Your hands do provide an initial indicator of where to look deeper!

[Gary]

Human skin is incapable of judging less than 10F temp difference... however hands are a pretty good judge of rate of heat transfer. You can feel liquid surging through a suction line because the heat transfers from your hand to the pipe much faster than if the pipe contains only vapor.

[Refrigerologist]

Human skin is incapable of judging less than 10F temp difference... however hands are a pretty good judge of rate of heat transfer. You can feel liquid surging through a suction line because the heat transfers from your hand to the pipe much faster.

That may be true Gary, but you can still feel if a liquid line is too hot, I am not arguing that if it is only marginally high you will detect it but you will feel the difference between a unit with a clean condenser and one that is fouled.

[Refrigerologist]

Forgot to add: Wise man once say: "many ways to skin cat"

[Refrigerologist]

Our industry seems to have a pressure addiction. It's kinda like being "huked on fonix". It works... sorta... sometimes... maybe.

But what do pressures (or temperatures for that matter) tell us? Nothing.

Every bit of useful information about any system comes from comparing temperatures. The temperatures/pressures by themselves are meaningless.

For example, consider head pressure. On a hot day we would expect the head pressure to be higher. On a cold day we would expect the head pressure to be lower. Right? Whether we realize it or not, we are comparing two temperatures. Without also knowing the outdoor temperature, head pressure doesn't tell us anything.

Similarly, in order to judge the suction pressure, we also need to know about the heat load. A freezer full of warm products (heavy heat load) is going to raise the suction pressure. Light load (fan out, filter dirty, coil dirty or frozen, etc.) is going to bring down the suction pressure. Whether we realize it or not again we are making a comparison. Suction pressure by itself tells us nothing.

The examples go on and on... delta-T, TD, subcooling, superheat, approach...

The point is: Every bit of useful information comes from comparing one temperature to another temperature.

Actually what you are saying is not quite right, as far as refrigerant goes, it is always the pressure/ temperature relationship. Temperature is no good on its' own for it doesn't tell us anything without knowing the pressure. There are too many variables.
The only temperature that is valid on its' own is the room temperature as that is what we are trying to lower. And that is what is important to the customer. We are always looking for subcooling or superheat values so we must know the pressure and then compare against the temperature and vice versa. One is useless with out the other.

Also you state that a warm load in a freezer will raise the suction pressure. That is not strctly true! It will raise the air temperature, which in turn will raise the suction pressure. Excepting of course for high latent loads and some minor amount of radiated heat. So generally speaking if we know the room temperature we can usually guesstimate the expected suction pressure. Surely the warm food will just mean the pull down rate will be longer.

[Gary]

Let's imagine that I am working on an R22 system. I put my gauges on the system and find that the pressure numbers have magically disappeared from the face of my gauges. In fact, the only numbers I can see on my gauges are the R22 scale. It's okay with me, because these are the only numbers I am interested in. I don't care what the pressures are.

If you are working on a system and you tell me what the SST and SCT are, then I not only don't care what the pressures are, but I don't care what refrigerant is in the system, either. Those temperatures are what I need to know... and they don't tell me anything useful until I combine them with other temperatures.

[Gary]

So generally speaking if we know the room temperature we can usually guesstimate the expected suction pressure.

Why would I care or even bother to guess at what the expected suction pressure is? If the dT's, TD's, subcooling and superheat are right, then the system is right... and the pressures are whatever they are... and I don't care what they are.

[Kh1971]

Gary , thanks for the info you give

[paul_h]

Whats the first thing you need to know when someone gives you pressure readings?

You need to know what refrigerant so you can work out the SST and SCT.
Without knowing the refrigerant, your gauges are useless.
Thats all you carry your gauges around for, not to read the pressures but to work out the SST and SCT.

[Gary]

Whats the first thing you need to know when someone gives you pressure readings?

You need to know what refrigerant so you can work out the SST and SCT.
Without knowing the refrigerant, your gauges are useless.
Thats all you carry your gauges around for, not to read the pressures but to work out the SST and SCT.

Exactly so... and even the SST and SCT are themselves a means to an end... mere stepping stones... they are just two of the temperatures we need in order to work out the various combinations which tell us what is wrong with the system.

Pressures and temperatures are the uninformative crap we go through in order to get to the really useful stuff (dT's TD's, subcooling, superheat and approaches). These tell us everything we need to know.

