Pressure Reading

Like the thermocouple amplifiers, I chose a pressure sensor that would work with the I2C interface of the RPi.  The Honeywell MPRLS family of sensors fit the bill.  I chose the MPRLS0025PA00001A, which features a 24-bit ADC and a maximum pressure scale of 25 PSI.

 

Since we are working with a vacuum, we need a pressure sensor that will operate at low pressures.  The HIGHEST pressure we can use in a Freeze dryer is 612 pascals.  Atmospheric pressure is around 100,000 pascals.

We also need a sensor that reads absolute pressure.  Most common sensors read gauge pressure which is relative to atmospheric pressure.  Atmospheric pressure can fluctuate by more than the operating pressure of the freeze dryer. Gauge sensors are not viable for this project.

Differential pressure sensors measure the difference between two pressures.  Since we do not have a perfect vacuum to reference from, a differential sensor is no good either.

An absolute pressure sensor has an integrated pressure standard and its output is scaled so that a perfect vacuum will output a known value (typically zero.)

With an absolute pressure sensor, there is no negative pressure.  Atmospheric pressure is approximately 14.7 psi, when measured in absolute terms.

The transfer function for this device scales its full pressure range over a 24-bit scale.  However, it reduces resolution by 20% (10% on each end of the pressure scale.)  So, instead of scaling to 2²⁴ = 16,777,216 counts, it scales to 13,421,773 counts.  This leaves 1,677,721 counts for pressures that are over range and an additional 1,677,722 counts that are under range.

Even with the reduced resolution, this leaves a resolution of  536,871 counts per PSI.  This is complete overkill for the sensor involved, since it only has an accuracy of 0.25% (0.0625 PSI) but this same device can mount several different transducers with different output units and different pressure ranges.