RF Ammeter resistance

AM Radio discussion. Directional arrays are FUN!
kcbooboo
Posts: 518
Joined: Sat Dec 10, 2011 4:53 pm
Location: East of the Mississippi

RF Ammeter resistance

Post by kcbooboo » Thu Nov 01, 2018 3:27 pm

I'm testing a Wenschel 15A internal thermocouple-type RF ammeter. It seems to be accurate enough, within 2%, however with 12A DC flowing through it, the back of the meter, the terminal studs, and the metal straps between the meter and J-plug are getting hot, and I don't think that's normal. This doesn't feel like milliwatts; it feels more like watts of heat.

I was told the metal straps are silver-plated copper, however they won't let me bend them, springing right back to the bends that are now in them, acting more like hacksaw blades or spring steel than copper. Also there's no tarnish on the straps like I'd expect with silver-plating.

I'm seeing a slight drift in meter indication, but with 12.0A flowing through the meter, it changes from 12.05A to 11.95A.

What sort of resistance should I expect at the meter terminals? I'm thinking perhaps there's a bad connection inside the meter that's resulting in a higher than normal resistance and possibly affecting the current reading, if left in that condition long enough.

Bob M.

ChuckG
Posts: 989
Joined: Tue Mar 11, 2008 11:12 pm
Location: Moo
Contact:

Re: RF Ammeter resistance

Post by ChuckG » Thu Nov 01, 2018 7:26 pm

The only one I Have on hand is a Weston 0-1A, which measures 1 ohm. I believe higher current meters will measure less..fractions of an ohm. If it's that hot you already have your answer I think.
<><><><><><><><><>
Chuck Gennaro
Central Wisconsin

kcbooboo
Posts: 518
Joined: Sat Dec 10, 2011 4:53 pm
Location: East of the Mississippi

Re: RF Ammeter resistance

Post by kcbooboo » Fri Nov 02, 2018 7:30 am

Thanks for the 1 ohm resistance of the 1A meter.

One thing I noticed is that my dynamic load shows only 4.5V across it at 12A flowing. I know the supply isn't drooping; it's good for 70A. The wires are all pretty thick. That 1/2 volt drop at 12A is 6 watts, which is plenty of energy to make things hot. I just have to find where it's being lost. Could be the clips I'm using to connect to the strap. I'll remove the meter terminal nuts and go directly to the studs to see if that cuts down the resistive loss.

Bob M.

ncradioeng
Posts: 53
Joined: Wed Nov 18, 2015 3:58 pm

Re: RF Ammeter resistance

Post by ncradioeng » Fri Nov 02, 2018 8:02 am

I would try different straps - if the connections between the straps and the meter studs are not making a solid connection, then that is where the heat is coming from. That could also explain the drifting.

kcbooboo
Posts: 518
Joined: Sat Dec 10, 2011 4:53 pm
Location: East of the Mississippi

Re: RF Ammeter resistance

Post by kcbooboo » Fri Nov 02, 2018 8:47 am

All the connections seem to be tight. I had loosened and removed the nuts and washers and pulled the straps off the threaded meter studs but they sprung right back to where they were. This is the strangest metal I've ever encountered. It's almost like the spring inside a wind-up clock. Perhaps it's stainless steel. It may also be temperature-compensating. I reinstalled the hardware and it's good and tight, but I'm going to pull it off again and clip directly to the threaded studs.

I was told the meter seems to drift between 9.5 and 10A out in the field, which I'm not seeing here on the bench.

Bob M.

kcbooboo
Posts: 518
Joined: Sat Dec 10, 2011 4:53 pm
Location: East of the Mississippi

Re: RF Ammeter resistance

Post by kcbooboo » Fri Nov 02, 2018 10:38 am

I measured about 0.25V drop across the meter terminal studs with 12A flowing, so there's 3 watts to generate some heat. The metal straps are also losing some voltage but I can't be sure if the connection to them is good. They're very hard and the clips aren't making a good solid (scratch the surface) connection, at least not as good as I think is required to measure the resistance. I was getting unstable readings between 0.05 and 4 ohms on each strap.

I tried my 5A meter in the same test circuit (no straps involved) and there's absolutely no drift, plus the meter indication is 100% reproducible. It too heats up slightly after having 5A flow through it for several minutes.

Something I thought about though. A thermocouple is designed to produce an output voltage or current when it heats up, and it's doing that when DC (or RF) is flowing through it. When that input current goes away, the thermocouple is still hot, or hotter than the ambient temperature inside the meter case. Doesn't it still produce an output voltage or current due to that residual heat, and if so, wouldn't that cause the meter to deflect upward somewhat, rather than return to zero, with no input current? This might explain why I'm chasing my tail trying to get the meter to read zero with no input current under all situations. If so, I should set it to zero when it's cold, then not touch it again during or after use.

Bob M.

User avatar
Jim Sofonia
Posts: 146
Joined: Fri Jan 30, 2015 11:15 pm

Re: RF Ammeter resistance

Post by Jim Sofonia » Fri Nov 02, 2018 1:48 pm

I would recommend using a Delta TCT transformer and meter. No loss to your rf chain and more accurate.

ncradioeng
Posts: 53
Joined: Wed Nov 18, 2015 3:58 pm

Re: RF Ammeter resistance

Post by ncradioeng » Fri Nov 02, 2018 2:08 pm

It doesn't take more than a second or two for the thermocouple to cool down to ambient after the current is removed. A thermocouple works by using dissimilar metals in contact at different temperatures so no current is generated when all the parts are at the same temperature.

kcbooboo
Posts: 518
Joined: Sat Dec 10, 2011 4:53 pm
Location: East of the Mississippi

Re: RF Ammeter resistance

Post by kcbooboo » Fri Nov 02, 2018 4:32 pm

I would agree, a Delta CT makes more sense, but this is a meter that's swapped around into the J-plugs in the tuning houses of several towers, and Delta CTs are a lot more permanent, requiring one to disconnect the current-carrying conductor that passes through the CT to relocate it. They're great for a permanent installation though.

