Agilent 7890A GC Pressure and flow problem

[Stef261]

I'm hoping someone can shed some light on this.

I'm having some issues with my GC. Here is my setup
Purged Pack inlet (no auto-sampler)
Capillary column (60m or 30m x 0.53 x 0.50)
TCD detector
Chemstation B.04.02 with A.01.14 firmware on Windows XP.

The problem is the same in the front and the back inlet: when I put the system on Pressure Constant Mode and I ask 2mL/min the system gives me 3.074 psi which is correct but it says my Flow is 0mL/min. I take my flowmeter and measure 2mL/min. So in theory I could still use it and get the correct data. The panel is just reading wrong.

When I put the system in Constant Flow Mode and I ask 2mL/min it says it's giving me the right flow but the pressure shoots up to 8psi and the flow that I actually read with my flowmeter is 6mL/min.

I would really rather use the system in Constant Flow but I can't… I've tried dealing with it on my own but I wasn't getting anywhere so I called Agilent and they sent the technician (good thing the GC is still under contract).

We checked for leaks, no leaks
We changes the back EPC, no change
We changes the Logic Board, no change
We upgraded the Firmware to A.01.15 , no change

The next step in changing the power and analog board!

I'm thinking of upgrading my Chemstation to B.04.03 but I'm not convinced that it would help since I have another identical GC on the same computer and its working fine.

Any ideas ?

[GasMan]

Do not use the packed inlet in constant flow mode if you are using capillary columns. Just set the packed inlet to work in constant pressure. The important step is to configure your columns correctly. Configure column1 to the length, internal diameter and film thickness. Then give the inlet and detector that you are using and then set the flow for the COLUMN, not the inlet. The 7890 now knows which inlet you are using and will set the pressure of that inlet to give you the correct flow. Repeat for your second column.

Gasman

[Stef261]

Do not use the packed inlet in constant flow mode if you are using capillary columns

May I ask why? I've always used this system in Constant Flow and it's never caused any problem?

My columns are configured correctly (length, diameter and thickness) and that is why I measure the correct flow coming out when I am in Pressure Constant even if the reading on the panel says 0mL/min. My other 7890A connected to the same computer and the exact same setup works perfectly well in Constant Flow?

[GasMan]

The flow in capillary columns is actually controlled by pressure. The flow you want is calculated by setting the pressure knowing the length, internal diameter of the column, the visosity of the carrier gas which also relies on the temperature of the carrier gas. If any of these values are incorrect, the ACTUAL flow will not be the indicated flow in the column. Pressure sensors are more accurate over their entire range than flow sensors, and it is probably true to say that any sensor is usually not so accurate at it's low or high end. The flows in capillary columns are low compared to packed columns and you will therefore find that a flow controller at 30 ml/min (packed columns) will work better than at 2 ml/min. Having said that, if you had a flow controller designed to give a maximumflow of 5ml/min, then it would probably work OK for your 2ml/min.

If you are using a PLOT column, you should calibrate the column, as the film thickness is rarely what is on the box...........

Sumarizing, your COLUMN can be in constant flow mode, but the device supplying the gas to that column must be in PRESSURE mode.

Gasman

[Stef261]

Ok, I get where you are coming from. I understand that in this system the pressure is giving and the flow is calculated and the precision of the Flow when it is Constant Flow is relative.

My usual methods use a 4mL/min column flow, 23mL/min reference gas and 10mL/min make-up gas. These last 2 are not giving me any problems and I have tried other values of column flow with the same discrepancies.

If you go back to the basics though and say PV=nRT, this mean that when you have a temperature program and the temperature goes up in a Constant Pressure mode the flow and velocity will go down. In Constant Flow the pressure will go up with the temperature and so will the average velocity and that is much better situation (for peak shape and overall results).

I really appreciate your input but I’m not looking to change my setup just fix the machine so it works the same way that when I got it and the way that the Agilent application specialists recommend using it.

[Peter Apps]

As a diagnostic test, set constant flows of 2, 4, 6, 8 , 10 ml min and record the panel indication and the actual flow. That will at least show whether there is a consistent bias in the panel flow reading, or whether it is just stuck on zero when the flow is controlled.

No matter how you have the gas control configured there is clearly something seriously wrong when the flow reads zero for a 2 ml/min flow.

Peter

[GasMan]

Stef261,

You are correct about temperature programming. Here is the misunderstanding. In ChemStation you set the COLUMN to be in constant flow mode. When you set this so, the system will automatically change the pressure to keep the flow constant as the temperature changes in the oven. To do this, the device supplying the carrier gas MUST be in pressure control mode otherwise you will have the situation where you are trying to change the pressure on a device which is trying to control a flow. The packed inlet has two sensors, a flow sensor and a pressure sensor, but only ONE can be used at a time for control purposes. If you have the packed inlet in FLOW control mode, the flow sensor is used for CONTROL and the pressure sensor is just a readout. Conversly, when in PRESSURE control mode, the pressure sensor is used control and the flow sensor is just readout.

Have you gone through the process of zeroing the pressure and flow sensors for the packed inlet?

