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.
On Fri, Jun 30, 2017 at 11:55 AM, Steve Tuzeneu <firstname.lastname@example.org> wrote:
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.Â 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