[Refrigerologist]

Let's imagine that I am working on an R22 system. I put my gauges on the system and find that the pressure numbers have magically disappeared from the face of my gauges. In fact, the only numbers I can see on my gauges are the R22 scale. It's okay with me, because these are the only numbers I am interested in. I don't care what the pressures are.

Please could you provide an explanation of how you would work out the actual sub-cooling value for an R22 system from the following parameters:

Discharge temperature: 68 degrees C
Liquid Line Temperature: 35 degrees C
Evpaorator Temperature: -4 degrees C
TD Across evaporator: 6 K
Room temperature: 4 degrees C
Ambient temperature: 27 degrees C
TD Across condenser: 10K
Temp at TXV Bulb: 2 degrees C

[Refrigerologist]

Whats the first thing you need to know when someone gives you pressure readings?

You need to know what refrigerant so you can work out the SST and SCT.
Without knowing the refrigerant, your gauges are useless.
Thats all you carry your gauges around for, not to read the pressures but to work out the SST and SCT.

Please could you provide an explanation of how you would work out the actual sub-cooling value for an R22 system from the following parameters:

Discharge temperature: 68 degrees C
Liquid Line Temperature: 35 degrees C
Evpaorator Temperature: -4 degrees C
TD Across evaporator: 6 K
Room temperature: 4 degrees C
Ambient temperature: 27 degrees C
TD Across condenser: 10K
Temp at TXV Bulb: 2 degrees C

[Gary]

To give just one small example:

We convert high side pressure to SCT. We measure liquid line temp at the receiver outlet. We subtract LL temp from SCT, which tells us the subcooling.

Does the pressure tell us anything important? No.

Does the SCT tell us anything important? No.

Does the LL temp tell us anything important? No.

Does the subcooling tell us anything important? Yes.

Assuming the condenser has good airflow, if the subcooling is excessive the system is either overcharged or has air in it... and that's the kind of thing we need to know.

[Refrigerologist]

Why would I care or even bother to guess at what the expected suction pressure is? If the dT's, TD's, subcooling and superheat are right, then the system is right... and the pressures are whatever they are... and I don't care what they are.

Gary, I want to know the suction pressure, or preferably the pressure at the evaporator so that I can accurately work out the amount of superheat in the evaporator. But then I guess Sporlan and a few other manufacturer's and my college lecturer's plus the engineers' I was trained by, were all wrong. They all say put your suction gauge on. Preferably as close to the evaporator outlet as possible. If not allow for the presssure drop. Fix a calibrated thermometer, or temperature probe, as close to the TXV bulb as possible. Using a comparator, (pressure/tempurature relationship table), read off the expected liquid saturated temperature and compare against the temperature reading. The bulb temperature should be 5 to 7K higher than that for the saturated liquid. All I can say is I have tried reading the inlet and outlet temperatures, but the inlet temperature tends to fluctuate to much to obtain a useful reading.

[Gary]

Gary, I want to know the suction pressure, or preferably the pressure at the evaporator so that I can accurately work out the amount of superheat in the evaporator.

And that's exactly what I am saying. The pressure is not important, it is the superheat that is important. The pressure is just how you get to the good stuff.

[Refrigerologist]

To give just one small example:

We convert high side pressure to SCT. We measure liquid line temp at the receiver outlet. We subtract LL temp from SCT, which tells us the subcooling.

Does the pressure tell us anything important? No.

Does the SCT tell us anything important? No.

Does the LL temp tell us anything important? No.

Does the subcooling tell us anything important? Yes.

Assuming the condenser has good airflow, if the subcooling is excessive the system is either overcharged or has air in it... and that's the kind of thing we need to know.

Gary! You just made the point! You needed the pressure to be able to obtain the expected temperature for the refrigerant. Without the pressure the temperature would remain useless! You had to convert the pressure to a temperature! So as I said before one is useless without the other.

[Refrigerologist]

And that's exactly what I am saying.

But Gary, You are telling people pressures are meaniningless, and of course they are, when used in isolation. Just as temperatures are, when used in isolation. You cannot fault find a refrigeration system without resource to both sets of parameters.

[Refrigerologist]

I believe that the words: REFRIGERANT PRESSURE TEMPERATURE RELATIONSHIP say everything I wanted to say on this matter!

[taz24]

I'l tell you when pressures are not meaningless.

