From: bill.sloman@ieee.org   
      
   On 27/12/2025 1:13 am, Liz Tuddenham wrote:   
   > Don Y  wrote:   
   >   
   >> On 12/26/2025 3:27 AM, Liz Tuddenham wrote:   
   >>> Don Y  wrote:   
   >>>   
   >>>> Any pointers regarding deployment of pressure sensors measuring   
   >>>> air pressure?  I'm looking at nominally an inch of water   
   >>>> (pressure/vacuum)...   
   >>>   
   >>> Washing machine/diswasher water-level switches?   
   >>   
   >> No, I'm looking at pressures in air handling systems.   
   >   
   > They operate on air pressure.  The rising water level closes off the   
   > bottom end of a chamber; thereafter any further increase in water level   
   > causes an increase in air pressure within the chamber.  A very flexible   
   > rubber diaphragm presses aginst a spring and eventually operates a   
   > microswitch.   
   >   
   > By substituting a movement sensor for the microswitch (and with the use   
   > of a suitable spring) you could make a proportional air pressure sensor   
   > of good sensitivity which held its calibration to a reasonable degree of   
   > accuracy.   
   >   
   > I have also used a differential air pressure sensor that used a thin   
   > silvered tensioned plastic diaphragm as a concave/convex mirror with a   
   > light emitter and sensor on each side, connected to a balancing circuit.   
   > It was exceptionally sensitive but not particularly robust or   
   > repeatable.   
      
   Slightly stretched beryllium copper membranes are pretty robust.   
   They have to be stretched enough (well within their elastic limits) to   
   keep them flat, and clamped between suitably dished mounting plates to   
   prevent them being stretched beyond their elastic limits by large   
   over-pressures.   
      
   Capacitative sensors can be used to track the movement of the membrane   
   accurately and with good stability. They've been commercially available   
   since the 1950's, but they weren't cheap.   
      
   Fast modern integrated circuits will have made the electronics cheaper.   
      
   For small pressure differences you can use charged plates on either side   
   of the grounded diaphragm to compensate for the pressure difference   
   across it. A big enough voltage difference to compensate for atmospheric   
   pressure or anything close to it would break down the gaps.   
      
   --   
   Bill Sloman, Sydney   
      
   --- SoupGate-DOS v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)