Gordon Stelter's post in MMD 21.03.12 presents an interesting approach
to a valve calibrating machine incorporating more modern-day electronic
components. While it sounds feasible, it would have been beyond 1920s
technology.

The salient patent [Ref. 1] describes a purely mechanical apparatus
that on cursory inspection uses a classic wedge pneumatic to provide
the motive force for pressing the metal valve cover into the valve
body until the desired volumetric flow through the valve was achieved.
It's unclear how fast the device would have operated, but my guess is
that several "cycles" per minute would have been possible.

I have not yet studied the details of the patent to determine how
that flow parameter was measured or displayed, but, again guessing,
it probably resembles a mechanical/pneumatic analog of an electrical
"Wheatstone bridge" which is used (mostly) to compare two voltages and
adjust one relative to the other (or standard) value.

More analysis to come, stay tuned.

John Grant

 [   Ref. 1
 [ https://patents.google.com/patent/US1684015A/en?oq=1684015 
 [   Patent US1684015, Fig. 1
 [ https://www.mmdigest.com/Attachments/21/03/14/210314_152833_US1684015-1.jpg 
 [   Patent US1684015, Fig 2&3
 [ https://www.mmdigest.com/Attachments/21/03/14/210314_152833_US1684015-2.jpg