When your trying to take the oil plug out of the cranckcase of the compressor and you have forgotton that there is 4b (60psi) of preasure in the comp.

taz.

[paul_h]

I believe that the words: REFRIGERANT PRESSURE TEMPERATURE RELATIONSHIP say everything I wanted to say on this matter!

Yes, that's why you use gauges to get the SST and SCT.
But you don't diagnose a system with the pressures. eg, so many people will say if a r22 split is running at 400kpa SP and 1700kpa HP, then nothing is wrong with it and no further temperatures or diagnostics are required. All they care about is the pressure, they have expectations that certain refrigerants in certain applications will always be a certain pressure.
The same people may **** bricks when they visit the same system on a 40C day and find out the sp is 600kpa, they may think something is wrong with the system because the suction pressure is high.

[Refrigerologist]

I think it would have been better to have started the tread as: Pressures are only part of the story! or something in that vein.

[Refrigerologist]

I'l tell you when pressures are not meaningless.

When your trying to take the oil plug out of the cranckcase of the compressor and you have forgotton that there is 4b (60psi) of preasure in the comp.

taz.

Nasty! Dirty Oil!

[Refrigerologist]

Yes, that's why you use gauges to get the SST and SCT.
But you don't diagnose a system with the pressures. eg, so many people will say if a r22 split is running at 400kpa SP and 1700kpa HP, then nothing is wrong with it and no further temperatures or diagnostics are required. All they care about is the pressure, they have expectations that certain refrigerants in certain applications will always be a certain pressure.
The same people may **** bricks when they visit the same system on a 40C day and find out the sp is 600kpa, they may think something is wrong with the system because the suction pressure is high.

And that is the point I have been trying to make you must use both sets of parameters. pressure & temperature. That is why we carry comparators. Isn't it? Although I do differ slightly in my interpretation. Pressures must be used in diagnosing problems and/or efficiency, BUT only in condjuction with temperatures in or around the system. The same is true of temperatures on their own, they do not give the full picture! So pressures are not meaningless. Due to the nature of the thermodynamics, we must know the operating pressure of the refrigerant, then obtain its' expected temperature for a given pressure, by using a comparator. From that and the actual temperature taken by using a thermometer, we can ascertain if the refrigeratnt is saturated, superheated or sub-cooled, and by how much.

[paul_h]

So many people regard the pressure as all important and don't even look at charts is my point. You said it yourself, because of the pressure/temperature relationship, the pressure tells us something if we look up the comparators. Without the p/t chart, the pressure wouldn't be telling us anything. For example, like I said before, how useful would pressure be if we didn't even know what refrigerant in the system? Pressure gauges are just a means to an end, to extract data from, not to speak for themselves.

You don't use pressure as a value, you use it to get the SCT and SST as a value. With air on/off temps it means something.

edit: VVVV
nice one, but we are talking about diagnostics here, not opening up a system.
Try this:
I've got a split system here that has 400kpa SP and 1700kpa hp, but it's not cooling. The compressor is good, the r/v is good, indoor air on is 23C, air off is 20C, whats wrong with it?
That's about all the info you get from the average tech that only cares about pressure and basic operation.

[Refrigerologist]

Nasty! Dirty Oil!

I'll give you another instance when pressures are important. When you find the 4th year apprentice removing the liquid line drier, form a York chiller with oxy-acetylene and he hasn't removed the refrigerant from the system. Put a gauge on and frightened him just a little bit.

[Springbok]

Its always a battle between refrigologist and gary,but 98% of the times,they are both correct,and usually make the same point,just seeing it at different angles.

Gary is just giving us his opinion and the way he sees it,which makes a lot of sense.Refrigologist also makes some good points.At the end of the day,every good service engineer who knows his job,whether looking at just the pressures or just take note of the temperatures,would still find the source of a problem...

[nike123]

Hi, Refrigerologist!

