I have been answering questions regarding gas/propane pool heaters for a while now and thought it might be time to add a knowledge base to some of these "mysteries". AC voltage is a sinusoidal wave form that is continuously variable. It can deformed by different variables in the circuit so that it is no longer a pure sinusoidal wave. Pure sinusoidal AC waveforms can be accurately read by both averaging and true RMS multimeters. NonPure AC waveforms can only be read accurately by a true RMS multimeter. See What is true-RMS?. When flame rectification occurs to prove a flame it is not an actual DC current even though it can be read as a DC current. See Troubleshooting a flame rectification system (note: this reference has limited access and will disappear if not copied and pasted). What is actually happening is that you are getting essentially a half wave AC voltage with a small blip on the opposite side of the wave because although the opposite side of the wave is resistant to flow because of the difference in surface area of the contact points there will still be a a small flow. It seems to me that most pool heater ignition modules are Fenwal ignition modules. Fenwal states that flame current can be read at their FC+ and FC- terminals with an analog DC ammeter or as voltage with a true RMS multimeter. True RMS multimeters can be somewhat expensive. For a cheap substitute you might try this alternative to a true RMS multimeter; Amazon.com: Mecion DC 0-50μA Ammeter 85C1 Pointer Type Analog Current Panel: Home Improvement, but realize that this is meant to be mounted in a panel and the most accurate reading would be if it was read in the upright position as if mounted in a panel and the readings that you will be getting are at the lower end of the scale but should suffice ( a 0-30 scale would be better). For a true RMS meter without spending hundreds of dollars since you will not be using it on a daily basis you could try these which Amazon just started offering: (1) Amazon.com: AmazonCommercial 4000 Count Compact Digital Multimeter, IP67, True RMS, CATIV 600V: Industrial & Scientific or (2) Amazon.com: AmazonCommercial 6000 Count Compact Digital Multimeter, IP67, True RMS, CATIV 600V: Industrial & Scientific. The 4000 count meter will be less accurate than the 6000 count meter, but should suffice for what you are using it for. Also both of these meters will measure capacitance so you will be able to check the capacitors on your pump motor if it won't start and just sits there and hums before shutting down from overheating. For a little more information on flame rectification you might try these sources: Flame Sensors — Revisited and CBFF - Flame Rectification. But realize that though the UK article is talking about 240 volt supply ( and obviously 50HZ [ cycles per second]), it would also hold true for US 120 volt 60Hz supply.
