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Re: Bird Meter Calibration
To: CRTech <crtech@crtech.org>
Subject: Re: Bird Meter Calibration
From: Alan Kilgore <wrvm.engineer@gmail.com>
Date: Sat, 01 Jul 2017 08:10:33 -0500
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Hello, John, Steve and other readers.

You can send the slug in for its own calibration. Unless you have had some damage to the line section or meter and your readings no longer seem normal, the slug is usually all that needs to be re-calibrated. But, it's not cheap. I instead bought replacement slugs. Of course, this assumes that you are talking about the legacy slug sample system.

The slugs for measuring the output of our 30kW Tx seem to change calibration far more often than for any other location. Being left in the active line section seems to be a symptom. I have not figured out why but this is our only transmitter near this power level.

Any slugs that stay on the shelf seem to remain stable. So, I learned to keep a new set of slugs on the self as my "standard" to periodically compare to the active pair of slugs. When the active pair drifts significantly from typical readings, I pop the cover off of the active slugs and adjust the internal potentiometer for the active slug to duplicate the reading from the slugs used as the on-shelf standards. Then I press the cover back in place on each active slug, being careful to observe orientation.

You can check calibration of the meter movement by observing the full scale reading at its stated rating (example: FS=100uA). Apply that amount of current across the meter terminals with the cable to the line section disconnected. I calculate the resistance needed in series with a regulated DC Voltage and use a fixed resistor in series with a trim pot. I adjust the pot for exactly full scale on the Bird meter movement. Then I use a trusted digital multimeter to measure the DC Voltge from the power supply (while still connected to Bird meter) then measure the series resistance between the power supply and the meter movement (after disconnected from Bird meter). Ohms Law (I=E/R) tells me the actual current for FS so that I can determine the meter's measured error:

I[meas] = measured current
I[fs] = stated FS current on meter

% Measured Error = (I[meas] - I[fs]) * 100% / I[fs]

Alan Kilgore, CPBE
WRVM Chief Engineer

At 01:07 AM 7/1/2017, you wrote:
Are you relying on the Bird for remote metering? We used one mainly for high VSWR alarms & protection. Bird told me they couldn't guarantee a calibration unless I sent in the whole thing, including the line section. There was nothing to replace the line section, so unable to send in for calibration.

A Bird Wattmeter is really only measuring current. Current is related to power, of course, assuming the impedance is exactly 50 Ohms, or whatever original specs might be.

John S.

On Fri, Jun 30, 2017 at 11:55 AM, Steve Tuzeneu <stuzeneu@gmail.com> wrote:
Hey Guys:

I'm looking for your thoughts about how often you get your Bird (in line) watt meter calibrated at your transmitter site(s).

The factory says they recommend sending in the meter once a year for calibration. We don't do it quite that often, but I don't want to say until I read your thoughts.

How long do you wait before sending yours in for calibration? And why?

Steve Tuzeneu,  CBT

References: Bird Meter Calibration
(Steve Tuzeneu <stuzeneu@gmail.com>, 30 Jun 2017 15:55:28 -0000)
Re: Bird Meter Calibration
(John Stortz <ka4flx@gmail.com>, 1 Jul 2017 06:07:24 -0000)
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