Well, I was trying to be helpful. Instead, I am condescending.

I have been a ham longer than 41 years, and I learn things all the time.
Sometimes, I forget simple ways to do things.

The Bird is not a model of accuracy. Accuracy is specified as a % of full
scale. I do not have my books handy, but I recall it is 5% of full scale.
If my recollection of 5% is correct, that means that it could be off (oh,
oh, here is simple math) 125 watts at full scale or 125 watts when you are
reading 500 watts. Using another Bird is not a good method of setting the
accuracy.

You have to use the dvm to check out the dummy load, anyway. If the dummy
load is off in resistance, the Bird wattmeter will not read correctly -
since it is assuming that the load is 50 ohms. I have checked dvms and
found even the cheap ones are quite accurate - much higher than the accuracy
specified for the Bird. I have also check dummy loads and found they are
often off. A friend asked me to come over because he was measuring 3200
watts out of his amplifier and thought it was high. It was - the dummy load
was also high.

If you are interested in an accurate reading slug, then calibrating it
against another Bird slug does not make much sense - since that might be
inaccurate. Forget the surplus rf voltmeter and simply measure the heat
rise of the dummy load. That will be an exact measurement of the power
going in and yield much more accurate readings than the Bird. It will
simply take a thermometer and a bit of math. I will not detail how to do
it, because that might be condescending.

I have a number of Bird wattmeters and slugs. I use them for quick and
crude measurements - because they are handy. Before I could afford the
Birds, I designed and built my own - and calibrated them. In fact, I wrote
an article for QST in 1973 on how to build your own using ordinary plumbing
parts. They were hand calibrated and as accurate as any Bird meter.

If, you want your meter to be calibrated like Bird would do it, send it to
them.

Colin K7FM