# Measuring Capacitance on a Running System

## Measuring Capacitance on a Running System

In a collaboration with TruTech Tools, Bryan shows us how to measure capacitance on a running system with the Testo 770-3. He also clears up a common misunderstanding about capacitors.

Before Bryan takes the measurement, he reads the compressor capacitance reading. We are dealing with a 70-microfarad capacitor at +/- 6%. However, every device has some uncertainty, so a 10% range is an acceptable rule of thumb.

First, Bryan takes the amperage of the wire going to the compressor start terminal (HERM terminal on the run capacitor). The Testo 770-3 picks up 8.3 amps. To find the capacitance, we multiply that 8.3 by the constant of 2652 to get a product of 22011.6, which we then divide by the applied voltage across the capacitor (between C and HERM). That voltage is 295.4, so we divide that into 22011.6 and get a capacitance value of ~74.5, which is slightly high but still within range.

Run capacitors can run weak, so if the capacitance is out of the 10% range, then we typically recommend a run capacitor replacement to the customer. The idea that run capacitors are either good or bad simply isn’t true, and the capacitance reading allows us to see the nuance in capacitor performance.

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