Gasman

[Stef261]

Sorry for being so thick Gasman! I figured out what you are saying… I've always used it in constant flow on both pages (inlet and column). That's the training I got when we bought the machine, the guy made a point of telling me to make sure that both pages are in the same mode!

Now I did change the inlet to Pressure and left the column to Flow. This is reflected in the digital panel. Alas no change to the flow I am reading on the panel and software (still 0). Like I said my other GC is set to constant flow on the inlet and the column and all the readings are correct.

If done extensive testing since I bought those GC and the constant Flow (inlet and column) gives much better chromatograms then Pressure constant (inlet and column).

When you say: "Zeroing the pressure on flow sensors" I'm not sure what you mean. I don't have a no hole ferrule to do a Pressure decay test.

I did as Peter suggested put the GC in constant pressure (inlet and column) and ask for a specific flow. The Pressure adjusts itself and the flow read with the flowmeter is correct but the flow read on the machine and software is wrong.
If I ask F=2 actual flow reading is 0
If I ask F=4 actual flow reading is 2
If I ask F=6 actual flow reading is 4

I think I'm just going to let the Tech work it out and keep using my Varian 3300 in the meantime. They are so much more reliable than the new technology!!!

[Peter Apps]

You have a consistent -2ml bias on the flow measurement, which is strong evidence that the EFC needs to be re-zeroed. There should be a procedure in the operators manual, but since you have a technician on hand get them to show you how to do it. I can't help but mention that this should have been one of the things that the technician did before the ritual of swapping hardware got underway.

Peter

[Stef261]

This has been a fruitful morning…

When I got in this morning I noticed that my He cylinder was really low, which is weird since I changed it last Friday. I Snoop the heck out of my GC and I found 2 leaks in the front and back EPC gas connections. The tech used is leak machine to check for leaks and assured me all was good but I guess he missed those! I corrected the problem and now my machine is giving the right flows by flowmeter in constant Pressure and constant Flow mode. The readouts were still wonky though.

I read up in the manual on zeroing the sensors and tried it. It's better but not quite there yet. So I have some more questions…

When I do the Flow sensors, gas has to be flowing but should I put the machine is P or F constant?

When I do zeroing of the pressure sensors it says to disconnect the gas supply line (not the valve because it could leak). Does that mean that the pressure in my machine as to be 0? I tried closing the valve (it doesn't leak!) and the pressure stays stable and I Zero and the machine gives me an error (saying I am over the calibration limit!).


Stéphanie

[GasMan]

To zero the pressure sensors, you MUST remove the pressure to the inlet at the rear of the GC. It is also important to wait until the pressure reading becomes steady, which indicates that any pressure within the system has bled away.

The flow sensors can be set up to zero at the end of every run. To do this, press the Option key on the front panel, then choose Calibration. Scroll to the inlet you wish to work with and press Enter. 'Auto flow zero' is the first line of the display, set this to ON.

Gasman

[Stef261]

Thanks Gasman, my Auto Flow Zero was already On and I did the gas sensors Zero correctly this time.

Right now with the Inlet at P and the Column at F (like you recommended) I'm asking for 4mL/min and 5,39psi (that's for a 60m x 0.53 x 0.5 SB-wax column at 40ºC). The readout on the software and the panel is 5.39psi and 3.5mL/min but I am measuring with my flowmeter 4mL/min.

The training that I received goes contrary to what you are recommending so I did some tests with a temperature program (Inlet=P and Col=F) and I measure and read the whole way and you are correct in saying that the pressure rises and the flow stays constant… Like I said that is not what I was though and I find it very weird that I've been using Inlet=F and Col=F with perfect readouts and results for 4 years (still using it right now on my other GC). So I'm quite confused and don't know what to tell the tech tomorrow when he comes…

If constant flow is Inlet=P and Col=F does that mean that Constant pressure is Inlet=F and Col=P?



P.S I've been reading the Chemstation manual and it states that the Inlet is to be set at P if the column is defined and at F if the column is undefined. The Constant Flow or Pressure comes from the Column page... I hate it when I'm wrong

[GasMan]

Stef61,

Basically only use constant flow with PACKED inlets if you are using packed columns. Your tech could be used to using a capillary inlet with capillary columns where the flow into the capillary inlet is flow controlled. If there are problems with your tech, tell him/her to contact me at Gasman41@gmx.de

The misunderstanding with many people is the use of the packed inlet. If you are using capillary columns, the flow is ALWAYS actually controlled by pressure. The difference then between constant flow and constant pressure with capillary columns is that in constant flow mode the pressure will be automatically adjusted to keep the flow constant as temperature changes, but in constant pressure the flow will reduce as the temperature rises as there is no pressure adjustment.

If constant flow is Inlet=P and Col=F does that mean that Constant pressure is Inlet=F and Col=P?

No

Constant flow is Inlet=P and Col=F and Constant pressure is Inlet=P and Col=P


Gasman

[Stef261]

Thanks for all the help, not only is my problem solved I have a much better understanding of my machine !

[Peter Apps]

Good result ! To celebrate go out and buy yourself a leak seeker - using Snoop and similar liquids on GC plumbing can contaminate the whole gas system.

Peter