From: blockedofcourse@foo.invalid   
      
   On 12/26/2025 7:13 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.   
      
   Yes, I know that from tinkering with old washing machines.   
   But, the mechanism was large (the size of a lemon).   
      
   > 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.   
      
   But, there are semiconductor devices already that do this for a   
   buck or two (in tiny quantities).  They would be considerably   
   easier to deploy -- in large numbers.  E.g., one typically   
   instruments an AHU in many places as the air is "processed"   
   within -- each observation point lets you close a loop   
   or infer something about that process.   
      
   > 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.   
      
   The same problem (as above).  You can use a chilled mirror to   
   detect dewpoint -- or, by changes in capacitance of polymers.   
   Big difference in size, cost, precision, durability, etc.   
      
   When you move away from thinking of sensors as needing "calibration",   
   you can approach problems with more flexibility.   
      
   --- SoupGate-DOS v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)