As a technician, you constantly handle different heating, ventilation, air conditioning, and refrigeration equipment and appliances. One tool that you can’t leave behind any time you’re going for repairs or checkups is a refrigerant gauge – it should be your most valuable asset! You will agree with me that you use your gauges a lot.
You can determine or troubleshoot the condition of an HVAC/R system without a set of gauges, don’t get me wrong, they are very useful tools and excellent diagnostic tools and will accurately read the pressures any refrigerating system. But don’t get hung up on the manifold, a lot of things become second hand to you as you become more and more familiar with systems and how they operate.
An example is a suction line coming back to the compressor, its frozen solid for a regular walk in fridge for example. You don’t need a set of gauges to tell you something is wrong, most likely, the evaporator is frozen over or there’s a piece of plastic blocking the air to the evaporator, something simple.
It doesn’t mean you can’t put your gauges on it and say, “Is it low?” and this is where explaining to people why you don’t HAVE to put your gauges on every system within reach can become tricky. As the refrigerant gets “hot” air blown over it as it’s in the coil of the evaporator, the refrigerant boils off and turns into vapor, it’s introduced as a liquid, this is all it really does.
So if you don’t have “warm” air blowing over the coil (due to something obstructing airflow), the refrigerant remains very cold and eventually continues to cool and will eventually reach below freezing temperatures as there is no heat added to the system like it should be. Then the suction line goes below 32*F and you get frost on the pipe and eventually ice.
You could do it both ways, but realize, if you switch refrigerants, you have to purge, if you hook into a system, you have to purge, if you unhook, you have to bleed high to low to keep the refrigerant in the system. Every time you hook in, a potential leak happens at the Schrader. If you can minimize the risk of contaminating the system and creating leaks, wouldn’t you want to?
A refrigerant gauge won’t be useful if you don’t know how to read it and, above all, interpret the readings.
What is a Refrigerant Gauge?
A refrigerant gauge is a device used to measure pressure and convert that into a temperature, that’s all it does, some have clamps that measure pipe temperatures as well, but that is ALL it does, it does not tell you if the system is happy, if it’s working well. It gives you DATA, you take the data and draw a conclusion.
The manifold has 3 or 4 ports, the utility (yellow), high-pressure (red), low-pressure (blue) and on some, a vacuum port (black). Each port has been designed to offer control over flow, meaning they have valves on the manifold to allow you to keep things separate. If you had the high side open and the low side as well as the utility open, you’d be short circuiting refrigerant from the high side directly into the suction line, which is a waste of energy and time and doesn’t do anyone any good, so having the valves and paying attention to what’s on and off is important.
Types of Manifold Gauges
There are a few types or styles of manifold gauges available today… analog and digital ones, stubby analogs and stubby digitals. Although the gauges are different, they have been designed to work in the same way. Let’s look at them.
To understand the entire concept of the manifold gauges, a little information is needed.
I hear this lot “I have a 404A freezer, what pressure should I see?” so what does that tell me?
The guy is either new in the field or never learned the basics of Refrigeration and A/C.
The correct question would be “I have a 404A freezer, what temp does my refrigerant need to be?”
Now you go, “What?” well hear me out, the entire concept of refrigeration is seemingly very complex, but it’s actually pretty simply, I’ll lay it out.
A Refrigerator needs to be what temperature? 38*F let’s say.
The outside air in Florida is what, on the hottest day? 100*F maybe.
Okay, so why is that important? Heat transfer. In order for me to cool my box down, I need refrigerant that’s a lower temperature than 38*F, if it’s higher, it won’t cool it, if it’s 38*F exactly, it won’t cool it. So I need my refrigerant to be around 30*F or even 28*F If I’m adventurous, now what is this called? Delta T. Difference in temperature between the coil and desired temperature of the box, it’s either 8*F or 10*F. This allows heat transfer to work and lets me cool the box down.
