Millivolt readings - Drop-out (alternative)

Millivolt readings - Drop-out (alternative)

• A. 27 mv
• B. 13 mv
• C. 4 mv
• D. 28 mv

Answer: (C)

The idea being tested is how much millivolt is required to keep a millivolt gas valve open—the drop-out voltage. In these systems, the flame heats a thermocouple (or thermopile) that generates a small DC voltage to hold the valve energized. There is a minimum voltage, the dropout, below which the valve will release and the gas supply will shut off. That threshold is very low, typically only a few millivolts. About 4 millivolts is a common figure for dropout, so this reading represents the point where the valve would drop out of conduction if the flame weakens further. The larger readings (around tens of millivolts) would keep the valve held open and are not the dropout value. If you’re troubleshooting, seeing drop toward this low level suggests the flame or thermocouple isn’t delivering enough heat, so you’d check the pilot flame stability, clean or replace the thermocouple, and ensure proper flame contact with it.

The idea being tested is how much millivolt is required to keep a millivolt gas valve open—the drop-out voltage. In these systems, the flame heats a thermocouple (or thermopile) that generates a small DC voltage to hold the valve energized. There is a minimum voltage, the dropout, below which the valve will release and the gas supply will shut off. That threshold is very low, typically only a few millivolts. About 4 millivolts is a common figure for dropout, so this reading represents the point where the valve would drop out of conduction if the flame weakens further. The larger readings (around tens of millivolts) would keep the valve held open and are not the dropout value. If you’re troubleshooting, seeing drop toward this low level suggests the flame or thermocouple isn’t delivering enough heat, so you’d check the pilot flame stability, clean or replace the thermocouple, and ensure proper flame contact with it.