|[AMRadio] Re: Thermocouple rf ammeter|
k4kyv at charter.net
Sat Feb 10 13:04:46 EST 2007
Usually it will be stamped on the meter to indicate whether or not an
external thermocouple is required. If the back of the meter is brown
bakelite instead of black, that usually (but not always) indicates a
Otherwise, try measuring the resistance using an ohmmeter or DVM. The
thermocouple should read no more than a fraction of an ohm. The DC meter
movement will read in the tens of ohms. Be sure to use the highest
resistance range available, then work down to the next lowest, etc, since
some ohmmeters run enough current in the lower resistance ranges to destroy
a sensitive movement. If the meter deflects positive with the ohmmeter
connected, then deflects backwards when the leads are reversed, that is a
sure sign that the meter requires an external thermocouple.
For open wire balanced feeders, ideally you would use a separate meter in
each feeder. If a matched pair of meters is not available, one meter in one
leg would be ok. With a 4PDT knife switch, you could rig up a switching
circuit to switch the meter from one side to the other. Be sure to
disconnect the meter when not in use, since it doesn't take much of a
lightning jolt to destroy an rf ammeter. I lost my very best one a couple
of years ago when lightning turned thermocouple, meter movement and all,
into a block of charcoal.
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