So now we go past the fridge and say, well, what do I do with all this heat? I just compressed all this gas from the fridge, it’s hot, where do I put this heat? Outside of course. So how does that happen? Well we know when the refrigerant leaves the compressor, it’s a certain temperature or pressure, what temperature should it is? Let’s say 10*F above the outside temperature. So we’re shooting for 110*F.
Now you are probably wondering why on earth I’m not saying pressure but temperature? Why? Because I couldn’t care less about pressure, what matters is temperature.
Why is temperature better? Because it applies to every refrigerant, instead of listing out 17 different suction pressures for 17 different refrigerants, I gave you 1 number to find on 17 different pressure/temperature charts that gives you that pressure you look for.
Yes. It really is that simple, this is what I look for when I hook in, that and additional things, but that is the BASICS of it. A lot of techs don’t get it still, I didn’t for 2 years until it clicked, now I merely glance at the PSI reading, I care about the temperatures.
These are the most used gauges – they are easy to read and handle. In the analog gauges, the needle position indicates the pressure, there’s usually multiple colored areas on the face of the dial which tells you the temperature of the refrigerant at that pressure, making it a little easier than pulling out a P/T chart and looking it up every time the needle moves.
I hear they are reliable and accurate, I think any wrong numbers is done in the calculation by the tech, not the manifold.
Compared to the analog gauges, digital gauges are more efficient – they provide the pressure readings in numbers, down to 1/10th of a PSI. You don’t need to convert the readings, reducing the chances of making an error. Talk about efficiency!
Digital gauges is what I personally started with, I’ve used analogs when nothing else was available, what makes Digital gauges useful is the additional features, automatic calculations of Pressure to temperature, most of them have either Bluetooth temp clamps for the pipes like the newer Testo ones or the Fieldpiece SMAN3 which is what I started with, which had wired temp clamps.
When it has these temperature clamps built into it, it’s able to again do automatic calculations of Superheat and Subcool.
Testo and Fieldpiece’s SMAN both have vacuum features, although a little different, the SMAN has the sensor built in and automatically switches to vacuum when it’s reading a vacuum.
The Testo 550S has a separate probe (which I think is brilliant) and it’s a stubby, so it’s easy to use and can be put away from the vacuum pump in the system, making it more accurate.
What makes the digital gauges less popular is that they come with some added features that might complicate their readings. Techtown members have reviewed some of these digital manifolds in our review program, check out one of these reviews here.
Reading Gauges: A Step-By-Step Guide
Before moving on, we need to get into what to look for, on a manifold, High/Discharge/Liquid all refer to the higher pressure side of the system, its color is red, always.
The liquid line, or Discharge or whatever you want to call it, is the smaller of the pipes going into the compressor. Suction line is always bigger than the liquid line.
Suction or low side is the Blue side, always; it’s the lower pressure side of the system.
Let’s learn how to read a refrigerant manifold.
Step 1: High-Pressure Connection
Connect the red hose (on your gauge face if it’s analog it will be red) to the high-pressure part of your system, this will again, be the smaller of the pipes, sometimes insulated but not very often.
This is usually done with small Schrader fittings which are like what’s in your car tire or bicycle tire, its auto sealing, so when you screw on the hose, there’s a part in the hose that pushes in the Schrader allowing the flow of refrigerant to enter the hose and gives you a pressure reading.
You can also have service valves at the receiver or on the compressor which require a service tool to turn, or a socket, but really the service wrench is the way to go… it doesn’t damage the stem like a crescent might…. When you leave the wrench somewhere else… Moving on.
If you crack this guy open, which is about to be confusing because you turn it clockwise, it has 3 positions,
1. Closed to the port side (where your manifold hose hooks in) and closed flow (the pipe that connects to the valve) this is all the way clockwise, fully shut and closed.
2. Open on the port side and flow side, this is done when you turn it clockwise in just a hair, it allows you to get a pressure reading without closing off the system or disturbing it too much.
