Millivolt question.

[wishful]

I'm working on a 250K btu pool heater. I use to have an old millivolt meter that told how low your reading could be and still hold in a 750 mv valve. The meter is long gone, I installed a new power pile and get around 590 to 600 mv when connected to the valve, but the pilot goes out when trying to energize the valve. I am going to change the valve, how low can the mv's be and still hold the valve in?
Been awhile since I've done a millivolt system. Thanks.

[Milk man]

I thought it was pretty low, like 200mv. But I may be way wrong. It might also depend on the valve it's self. It may have trouble opening.

[mike3]

My first instinct says resistance somewhere. Try disconnecting all the wires from the gas valve except the PG and than using 2 jumpers jump the 2 middle terminals and than the 2 opposite the PG. Meaning one of the middle terminals will have 2 jumpers. The middle jumpers are acting like a buss. These last 2 are acting like the t'stat. If the valve fires off than you know the problem is elswhere. These problems normally are corrected using a meter unless you get lucky. Most millivolt valves will dropout around 60 mv. Do not confuse the the dropout in a millivolt gas valve with that of a power unit (magnet) in a 24 volt valve or a water heater control.