Updated: Oct 25, 2022
Can you check the charge on a system without checking pressure? Sure, remember that the only reason we check pressure is to convert it to saturated temperature. For single and two speed compressors systems, we use the subcooling and total superheat charging methods. I will briefly explain these charging methods using pressure before diving into reading temperatures only in order to check the charge.
In order to measure subcooling, we measure the saturated temperature in the middle of the condenser coil. Normally, at the outdoor unit, we measure the pressure on the high side liquid port and convert this pressure to saturated temperature. Then we measure the temperature on liquid line within a few inches from the liquid port. Subcooling is the saturated temp minus the liquid line temp. In the example below, the subcooling is 5°F
On the red gauge face in the picture above, the pressure measured is 318 PSIG. The pressure needle intersects at 100°F saturated temperature for R-410A. Therefore, the saturated temperature inside the condenser coil is 100°F. Calculate the subcooling based on the picture:
Sat Temp - Actual Liquid Line Temp= Subcooling
100°F - 95°F = 5°F of Subcooling
In order to measure total superheat, we measure the saturated temperature in the middle of the evaporator coil. Normally, at the outdoor unit, we measure the pressure on the low-side vapor port and convert this pressure to saturated temperature. Then we measure the temperature on vapor line within a few inches from the port. Total superheat is the temp on the line minus the saturated temp. In the example below, the total superheat is 15°F.
On the blue gauge face in the picture above, the pressure measured is 118 PSIG. The pressure needle intersects at 40°F saturated temperature for R-410A. Therefore, the saturated temperature inside the evaporator coil is 40°F. Calculate the total superheat based on the picture:
Actual Vapor Line Temp – Saturated Temp = Total Superheat
55°F - 40°F = 15°F of Total Superheat
There is a difference between a superheat measurement and a total superheat measurement. A total superheat measurement is taken at the outdoor unit, whereas a superheat measurement is taken at the indoor coil. Total superheat includes any change in the superheat from where the refrigerant exits the indoor coil until it enters the outdoor unit. Below are examples of superheat and total superheat.
Superheat is shown in the picture below. 54°F - 40°F = 14°F of superheat
Total superheat is shown in the picture below. 55°F - 40°F = 15°F of total superheat
If you want to learn more about the refrigeration cycle, check out this video: https://youtu.be/yjmrCEA1zIA
If you want to learn more on the complete total superheat method, read this article.
If you want to learn more on the complete subcooling charging method, read this article.
Getting on with it!
Now that we have covered what is normally done to measure both subcooling and total superheat, let's explain how to measure the saturated temperature without measuring pressure. Let’s start with subcooling.
In order to determine the saturated temperature in the middle of the condenser coil, we need to figure out where to take our measurement. We must attach our temp sensor onto the condenser coil tubing in a location where the saturated refrigerant is located and where there is not much air moving through which could affect our temp measurement.
The picture below shows the refrigerant changing as it moves through the outdoor coil. As the refrigerant enters the condenser coil, it is a high temp superheated vapor. As it moves through the tubing, the refrigerant changes to being saturated. Next, the refrigerant subcools. In the example below, the high temp superheated vapor is 170°F. The saturated refrigerant is 105°F, and the subcooled liquid exiting the condenser is 93°F.
In the example above, you see that the refrigerant is in the saturated state at 105°F during the majority of the time while traveling through the coil. Now let's look at where to find this location in real life.
Below, you will see several pictures of the inside of an outdoor unit. In this case, the compressor is wrapped with a black sound shield. Note where the tubing is exiting the compressor and entering the condenser coil. Also note where the tubing is exiting the coil and then exiting the outdoor unit on the small liquid line.
On the coil of this unit, the high pressure superheated vapor from the compressor is distributed into the coil in multiple locations. The subcooled liquid is also collected in multiple locations. Between where the superheated vapor enters the coil and where the subcooled liquid exits the coil, we see three elbows in most of the areas and two elbows in one area. This may be different from unit to unit that you work on. If a temp sensor was mounted to the middle elbow, this would be where the saturated state is located. Insulation can also be mounted to the outside of the sensor to make sure that the measurement is accurate.
My favorite tool to take temperature measurements with is the ST4 digital dual temp meter equipped with k-type bead temp sensors: http://amzn.to/2wc1ME3
K-type bead temp sensors can be taped onto the tubing to get an accurate temperature reading. Clamps can also be used but for small locations, I find k-type bead temp sensors work best. Bead temp sensors are so small that they can even be pushed through a hole made by a zip screw, if needed.
In this example, while this system was running, the temp on the elbow for the saturated refrigerant measured 92.2°F. The actual temp on the liquid line read 84.9 . To find the subcooling: 92.2°F sat temp - 84.9°F line temp = 7.3°F subcooling.
Another way to find the location of the saturated refrigerant is to look down through the top of the outdoor unit in order to find and count the elbows between the superheated vapor and the subcooled liquid refrigerant on the condenser coil. Then place a temp sensor between the fins on the tubing in the correct spot where there is a lower amount of air crossing the fins.