3. Closed on the port side but open on the flow side, this is when the valve is fully turned out counterclockwise, in other words, as open as it will get. This closes the port side but leaves the flow side open so the system can function; this is how you will first find it when you go to work on it.
Step 2: Low-Pressure Connection
The next step is to connect the blue port to the low-pressure side. Just as you did for the red one, connect the blue hose to the low side pressure port (Schrader or service valve).
Step 3: Vent/Purge
When you hook into the system you want to purge a little, so as you screw on your hose to the Schrader connection, you want to open your manifold so that you are not leaving any air trapped in your hose that would then enter the system. The idea is to use the already purged manifold with some refrigerant still in it, to push out from the hose while making the final connection to the system.
This should be done on the high side as well, but if you ever tried hooking into a high side when the system is on, getting it on without getting frostbite is good enough for me.
Step 4: Check the Connections (Especially on stubby’s)
Here you want to make sure everything is good and tight, verify your hoses at the manifold side, verify it at the system side and just make sure nothing is loose.
Stubby tools especially can vibrate loose and leak, I’ve had a reach in True fridge loose all it’s refrigerant when I walked away for a couple of hours with my tablet data logging data for me, it vibrated loose and just gently hissed out in an hour or 2.
Step 5: Reading The Measurements
That’s about all there is to it, there’s no Siri, no Alexa to tell you if it’s right or wrong. The manifold only provides you with information it’s not going to tell you everything you need to know but it will provide the information you need to know in order to determine what is wrong.
Step -0 HG: Reading Vacuum
When repairing a system and it requires you to open it up, pulling a vacuum should always be done, down to below 500 Microns. Most vacuum sensors, if separate from the manifold, should never be pressurized more than 1-10 PSI. You also don’t want to connect your vacuum pump and put pressure on it as it can blow oil out the exhaust of the pump.
If you are going from vacuum to charging, close off the pump and add refrigerant, just enough to break the vacuum and THEN remove your vacuum probe. If you try to remove it from a Schrader during a vacuum, the moment the vacuum probe lets go and the Schrader closes, it will suck air in and contaminate the system. Sure, it’s minimal, but it’s unnecessary and sloppy.
Same goes for switching hoses from your vacuum pump to the refrigerant cylinder, it’s dicey, one accidental flick of the valve and you are letting air in and have to redo the vacuum.
Operating Pressures of the Most Common Refrigerants
So here is the part you’ve all been waiting for, I type out the numbers you need to see on your manifold and all is well.
I’ll do you one better.
Figure out what your desired box temperature is, room temperature, whatever it is, find that and you can figure out the rest.
- A/C is about a 10-20*F delta T I believe, so if you want the room to be 70*F you normally always supply 55-60*F air because you also want to dehumidify.
- Refrigeration is about 10*F, rule of thumb, 10*F will at least get you somewhere.
Then look at the pressure temperature chart of the refrigerant and there is your answer. Let’s do an example.
A walk-in fridge or reach-in fridge with R22 has a desired box temp is 38*F. What is 38*F on a P/T Chart? Around 63 PSI. But we don’t want 38*F, we want 28*F. So what’s 28*F? Around 49-50 PSI. So the suction side should be around 50 PSI.
High side, well it’s Florida, design temperature would be 95*F based on what I said earlier. So 10* above that is what? 105*F, okay so we want the high side pressure to be dictating 105*F, so that’s 211 PSI.
Now as for high side, it can be lower but not too much lower but also not higher than the 211, the higher the high side, the harder the compressor is working. Low side can be slightly lower and slightly higher but it should be right around there.
Pipe temperatures, what do they mean? The discharge line of a compressor should never exceed 225*F as it begins to cook the oil.
Suction line should be a maximum of 65*F and not any higher or you are not cooling the compressor properly.
On most compressors, 20*-40*SH, not any lower, not any higher. Or again, it’s too much heat.