ChemStation CDS Data Acquisition and Single GC Runs

[mk200]

Hello, my gas lab recently upgraded to Chemstation CDS and I'm having some trouble understanding how the sequence template and the master method work together. We employ two methods--one for measuring the CO2 mixing ratio of our gas samples and one for baking out our GC column. Pretty simple.

Previously we never used sequences. We used (and still use) an undergraduate technician to manually inject gas samples into our GC. When I Queue Method from the RunControl drop-down menu the Method is added to Sequences in the Active Queue window and the method runs perfectly--except the Data Files are overwritten on every single run!

Do I need to create an Easy Sequence to save each single run with unique file name (I need to export these as a CSV file too)?

Additionally, with this version of Chemstation a single press of the F5 key will no longer initiate gas injection via the rotation of our pneumatically controlled gas sampling valve. Any thoughts?

Thanks!

[GasMan]

Here is the information that I give to engineers regarding the setting up of valves with ChemStation.

Configuring valves on the 7890.

Valves can be configured into the 7890 several ways, with some confusion. The choices are as follows:

Gas Sampling
Switching
Other
MultiPosition
Not Installed.

Multiposition.

Used to configure a valve as a stream selection valve. Only ONE valve on the GC may be configured as a multiposition valve.

Not Installed

Used to uninstall a valve.

Other

Used to define something that is not actually a valve. For example the valve control could be used to switch a relay which controls some other piece of equipment. This still acts as a valve control for all intents and purposes.

Switching

It’s used to control a valve. Valve can be controlled from the run table or from the keyboard. Turning the valve on has no effect on the GC status (see Gas Sampling). Valve will NOT turn to it's original position at the end of the run. To do this you will need a Run Table entry.

If you have a valve hardware configured as a gas sampling valve but configured in the 7890 as a switching valve, you cannot start the run remotely from the GC ChemStation.

Gas Sampling

Now this is where the confusion starts. If a valve is configured as a Gas Sampling valve, the following will happen.

• The GC can be started remotely from the GC ChemStation providing the injection source has been set to "GC Valve".
• An automatic sequence can be set up.
• The valve will return to the OFF position at the end of the run.
• If you press "START" on the GC, the GC program will start but the Gas Sampling valve will NOT inject the sample.
• If Valve 1 is configured as a Gas Sampling valve and from the GC keyboard you enter Valve 1 ON, you will inject the sample AND start a GC run. This can be a nuisance if you are trouble shooting, as all events in the Run Table will also execute.
• The switch on time of the Gas Sample Valve will NOT appear in the Run Time Table.
• You cannot stagger injection times if you have two or more valves injecting samples.


So what is the best way to set up?

The 7890 has eight valve control functions. It is rare that all are used up. Normally valves 1 to 4 are used to actually control valves on the GC. This leaves valve controls 5 to 8 free. So configure all of the valves in your valve system as switching valves and configure one of the unused positions as a Gas Sampling Valve. This will give the following advantages.

• You can control the GC remotely from the GC ChemStation.
• You can set up a sequence.
• If you press "START" on the GC, the GC will start AND your sample will be injected.
• If you enter "Valve 1 ON", valve 1 will switch on but the GC will NOT start a run.
• All switching times will appear in the Run Table.
• You can stagger injection times.


Gasman

[Bigbear]

Don't know about valves, but I do know that there is a check box on the sequence start page that will over write files. Make sure it's unchecked.

[Consumer Products Guy]

When I Queue Method from the RunControl drop-down menu the Method is added to Sequences in the Active Queue window and the method runs perfectly--except the Data Files are overwritten on every single run!

Under Run Control, click "Run Method" (the first choice) and enter in a different filename for each injection. If I understand what you posted, you're making this way more complicated than it needs to be.