Chromatography Forum

Dear colleagues,

From this morning I'm facing with a problem that I haven't encountered before. For the first time today morning, I used the new air gas cylinder. The previous empty air cylinder was replaced with this new one couple of days before the New Year's Day . Since then the GC attached to the new air cylinder hasn't been used.
When I tried to set the air flow (in the FID parameters) to 450 mL/min, the display of the instrument (PE Clarus 500) showed only cca 380 mL/min air flow. The hydrogen flow was OK (45mL/min).
First thing that I've done was to check for air leaks along the copper tubing from the cylinder to the GC. Everything was OK, no leaking was observed.
Then I started troubleshooting this problem by searching similar problems described in the instrument manuals and this forum also. But I didn't find anything similar to this.
Clarus 500 GC has an electronically pressure control. I wouldn't suspect to the electronics fault because this GC has been recently (a month ago) qualified and verified by PE service.
I did some tests and this is what I got:
1. When I set the FID air flow to 450 mL/min, the actual flow was 377 mL/min
2. When I set the FID air flow to 400 mL/min, the actual flow was 377 mL/min
3. When I set the air flow to 370 and 300 mL/min, the actual flow was 370 and 300 mL respectively.

Then I suspected that maybe some occlusion has occured down the gas lines or (for which I am not convinced) the air cylinder valve cannot support higher flows than 377 mL/min.
The gas supplier is the same. The gas cylinder valve is fully opened and the manometer at the exiting tube shows a pressure above 300kPa.

Everything else seem to work properly, but I'm still confused from this problem. Have anyone ever had similar problems to this.
Your advices are greatly appreciated!

Thanks,
Zoran

Hello Zoran

Did you get a flow of 450 ml/min with the previous air cylinder ?

When you changed the air cylinder did you turn down the pressure on the cylinder regulator and then turn it up again ?

What length and diameter of tubing do you have between the cylinder and the GC ?

Are there any other instruments using air from the same supply line as the GC ?

Peter

Hi Peter, thanks for your reply

Yes, I got air flow of 450mL/min with my previous cylinder (when the flow began to decrease below 450mL/min I checked the pressure of the old cylinder, and I was sure that it was empty, so I had to change it with a new (present) one)

When you changed the air cylinder did you turn down the pressure on the cylinder regulator and then turn it up again ?

I am not sure because the service guy did the change, but if I not mistaking I think that he only turned down the cylinder valve when changing the air cylinder, not the pressure regulator valve (with manometers, attached to the cylinder)

What length and diameter of tubing do you have between the cylinder and the GC ?

The length is approx. 5 meters and the tubing is 1/8 inch in diameter

Are there any other instruments using air from the same supply line as the GC ?

No. The line from the cylinder goes directly in to the GC

What could be the problem?

Thanks again

I could never imagine how simple was the solution for my problem. After several hours reading and trying to realize what is going on, finally I came up to the solution

As I mentioned previously this appeared to me like pressure/flow problem.
Now I am certain about that. The service guy who changed the air cylinder, after he had installed the new cylinder (I can recall that) he turned down and then turned up the pressure regulator valve, which proportionate between the pressure in the cylinder and the pressure in the gas line.
This morning the manometer showed a pressure cca 43 psig (300kPa) of the air in the gas line. When I tried to increase the pressure in the gas line (by turning up the pressure regulator/proportionate valve) simultaneously I observed increase in the actual flow of air in FID. When I set the flow to be 450 mL/min, the actual flow raised from the old value to the desired value (450mL/min). At that moment I read the pressure in the gas line from the manometer. It showed pressure between 50 and 60 psig.
So now I know that the required air flow in FID must be accompanied with higher pressure in the gas line

Although this is a very simple solution (maybe stupid) I think that it's worth posting it

Best regards

You have learned a valuable lesson. The manual of any GC may specify in the section on pneumatics the required pressure of gases that enable proper operation. To be conversant with the required specifications of your GC will help you in trouble shooting problems that may occur. I am sure that in the future you will not forget this episode and will apply your new experience when needed.

An experienced operator is more valuable than an inexperienced one. A lesson HR should remember that when they consider the value of their employees.

Congratulations to you in discovering the problem and correcting it. It always gives one a good feeling to solve problems when they arise.

best wishes,

Rod

Now take a marker and mark a line on your gauge so you can see at a glance what your pressure should be.

Yup, that's exactly what I'm going to do immediately

Thanks