Chromatography Forum

Does anyone have any suggestions as to why the pressure setpoint would be fluctuating on my Agilent 7890A GC? Running the GC in constant flow mode and the pressure setpoint is fluctuating by only 0.001 psi (between 9.765 and 9.766). When I set the method to run at a constant pressure, it has no problem holding the setpoint constant. Set point fluctuation seen no matter what injection conditions I use. Thanks.

Perhaps the inlet or oven temperature is fluctuating? The EPC will try to adjust in contstant flow mode as the oven temp changes. Contant pressure mode would not have this issue.

A co-worker had this problem recently - the inlet insulator was getting kicked off when he closed the over door. Every one was convinced the EPC was damaged, but it went awat after correcly securing the insulation.

It's a feature of 7890 with such high pressure reading resolution. Such small fluctuations should be neglected.
6890 has much lower pressure resolution (0.1 psi) and you really have no information what's happening on 2nd and 3rd decimal place.

It's probably because the real number it's working on is on the border of the two it's displaying. The pressure setpoint is calculated by the GC to give the flow you want so probably has more decimal places than are displayed. As there is no variation in constant pressure mode I'd say there is nothing to worry about.

0.001 psi is basically nothing, the 6890 only shows you one decimal for the pressure readings.

http://www.chem.agilent.com/en-US/Produ ... check.aspx here is a leak test you can do. There are a few different ways to do it, you can also just press service mode on the front panel, go into diagnostics, and run the automated leak test.

You have a good point aldehyde but the question here is about the setpoint - if the GC is wandering around with the setpoint then it would point to a software or electronic problem, which is then separate from any performance issues. I think that the lack of problems seen in constant pressure mode point to it just being a number that is on the knuckle of what the display can show.

You have a good point aldehyde but the question here is about the setpoint - if the GC is wandering around with the setpoint then it would point to a software or electronic problem, which is then separate from any performance issues. I think that the lack of problems seen in constant pressure mode point to it just being a number that is on the knuckle of what the display can show.

With the GC in constant flow mode the pressure is not strictly a set point - the flow is set and the required pressure is calculated as a function of the column dimensions, the type of carrier gas and the oven temperature. If the required pressure just happens to be where the rounding of the display jumps from one value to another, then the display will fluctuate (quite possibly without a corresponding change in the actual pressure).

Peter

You have a good point aldehyde but the question here is about the setpoint - if the GC is wandering around with the setpoint then it would point to a software or electronic problem, which is then separate from any performance issues. I think that the lack of problems seen in constant pressure mode point to it just being a number that is on the knuckle of what the display can show.

With the GC in constant flow mode the pressure is not strictly a set point - the flow is set and the required pressure is calculated as a function of the column dimensions, the type of carrier gas and the oven temperature. If the required pressure just happens to be where the rounding of the display jumps from one value to another, then the display will fluctuate (quite possibly without a corresponding change in the actual pressure).

Peter

EPC controls only one parameter: pressure.
Column flow value is the consequence of applied pressure.
Using Flow and current temperature (which alters gas viscosity) the pressure is calculated and then passed to EPC electronics.

Precisely. The flow is the setpoint specified by the user, but the pressure is the one the instrument works on.

Hi Rod,
this pressure is nothing, you needs to check the retention time of compounds, if it varies, then you needs to check set leakage, or EPC, bt EPC seems to OK,

bye take care