I give you example, where, without ever taking any pressure measurement, you could only by taking temperature measurement, find all necessary data to evaluate system performance and troubleshoot. Suppose that we have saturated condition in the middle of heat exchangers.
1. Take temperature measurement of gas/liquid mixture at the middle of condenser. That is your SCT.
2. Take temperature measurement of gas/liquid mixture in the middle of evaporator. That is your SST.
3. Take temperature measurement at inlet of expansion device and subtract from SCT and you got total subcooling.
4. Take temperature measurement at outlet of condenser and subtract from SCT and you got condenser subcooling.
5. Take temperature measurement at outlet of evaporator and and subtract SST from that and you got evaporator superheat.
6. Take temperature measurement at inlet of compressor and subtract SST from that and you got total superheat.
7. Take temperature measurement at compressor outlet and you got discharge temperature.
8. Take air in/out temperature at evaporator/condenser and you have dT of evaporator/condenser.
9. Take temperature measurement of air entering evaporator and subtract SST and you got evaporator TD.
10. Take temperature measurement of air entering condenser and subtract that from SCT and you got condenser TD.

You see, we don't need pressure at all.
But, because of impracticality and difficulties to obtain accurate saturation temperature we use manometers to obtain pressure, and from that pressure we acquire corresponding TEMPERATURE for our needs.
That is like measuring electrical circuit. If we want to measure tension, our instrument actually measure current, and from that measurement, converts in corresponding tension which is of our interest.
That is same case here. Pressure is of no interest to us, it is corresponding temperature which we want to know.
That's why is Gary said "pressures are meaningless".
Indeed they are, until we convert them in SST and SCT.

And if we want to know if system is pressurized, then pressure have meaning, but I am sure that is not what is Gary addressing here.

[Refrigerologist]

Hi, Refrigerologist!

I give you example, where, without ever taking any pressure measurement, you could only by taking temperature measurement, find all necessary data to evaluate system performance and troubleshoot. Suppose that we have saturated condition in the middle of heat exchangers.
1. Take temperature measurement of gas/liquid mixture at the middle of condenser. That is your SCT.
2. Take temperature measurement of gas/liquid mixture in the middle of evaporator. That is your SST.
3. Take temperature measurement at inlet of expansion device and subtract from SCT and you got total subcooling.
4. Take temperature measurement at outlet of condenser and subtract from SCT and you got condenser subcooling.
5. Take temperature measurement at outlet of evaporator and and subtract SST from that and you got evaporator superheat.
6. Take temperature measurement at inlet of compressor and subtract SST from that and you got total superheat.
7. Take temperature measurement at compressor outlet and you got discharge temperature.
8. Take air in/out temperature at evaporator/condenser and you have dT of evaporator/condenser.
9. Take temperature measurement of air entering evaporator and subtract SST and you got evaporator TD.
10. Take temperature measurement of air entering condenser and subtract that from SCT and you got condenser TD.

You see, we don't need pressure at all.
But, because of impracticality and difficulties to obtain accurate saturation temperature we use manometers to obtain pressure, and from that pressure we acquire corresponding TEMPERATURE for our needs.
That is like measuring electrical circuit. If we want to measure tension, our instrument actually measure current, and from that measurement, converts in corresponding tension which is of our interest.
That is same case here. Pressure is of no interest to us, it is corresponding temperature which we want to know.
That's why is Gary said "pressures are meaningless".
Indeed they are, until we convert them in SST and SCT.

And if we want to know if system is pressurized, then pressure have meaning, but I am sure that is not what is Gary addressing here.

I don't want to be pedantic, but Without the actual pressure value you could be way off base, for a start you could not be sure if there were contaminants within the system, let alone the fact that another engineer may have charged the wrong gas into the system!! If you do not have a pressure reading I don't see how you actually know what the saturated temperature of the gas would be. OK if it were a bowl of water at sea level, thats easy, but in a closed system, with heat and compression being applied? Obviously only to the water vapour for as we know you cannot compress liquid water!!

[Refrigerologist]

Hi guys! I think that by now many will think I am argumentative, and you are right! I call it debate; it is a good thing. I always tell my kids question everything, don't just except an answer, if you think it may be wrong ask for an explanation. For the record I CAN see Gary's point, all pressure readings must be converted back to temperature, except for perhaps one or two that spring to mind, and they are compression ratio's and differential oil pressure! It was just the thread heading that I found slightly disturbing, as we need pressure to tell us what the gas is doing in relation to its' temperature at the given ambient etc, before we can make a sound judgement on superheat or sub-cooling etc. Gary, I beleive is making the point that a lot of engineers' use only pressure to fault find, when of course this does not really tell us what is going on.