Thanks for the confirmation that the thermocouple would cool down rather quickly, although the back of this meter (bakelite case) does manage to stay warm for several minutes. I'm assuming the TC is mounted on some piece of metal that perhaps is retaining the heat a bit longer than a few seconds.

I've got a few more 20A meters arriving shortly to check out. Hopefully they don't have similar issues with zero position and repeatability. I can always create a calibration chart to go with each one if I find more than 2% error.

Bob M.

kcbooboo
Posts: 518
Joined: Sat Dec 10, 2011 4:53 pm
Location: East of the Mississippi

Re: RF Ammeter resistance

Post by kcbooboo » Mon Nov 05, 2018 2:58 pm

I tested some other (Weston) ammeters this morning. While three of them were about 50% off calibration, none exhibited the drift of the first meter I encountered.

I adjusted the 15A meter's zero adjust screw so it tracked within 2% from 6 to 15A. I set things up for 10.0A and connected the meter, which read 10.25A. It slowly reduced its reading until it got to 9.95A after 10 minutes, and it seemed to be stable there. When I removed the current, the meter indicated far below zero. I let it cool off for a couple of hours. When I went back and looked at it, the pointer was dead on zero. Applying current (the same 10.0A as before), it went right up to 10.25A and slowly fell to just below 10A. Removing the current let the meter fall below zero again. Even with just 10A flowing, the back of the meter as well as the connecting straps got warm to the touch.

I tried another, much larger 12A meter, and it read 9.90A and stayed there during the next 10 minutes. Removing the current, the meter fell right back to the same zero position it had been in previously. This meter did NOT get hot.

So there's something screwy with the 15A meter that makes it impossible to calibrate due to self-heating, which seems to also be causing the meter to indicate something other than 0.0A when no current is flowing through it.

Bob M.

ncradioeng
Posts: 53
Joined: Wed Nov 18, 2015 3:58 pm

Re: RF Ammeter resistance

Post by ncradioeng » Mon Nov 05, 2018 3:42 pm

Maybe it got a lightning hit.

kcbooboo
Posts: 518
Joined: Sat Dec 10, 2011 4:53 pm
Location: East of the Mississippi

Re: RF Ammeter resistance

Post by kcbooboo » Tue Nov 06, 2018 4:55 am

Shouldn't have happened. This meter is on a J-plug and is only used when fiddling with the ATU or making impedance measurements. It's out of the circuit and inside the xmtr/studio building hanging on the wall when not in use. I doubt the CE would have gone outside during a t-storm to measure base currents with this meter.

I guess they just don't last forever.

Thanks all for the comments.

Bob M.

ChuckG
Posts: 989
Joined: Tue Mar 11, 2008 11:12 pm
Location: Moo
Contact:

Re: RF Ammeter resistance

Post by ChuckG » Tue Nov 06, 2018 9:16 pm

Could be as simple as having been dropped at some point. It would be interesting to open it up if you haven't and see what there is to see.
<><><><><><><><><>
Chuck Gennaro
Central Wisconsin

kcbooboo
Posts: 518
Joined: Sat Dec 10, 2011 4:53 pm
Location: East of the Mississippi

Re: RF Ammeter resistance

Post by kcbooboo » Wed Nov 07, 2018 6:28 am

As for the original meter that changes with temperature, I gave that back with the caveat that the readings may not be consistent unless the meter is left running with the same 10A of current flowing through it for 10 minutes.

Three other 20A meters were way out of calibration, which is probably why they were put into a junque box. It's not economical to get them repaired or calibrated, so they were going to be thrown away. I grabbed them and will see if each one can be re-calibrated. Some time ago I got a Westinghouse 5A RF Ammeter going again by fiddling with the calibrating wire inside, about three inches of resistance wire between the thermocouple and the meter coil. I had to make it longer, so I threw in a length of #30 wire-wrap wire and kept cutting that back until the meter read the correct current. I will attempt that same fix with these three meters, although they are quite a bit smaller and I may have to pull the actual movement out to get to that calibrating wire.

Bob M.

User avatar
jthorusen
Posts: 11
Joined: Sun Sep 02, 2018 4:35 am
Contact:

Re: RF Ammeter resistance

Post by jthorusen » Thu Nov 08, 2018 7:45 pm

Greetings to the Forum:

kcbooboo writes:
This is the strangest metal I've ever encountered. It's almost like the spring inside a wind-up clock.
I'd be careful with this stuff. What you are describing sounds to me like Beryllium Copper alloy. Be is dangerous stuff... if you get any of the metal in a cut or scrape, it will take forever to heal and you probably should seek medical attention. If you abrade the stuff and breathe it in, it is as bad as Plutonium.

Unless the application requires the properties of BeCu alloy, I'd replace the stuff with copper strap. Just my $.02 worth; your mileage may vary.

Regards,
Jim T.
KB6GM

Post Reply