Principle of operation of Agilent EPC?

[Eluter]

Anyone have a simple description of how the Agilent EPC works, the flow controller for gas flow settings to an FID etc.?

I get the feeling it simply measures pressure drop across an controlled needle valve or orifice. The flow measurement is only correct if the downstream flow is relatively unimpeded.

In fact, it seems if the flow is totally blocked, it'll still report a flow as it'll adjust the down stream pressure to the setpoint for unimpeded flow. Kind of like when you simply adjust a gas regulator that is valved off. But in Agilent EPC case, you can drop the EPC flow setting and the pressure is relieved on the low pressure side of the orifice and "go to zero flow" whereas you have to vent the gas regulator to drop the pressure. If you follow what I mean....

[antonk]

AFAIK Agilent EPC is PWM (pulse-width modulator) for pressure.
Then installed calibrated restrictor (pneumo-resistance) which defines flow.
Machine controls the pressure.

[Eluter]

That's what I thought, the EPC just measures pressure drop across a calibrated orifice. But how does it fail when it's inaccurate, that's my question.

[daniel_aut]

It's a diaphragm based sensor(2 variable orifices):imagine if one of them has a small blockage then their diameter is screwed.
Now, in the advanced user guide is a pretty straigthtforward procedure on how to maintain a healthy EPC:make sure that 'autozero septum purge' and 'auto flow zero' are selected.For the pressure sensors see 'zero all pressure sensors in all modules'.