[paul_h]

I'm the same
I just hate people who don't deal in anything but pressures and nothing else. By themselves except for very bad values, they aren't important without other information
Gary had a point in that pressures only relate to the SST and SCT and wouldn't care if the gauges only showed that rather than pressure.
And I know pressures aren't completely useless, if you had a system tripping on HP or LP, pressures are all you need to know for the time being. Likewise if you had a system where LP and HP were the same while the compressor is running, that's all you need to know to diagnose a faulty compressor, r/v, bypass valve, valve plate etc. So I can see where your coming from, but that wasn't the spirit of the thread I thought, but maybe you were just in going after the title of the thread.

But anyway, I can see his point enough to know he's not talking about these blantant hp/lp faults, rather the instances where you need only sct, sst and other temperatures to diagnose an otherwise normally running system that may be underperforming

[Gary]

Back when we had only three refrigerants to worry about, I used to tell people to black out the pressure scales on their gauges and directly read the temperatures, so they would learn to think in terms of temperatures.

Now that we have all of these new refrigerants it is necessary to use a P/T chart, but it is even more advantageous to think in terms of temperatures.

If you have ten identical systems using ten different refrigerants, all you need to know is the SST and the SCT. You can then troubleshoot everything on each system even if the refrigerant is something you have never worked with before.

If you try to memorize all of the pressures for all of the refrigerants, you are doing things the hard way. If you think in terms of temperatures and have the right P/T chart in your pocket, you can work with ANY refrigerant.

Refrigeration is about temperatures, not pressures.
__________________

[nike123]

I don't want to be pedantic, but Without the actual pressure value you could be way off base, for a start you could not be sure if there were contaminants within the system, let alone the fact that another engineer may have charged the wrong gas into the system!! If you do not have a pressure reading I don't see how you actually know what the saturated temperature of the gas would be. OK if it were a bowl of water at sea level, thats easy, but in a closed system, with heat and compression being applied? Obviously only to the water vapour for as we know you cannot compress liquid water!!

That pedantic approach could also be used for pressures, and we could than say, that one cannot get accurate saturation pressures, because it is usually, that one taking pressure reading where is some pressure drop from place where the actual saturated condition is.

But that is just hair splitting, and most of good RE uses both as tool for evaluation of system performance and troubleshooting.

[Refrigerologist]

Back when we had only three refrigerants to worry about, I used to tell people to black out the pressure scales on their gauges and directly read the temperatures, so they would learn to think in terms of temperatures.

Now that we have all of these new refrigerants it is necessary to use a P/T chart, but it is even more advantageous to think in terms of temperatures.

If you have ten identical systems using ten different refrigerants, all you need to know is the SST and the SCT. You can then troubleshoot everything on each system even if the refrigerant is something you have never worked with before.

If you try to memorize all of the pressures for all of the refrigerants, you are doing things the hard way. If you think in terms of temperatures and have the right P/T chart in your pocket, you can work with ANY refrigerant.

Refrigeration is about temperatures, not pressures.
__________________

Let's put this one to bed.

Yes I do agree pressures are meaninless on their own and it is the temperatures we really need to get at. As I stated in a previous post it was really only the headline that bothered me And no I don't bother trying to remember the P/T chart for each gas. The older I get and the more I learn, I find the more I forget also!! The brain is a sponge, for years it soaks up the information and then, somehow, someone seems to be wringing it out

[Refrigerologist]

Why would I care or even bother to guess at what the expected suction pressure is? If the dT's, TD's, subcooling and superheat are right, then the system is right... and the pressures are whatever they are... and I don't care what they are.

I can now come up with one scenario when knowing about pressures can come in handy!

You will have to forgive me not posting the operating data as this happened nearly thirty years ago

I was called to Tesco's in Barking. The veg fridge was freezing all the produce. 2 guys had already been to site adjusted the case mounted thermostat and left site saying it would be OK. I check the system and put my meter on the thermostat. There is no voltage at the stat contacts. I trek all the way to the plant room and find the condensing unit still operating. Back down to the sales floor. Still no voltage at the stat. Back to the plant room, still running. mmmm! What is going on? I check the control panel, the stat has been disconnected at the outgoing terminals. So maybe it has been changed to pressure switch control. Checked for welded contactor, it's OK. Out come the gauges, everthing appears about normal. Sub-cooling OK, not great, but OK. I didn't bother to check the expansion valve as the lettuce was now past shivering and was complaining it was frozen. But why is the compressor running when the stat has been disconnected? So I checked the low pressure switch setting, R12 cabinet, cut out set 2psi below evaporating pressure. But the compressor never quite gets there. I did a compressor efficiency test, and LO! it is not pumping much below 5". Raised the switch setting. Ordered new valve plate. Installed valve plate, lowered LP switch to previous setting and all OK.

