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One controller called the primary is used to
provide the pressure source. While acting as the pressure source
the controller is called the ‘master’ controller. At the same time
the secondary ‘slave’ controller can refill and centre itself ready
for when the master controller exceeds its volumetric limit.
When this happens the secondary controller
takes over providing the pressure and thus becomes the master. The
primary controller becomes the slave and so can refill/empty and
re-centre itself. When this is complete the primary controller
resumes pressure control and becomes the master again and the
secondary controller re-centres.
The secondary pressure controller is only used as the master
temporarily (to provide pressure or volume change) when the primary
controller exceeds its volume capacity and thus needs to
refill/empty and re-centre itself.
Careful switching sequencing in the software ensures that valves are
energized for the minimum period necessary, and remain unenergised
for most of the test. The configuration also ensures any pressure
calibration differences between the primary and the secondary
controller are compensated for.
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