Take any chance you have to skip connecting refrigerant gauges to a system. Each time you attach your gauges, you remove a small amount of refrigerant from the system through your hoses. The longer the hoses, the more charge that’s lost. After a few times of doing this, you might end up chasing a leak that doesn’t even exist.

Using ball valve hoses you can get most of the refrigerant back into the low side of the system before disconnecting, but if you don’t need to connect in the first place, don’t. Smart probes are another option to minimize release.

Connecting gauges can also cause schrader cores to stick and release the refrigerant. The technician can contaminate a system if connecting hoses incorrectly. It’s best to avoid all these risks when you can.

I’m not saying it’s never a good idea to connect gauges to a system. On maintenances or a service call for an electrical issue there really almost never is a need to connect.

Superheat Without Gauges

Step 1: Getting the required superheat

Firstly we need to calculate what our superheat should be. You can use the superheat charging chart from the system, the Emerson Check and Charge app, or a table like the one above. Whichever you use, you will need the indoor wet bulb measured in the return air and the outdoor dry bulb measured in the shade by the condenser.

The return air wet-bulb for this system is 64.5°F, rounded to 64°F, and the outdoor temperature is 70°F at the condenser in the shade.

Step 2: Subtract evaporator design temperature difference from the indoor dry bulb

Next, we need to get the return air dry bulb. The return air dry-bulb temperature here is 75°F.

We subtract the evaporator design temperature difference from the return air dry bulb. For most residential and lite commercial cooling equipment, we use 35°F.

75°F – 35°F = 40°F

Evaporator design temperature difference is the difference in the evaporator saturation point (which you see on the low side of your refrigeration gauges) and the return air temperature. Essentially, subtracting the evaporator design temperature difference gives you the saturation temperature (which is your temperature you would read on your refrigerant gauges) of the low side of the system. 40°F from this example converts to 68.5 psig using the pressure-temperature relationship. This is an R22 unit.

Step 3: Calculate

Now to bring it all home we take the evaporator saturation temperature from step 2 and add the required superheat from step 1.

40°F + 21°F = 61°F

Remember the 40°F is what we would see on our gauges had you hooked them up. We got this number without ever having to do that.

Step 4: Confirm proper superheat

A system charged by superheat is considered correctly charged when within +/- 5°F.

Subcool Without Gauges

Step 1: Get condenser discharge air temperature

The condenser discharge air temperature will give the saturation temperature value of the refrigerant in the condenser. The saturation temperature can be converted to the refrigerant gauge pressure if needed using the pressure-temperature relationship by using a refrigerant slider.

Take the condenser discharge air temperature across a few areas of the outlet of the condenser. Generally, the highest number recorded after a few attempts is what’s used.

This system has a condenser discharge air temperature of 95°F.

Step 2: Liquid line temperature

Make sure to cover the temperature probe to protect from direct sunlight causing false readings. Record the liquid line temperature about 6” or so from the liquid line service valve.

This system has a liquid line temperature of 78°F.

Step 4: Calculate subcooling

The formula for calculating subcool is condenser saturation temperature – liquid line temperature

95°F – 78°F = 17°F subcooling (required subcooling of 16°F)

A system charged by subcooling is considered correctly charged when within +/- 1°F.

As mentioned earlier, the 95°F from step 1 can be converted to the refrigerant gauge pressure using a refrigerant slider or refrigerant slider app. 95°F for a 410A system converts to 299 psig using the pressure-temperature relationship.