It's important the device has enough energy to adequately achieve a reliable
separation and chute deployment. As all the energy of the device is contained within
the gas pressure, it’s important the device contains enough gas pressure for the
deployment event to ensure a successful outcome.
The pressure status of the device is now acquired by a full bridge strain gauge mounted
to the outside of the gas cylinder with the output fed into an INA122 instrument
amplifier whose output is in turn fed into a central microprocessor. An illustration
and theoretical equations behind the strain gauge and associated strains encountered
is detailed within the Stretch & Strain spreadsheet.
Previous methods of detecting the pressure status involved a homebrew pressure
switch mounted within the chamber. Not only was this considerably more onerous, it
was also limited to an on/off indication at a specific pressure. This was later superseded
to a quarter bridge strain gauge mounted to the chamber fed into a cheap Chinese
amplification module BF350-3AA/1.5AA available on all the various online market places
with access to China. Unfortunately, these modules weren't very stable - there was
dramatic drift within the 1st 20 minutes of power up. However, this provided a useful
proof of concept and learning curve to what is now the settled solution.
Accuracy (with the full bridge strain gauge & INA 122) is within ~3 Psi with a
slapped together 50 Psi step lookup table. No doubt this accuracy can be improved
with more care and more lookup values, but the ultimate objective here is to get
an approximate pressure indication from the device to detect if:
(A) Is there enough pressure within the device to adequately ensure a successful
deployment
(B) Is the device leaking gas and is the leakage at an unacceptable rate
(C) can also be used to detect the successful deployment of the device (if required)
