pressure always 0.01 below setpoint

[MichaelVW]

This is a new (to me) 6890.

If I set the injection port to 10 psi, it hovers around 9.99 psi. Set at 20 psi, I get 19.99 psi. (This is in the software - there aren't actually that many units on the instrument's display).

I wouldn't really care except that it won't run - injection port pressure not ready, detector (ECD) pressures not ready (they are set to 60.0 and continue to read at that pressure). .

I've been hunting for a leak but haven't found anything. I assume that's what it is because that's always what it is, but... any other thoughts?

[dblux_]

I doubt if this can help but I would try zeroing EPC first.

There is something like "initialize" in the 6890 config (access from front panel keyboard). Worth trying.
Is cylinder regulator set to sufficient pressure ?

[Peter Apps]

What happens if you make the set point 9.99?

[MichaelVW]

What happens if you make the set point 9.99?

I get 9.98!

I tried zeroing the EPC but that didn't work. I believe the supply pressure is near 100 psi.

[dblux_]

Then what if you change units from psi to bar ?

(Options>Keyboard&display>pressure units)

0.01 psi difference in bar is 0.000689 and this may be below resolution of GC.
That's about decimal places - worth trying - may be GC will truncate part of them and 0.01 psi difference expressed in bars will become 0.

[AICMM]

MichaelVW,

You zeroed the EPC for pressure or flow? Because for pressure it has to be disconnected from the supply.

Best regards,

AICMM

[MichaelVW]

I zeroed both flow and pressure. I also tried using different units, but the issue was the same.

I wondered if it was a quirk of the software (GC Chemstation B.04.03 [16]). I borrowed a computer with MSD Chemstation E.02.02.1431 and used that to communicate with the instrument.

Setpoints / Actual:
Pressure: 14.5 psi / 14.4 psi <--??
Total Flow: 64.5 ml/min / 64.4 ml/min

Splitless mode
Purge Flow to Split vent: 60.1 ml/min at 0.75 min
Gas saver off

There are two ECDs with 60 ml/min of N2 for purge. Only one is hooked up to the column, the other is just plugged and has the temperature turned off.

Column is set to constant pressure
pressure: 14.5 psi / 14.5 psi
flow: 2.1 ml/min / 2.1 ml/min
avg velocity: 43 cm/sec
post run: 14.504 psi <--??

So the pressure still seems to be slightly lower than it should be. It will inject a sample now, though. But I'm still not sure what's going on.

[James_Ball]

If you disconnect the computer software and just control through the front panel does it do the same thing?

I have one computer that any time you are editing a method and hit the Apply button it will reduce the flow setting by 0.1ml/min. Hit it again and it drops again. If you just set the flow and hit OK and save it is fine.

Some of these do some strange things especially if you have upgraded the computer above XP.

But this does sound more like a hardware problem on the instrument itself. Do you have an EPC board from another GC you can put in there and test?