Vacuuming an aircon system does more than remove air. Its main job is to pull moisture out of the system. So while the vacuum pump is running, how can you tell the moisture is actually leaving? This article walks through an Elitech test setup: a single-stage vacuum pump, a DMG-2SE digital manifold gauge and a VGW-MiNi digital vacuum gauge.
Why vacuuming removes moisture
Water boils at 100 °C only at atmospheric pressure. As the pressure inside the system drops, so does the boiling point of water. Once the boiling point falls below the surrounding temperature, moisture left in the system evaporates and the vacuum pump pulls the vapour out.
Reading the test setup
During the test, the VGW-MiNi showed pressure, the boiling point of water (T H2O) and ambient temperature on one screen:
- Pressure fell from 2,133 Pa to 1,307 Pa, about 16,000 to 9,800 microns
- The boiling point of water fell from 18.5 °C to 10.9 °C
- Ambient temperature was about 18.8 °C
From the moment the boiling point dropped below ambient temperature, the moisture in the system started to evaporate and was pulled out. These numbers are only the early stage of the vacuum. The system still has to go much lower before the job is done.
Why you need a micron gauge
A manifold gauge shows that the system is under vacuum, but the inHg scale is too coarse to tell whether the moisture is gone. For that you need a vacuum gauge that reads in microns or Pa.
How to hook up the vacuum setup
- Use fittings that match the system’s service ports. The VGW-MiNi connects directly to a 1/4 SAE port.
- Check the O-ring seal in every fitting. It should sit in place with no cracks or deformation. Tighten each fitting to cut the risk of leaks at the joints.
- Mount the vacuum gauge on a tee fitting with the gauge connection facing down, so dirty oil is less likely to run into the sensor and damage it.
- Connect the hose to the vacuum pump and check every joint again.
- To start, switch on the vacuum pump first, then open the valve. Watch the reading the whole time. The VGW-MiNi connects to the phone app over Bluetooth within 30 m, so you can follow the reading without standing next to the gauge.
- To finish, close the valve first, then switch off the vacuum pump.
Vacuum time depends on the size of the system. The system in Elitech’s installation demo took about 30 minutes. After the valve is closed, the micron reading rises slowly. Watching that rise is the vacuum hold test.
Elitech vacuum pumps and vacuum gauge
- NVP-3: single-stage aircon vacuum pump, 3 CFM, 2 Pa ultimate vacuum, 1/4 HP motor, 1/4 and 3/8 inch ports
- SVP-DC: cordless aircon vacuum pump with an 18 V lithium battery and brushless motor, 100 minutes of continuous running
- VGW-MiNi: wireless digital vacuum gauge, 1 to 19,000 microns, Bluetooth 30 m, rechargeable battery for 20 hours of use, 145 g
Related: Why analog manifold gauges drift on aircon jobs. See all manifold gauges, vacuum pumps and leak detectors on the aircon tools page, and find authorised sellers on Where to buy.
Choosing a pump: Vacuum pump price in the Philippines: what you pay for.
