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 Post subject: Radio Frequency Ammeter
PostPosted: Mar Mon 05, 2018 4:09 pm 
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Joined: Feb Thu 22, 2018 4:25 pm
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Can a "radio frequency ammeter" be used as an AC ammeter?

A brief google search tells me they were built to be more accurate in the field than other ammeters, but I don't see if they can be used as an AC ammeter.

I see quite a few RF ammeters on ebay in convenient ranges.

Thanks.


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 Post subject: Re: Radio Frequency Ammeter
PostPosted: Mar Mon 05, 2018 4:18 pm 
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Joined: Feb Sun 02, 2014 9:13 pm
Posts: 1836
Location: Roanoke, VA
It depends upon the meter. Some were relatively accurate at line frequency but some were not. The only way to know would be to run a calibration check.

Dale H. Cook, Radio Contract Engineer, Roanoke/Lynchburg, VA
http://plymouthcolony.net/starcityeng/index.html


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 Post subject: Re: Radio Frequency Ammeter
PostPosted: Mar Mon 05, 2018 4:27 pm 
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Thanks Dale. I guess the best bet is to see if there is online info for the specific ammeters before I bd on anything.


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 Post subject: Re: Radio Frequency Ammeter
PostPosted: Mar Mon 05, 2018 8:51 pm 
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Joined: Jan Mon 28, 2013 9:35 pm
Posts: 1088
Location: Santa Rosa, CA
Take a look on the meter face. If is says 'calibrated at 60 cps'
it'll obviously be OK for what you want. Most I've seen are
like this.
Steve

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 Post subject: Re: Radio Frequency Ammeter
PostPosted: Mar Mon 05, 2018 9:28 pm 
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Location: Roanoke, VA
zarco wrote:
If is says 'calibrated at 60 cps' ... Most I've seen are like this.

Most I've seen are not. I have about a dozen RF ammeters on hand and none say that. Bear in mind that mine are all thermocouple ammeters used at AM broadcast stations.

Dale H. Cook, Radio Contract Engineer, Roanoke/Lynchburg, VA
http://plymouthcolony.net/starcityeng/index.html


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 Post subject: Re: Radio Frequency Ammeter
PostPosted: Mar Mon 05, 2018 9:28 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 7885
Location: Long Island
Analog RF ammeters are thermocouple devices. The RF is passed through a heating element which is connected to a thermocouple. The DC from the couple is registered on a microammeter. They will work on anything from DC up to their rated frequency, only limitation is on AC frequencies of a few Hz. There heating and cooling of the couple on successive AC half-cycles could give rise to pointer jitter. Nearly all will work fine on 60-Hz, in fact that is one of the preferred ways to calibrate them as it avoids errors at RF frequencies due to capacitive effects.

Thermocouple meters have one big drawback. Most cannot tolerate overloading for any length of time; if this occurs the heater may burn out or they may go out of calibration. 100% overload for 10 seconds is a typical rating. Problem is, there is a lag between when the power is applied and when the couple warms up, and the heater could burn out before the pointer pegs. Unlike other kinds of meters, it is possible to destroy a thermocouple without even knowing it was in distress. For this reason one has to be careful when buying these meters used. There are a lot of them out there with burned out thermocouples.

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 Post subject: Re: Radio Frequency Ammeter
PostPosted: Mar Mon 05, 2018 10:42 pm 
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Thank you all for the information. When I started the day I didn't expect to learn something :D


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 Post subject: Re: Radio Frequency Ammeter
PostPosted: Mar Tue 06, 2018 12:14 am 
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Location: Maryland 20709, USA
zarco wrote:
Take a look on the meter face. If is says 'calibrated at 60 cps' it'll obviously be OK for what you want. Most I've seen are like this.
Not necessarily true.

You can put 10 amps 60 Hz through the meter and have it read 17 amps.

Calibration only means it reads what it is supposed to read at the specified excitation.

- Leigh

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 Post subject: Re: Radio Frequency Ammeter
PostPosted: Mar Tue 06, 2018 3:09 pm 
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Joined: Jan Mon 28, 2013 9:35 pm
Posts: 1088
Location: Santa Rosa, CA
As usual lots of opinions here. Took a look at a few of the RF
ammeters I have and 5 out of 6 had the 'cal'd at 60 cps'
on the meter face. All were WWII vintage and either GE or Westinghouse.
These are old meters and they should be checked before
using them. However I've used these at 60 cps in many applications
and they work fine.
Steve

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 Post subject: Re: Radio Frequency Ammeter
PostPosted: Mar Tue 06, 2018 4:21 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 7885
Location: Long Island
There are some bolometer and thermal bridge type electronic RF meters where calibration is inaccurate below a certain frequency, typically in the range of 100-kHz or so. Of course such instruments will give wildly inaccurate results if you try to use them on 60-Hz--if they even work at all.

From the context of the question it appears that the question pertains to thermocouple RF ammeters of the panel variety, and those have no problem opertaing at 60-Hz. There is one other consideration to be aware of, however. That is, in many cases the heater assembly was external to the meter. This was so the heater could be placed in the RF line and the meter could be elsewhere. Many of the WW-2 surplus RF ammeters were made that way. Often--but not always--the meter will say something on the face about an external heater or thermocouple, or an internal one. If an external couple is required but it is missing, all you have is a DC microammeter, not an AC or RF ammeter.

One clue is, the rear plates of RF ammeters with internal thermocouples are often deeper than ordinary meters, and they were sometimes made of brown Micalex rather than ordinary black Bakelite.

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 Post subject: Re: Radio Frequency Ammeter
PostPosted: Mar Tue 06, 2018 4:40 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1998
Location: Oswego, NY, USA
Chris108's advice is something I could have used 25 years ago, when we had our WWII vintage Lionel O-scale train set up for our children to enjoy. I had a couple Weston (i.e., very good) RF 20A, thermocouple-based ammeters monitoring the current; but in short order, we burned out two of those meters, from the train start-up current surges. These classic analog RF Ammeters are high quality "works of art", but they need to be used correctly....I learned the hard way.

Fred


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