By private email a week or so ago I asked Robbie some questions about
pneumatic roll reading.  I did not go through MMD since I was not sure
that there was still general interest in the subject.  There obviously
is, so I now will now continue the topic in the MMD.

First of all, on the subject of the pneumatic-electrical interface,
here is yet another design which Jim Heyworth and myself have come up
with.  One of its nice features is that it does not need a vacuum chest
-- it operates in the open air.

Think of a normal pouch, about 3/4" diameter.  Vacuum is feed to the
pouch through an orifice bleed.  There is also the usual tracker bar
connection.  Stuck to the centre of the pouch is a small (1/4" diam.)
magnet.  Stuck to the top of the magnet is a small brass disk upon
which is connected a small wire, thus making the lower contact of the
switch.

Located over the pouch is a steel screw, with the distance between the
steel screw and the magnet and brass disk being adjustable.  Connected
to this screw is a wire, thus making the upper contact.

In the normal position (i.e., the tracker bar hole closed) the vacuum
through the bleed sucks the pouch down and holds the electrical contact
open.  When air from the tracker enters the pouch, the vacuum in the
pouch is reduced to zero thus letting the magnetic attraction of the
magnet to the steel screw lift the pouch and make the electrical
contact.  We have run a number of very promising preliminary trials.

Because of the nature of magnet attraction this switch almost has
a snap-action feel, along with a static measured hysteresis of about
1" water gauge.  Also, because of the magnetic attraction between the
contacts, there appears to be no bounce.  (We have yet to put an
oscilloscope on it.)

Compared to some of the other very ingenious ideas this is low tech
in the extreme, and costs only pennies per switch to build.  If the
switch proves to be workable, the electrical contacts could be gold
plated.

There are a whole number of other questions/issues which I want to
eventually ask about, but my time is running out and I must get back
to the real (?) world.  My wife is returning home after a month-long
vacation, and she will not want to see this house in its present state.

Regards to all

Phil Dayson

 [ Another fine idea!  The magnet provides the desirable hysteresis.
 [ I hope that (after the house and the wife return to normal ;) you
 [ can compare the hysteresis with a few ordinary valves.  -- Robbie