In this example, while this system was running, the temp measuring the saturated refrigerant read 87.5°F. The actual temp on the liquid line read 82.6 . To find the subcooling: 87.5 sat temp - 82.6 line temp = 4.9°F subcooling. This actual subcooling would need to be compared to the target subcooling found on the outdoor unit rating plate. If the actual subcooling is lower than the target subcooling, add refrigerant. If the actual subcooling is higher than the target subcooling, recover refrigerant.
Now onto superheat and total superheat!
At the evaporator coil, the refrigerant enters the metering device as a high pressure, subcooled liquid. The refrigerant exits the metering device as a low pressure, 80% liquid 20% flash gas mix. Because the refrigerant becomes saturated immediately after exiting the metering device, it is easy to locate the section of tubing where the refrigerant is saturated. As you can see in the animated picture below, the temperature of the saturated refrigerant as it enters the evaporator is the same as where the refrigerant has traveled halfway through the coil. In this example, the refrigerant remains 40°F until the refrigerant changes completely into a vapor and starts to increase in temp. We see on this coil that the there is a 54°F line temp - 40°F sat temp = 14°F superheat.
In order to mount the temp sensor on the evap coil in the location of the saturated refrigerant, the evap coil box cover will need to be removed. Between the metering device and the coil are distributor tubes connecting to multiple locations on the coil. Usually, the distributor tubes connect at the bottom of the A-coil. This is where the low-pressure refrigerant enters the coil. The superheated vapor refrigerant exits at the top of the A-coil. The temp sensor can be mounted anywhere from the first tube elbow to about halfway up the coil as long as the elbows are all in the same line and not going in a different direction.
The temp measurement will also be more accurate if the bead temp sensor is insulated around the mounting location.
In the example below and to the left, if the temp on the elbow inside the evap coil measured 40°F and the temp on the vapor tube on the outside of the coil measured 51°F, the superheat would be 11°F.
51°F - 40°F = 11°F superheat
In the example below and to the right, if the temp on the elbow inside the evap coil measured 40°F and the temp on the vapor tube at the outdoor unit measured 52.9°F, the total superheat would be 12.9°F
52.9°F - 40°F = 12.9°F total superheat
As you can see, it is fairly difficult to locate and mount a temp sensor to the tubing which carries the saturated refrigerant. There is also the possibility of mounting the temp sensor in the wrong spot accidentally. In reference to checking subcooling at the outdoor unit, there may be some units that have an access panel that can be taken off for easy access into the inside of the unit. However, in most cases, there is no easy access door. The easiest access is to take off the top grille and condenser fan. We would want to avoid moving objects like the condenser fan just to take a simple measurement.
It would be great if a manufacturer provided us with a simple temperature measurement spot to measure the high side sat temp but up to this date in time, no manufacturer has taken the step to provide us with one. Maybe someday this will be considered. I look forward to that day but for now, you can see why we find it easiest to just measure pressure to find subcooling. Also, if the unit was severely undercharged or overcharged, the location on the tubing at the outdoor unit where the saturated state could be found may change. We know where the location is with an average refrigerant level in the outdoor unit but with a severe case, this could cause confusion for the technician if only one specific elbow temp was measured. At the indoor coil, it may be slightly easier to locate and measure the saturated temp to determine superheat and total superheat, but it is usually still not worth the time and effort it would take to mount the sensor.
Another point that must be considered is that we usually check total superheat and subcooling at the same time. When using the subcooling method, we also measure the total superheat to make sure that the TXV is controlling the superheat properly.
Most techs try to avoid measuring pressure to avoid contaminating the system with old oil which may be leftover in the refrigerant hoses of their manifold gauge sets. That is why quick connect test gauges and electronic test probes have grown in popularity. These tools provide a quick access to measure pressure without needing to fill long hoses just to read pressure.
Wireless test probe links:
Fieldpiece Wireless 6 Piece Job Link System Probe Kit JL3KH6 - https://amzn.to/2wr4Xbp
Testo Wireless Smart Probe Full Kit - https://amzn.to/2UU81ZH
Quick connect test gauge links:
Blue Vapor Gauge- https://amzn.to/2Ju7pan
Red Liquid Gauge- https://amzn.to/2sSFmGH
Quick Coupler for the Gauges- https://amzn.to/2sTk8Ze
ST4 Dual Temp Meter- http://amzn.to/2wc1ME3
If you want to learn more about all the fine details on charging methods and troubleshooting, check out our book which is available on our website and on amazon. The full outline and sample pages are available here. We have a 1,000 question workbook with an answer key that you can use to apply your knowledge as well.
If you want to learn about Delta T, check out this article!
If you want to learn the full Total Superheat Charging Method, check out this article!
If you want to learn the full Subcooling Charging Method, check out this article!
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Tools that we use: www.amazon.com/shop/acservicetech
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Published: 7/16/2020 Author: Craig Migliaccio
About the Author: Craig is the owner of AC Service Tech LLC and the Author of the book “Refrigerant Charging and Service Procedures for Air Conditioning”. Craig is a licensed Teacher of HVACR, Sheet Metal, and Building Maintenance in the State of New Jersey of the USA. He is also an HVACR Contracting Business owner of 15 years and holds an NJ HVACR Master License. Craig creates educational HVACR articles and videos which are posted at https://www.acservicetech.com & https://www.youtube.com/acservicetechchannel & https://www.facebook.com/acservicetech/