So understanding pressures can be important, and I believe that LP control is making a comeback for some high temp applications. And Gary I already know that you will say that I still had to have an understanding of the pressure/temperature relationship. And you are probably right, but having a feel for the correct pressures for a given set of conditions really helps to point me in the right direction of diagnosis, before I get to use my temperature tester. From the pressures I generally know at which end of the system I should be checking!

[philfridge]

I have to disagree pressures are not meaningless they are the information we use to diagnose faults. You cannot pump test a compressor with a thermometer. Also I have to agree with the use of the hand you can normally tell whats up by the temperature of the pipes in various parts of the system. Who in the REAL world starts getting a thermometer out when your fingers can do the job .

[US Iceman]

Who in the REAL world starts getting a thermometer out when your fingers can do the job .

There is a lot of truth in this statement, but unfortunately it seems people never quite pick this up.

We have five sense we should learn to use.

[750 Valve]

its pretty darn hard to find out suction line pressure drop without guages...

I don't agree that guages and thermometers are useless, they each have their place in AIDING us to diagnose a system. I do understand what Gary is trying to say and do agree to a certain extent. They help us get to where we need to be but fact is we couldn't get there without either of them

[WINJA]

Our industry seems to have a pressure addiction. It's kinda like being "huked on fonix". It works... sorta... sometimes... maybe.

But what do pressures (or temperatures for that matter) tell us? Nothing.

Every bit of useful information about any system comes from comparing temperatures. The temperatures/pressures by themselves are meaningless.

For example, consider head pressure. On a hot day we would expect the head pressure to be higher. On a cold day we would expect the head pressure to be lower. Right? Whether we realize it or not, we are comparing two temperatures. Without also knowing the outdoor temperature, head pressure doesn't tell us anything.

Similarly, in order to judge the suction pressure, we also need to know about the heat load. A freezer full of warm products (heavy heat load) is going to raise the suction pressure. Light load (fan out, filter dirty, coil dirty or frozen, etc.) is going to bring down the suction pressure. Whether we realize it or not again we are making a comparison. Suction pressure by itself tells us nothing.

The examples go on and on... delta-T, TD, subcooling, superheat, approach...

The point is: Every bit of useful information comes from comparing one temperature to another temperature.

its true to a point , some trane chillers have no preasure transducers and they work ok ,it had me fooled one day for a second too , turned up and it said it had a standing preasure of 400psi or so when it was stone cold, it was the temp sensor in the condensor had gone bad , had to decant the whole circuit to replace the sensor, im going out to the van right now and throw my gauges in the rubbish

[Gary]

From the pressures I generally know at which end of the system I should be checking!

The saturation temperatures will tell you the same thing.

Where did I say that people shouldn't use gauges? Gauges tell us the saturation temperatures.

[BigJon3475]

I agree...I find myself not even noticing pressures unless I think something is possibly wrong....then I'm looking at approach....

[Refrigerologist]

its pretty darn hard to find out suction line pressure drop without guages...

I don't agree that guages and thermometers are useless, they each have their place in AIDING us to diagnose a system. I do understand what Gary is trying to say and do agree to a certain extent. They help us get to where we need to be but fact is we couldn't get there without either of them

Total agreement from me. As Gary himself said and I quote "we only need the pressures to get to the good stuff", so by that alone it means pressures are not meaningless. Otherwise why would any one bother to manufacture them? Why are pressure temperature relationship charts available?

As I have said before the thread leader could have been different, something like 'Pressures are only part of the story'. Never the less it has provoked a fair old debate, so it has certainly brought attention to the fact that we do need to convert pressures to temperatures to diagnose most common problems.

[Larry2]

im going out to the van right now and throw my gauges in the rubbish

Please tell me where you're rubbish bin is located. I'm not to proud to dumpster dive for a treasure like this. I'll give them a proud home in my toolbox, even if I use fingertips, eyes, sense of smell and observations for a first cut on what has gone wrong.

[ammonia101]

You are 100% correct, superheat or subcooled the pressure will not tell you any of that. However old habits are hard to break>>