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Baseline disturbance TCD detector - Sine wave behavior - multiple systems affected similarly

kevin.gallagher3

I am experiencing sine wave behavior on the TCD signal for 4 different GC systems (all 7820A). I've attached two images for reference. All systems use He for column and reference/makeup flow. All systems operate under the same testing method, and so have the same detector temperature. These images show the signal "at rest" as in with no sample running, with full column flow and full reference/makeup flow. The "waves" do not appear synced across the systems.

My working theory is that we have a leak in our He lines. This could explain the behavior and why all the systems are affected. We have a primary gas manifold with 3 tanks hooked up which supply the He for all 4 systems. However, I checked the manifold thoroughly and could not find any leaks. There were also not any leaks I could find in any of the 3 tanks. The common thread for all the systems is the He source, so this is what I have been focusing on.

I am posting the following question: Is there another factor I could have overlooked which could be the source of this behavior? Again, these are TCD detectors, so could it be some other environmental factor?

Thank you.

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  • +1

    Update: After some investigation, I believe I found the cause of the issue, so I will add it here in case it is helpful to someone in the future.

    I mentioned that there are 3 He tanks hooked up to our manifold. For a while there were only 2 tanks hooked up because we were waiting for replacement He. When we received our new tanks, the 2 old tanks were not swapped out for fresh tanks. An analyst instead hooked up a third tank and left the 2 old tanks open to the system. Since the gauge only measures the pressure from the most-filled tank, the gauge pressure was reading 2500 psi as is typical with full tanks, even though 2 of the tanks were in fact near empty.. The two old tanks emptied beyond a typical changeout level (we typically use 500 psi) after the system ran for some time, which would not be evident from reading the gauge. This would release any contaminant higher molecular weight gases which were pooled at the bottom of the tanks into the system, creating a "mix" of gases. This could cause the sine wave effect for a TCD, due to inconsistent thermal conductivity of the "mixed" reference gas.

    What was learned from this is that a multi-tank manifold should have all hooked up tanks changed at the same time, and not in a staggered fashion.

Reply
  • +1

    Update: After some investigation, I believe I found the cause of the issue, so I will add it here in case it is helpful to someone in the future.

    I mentioned that there are 3 He tanks hooked up to our manifold. For a while there were only 2 tanks hooked up because we were waiting for replacement He. When we received our new tanks, the 2 old tanks were not swapped out for fresh tanks. An analyst instead hooked up a third tank and left the 2 old tanks open to the system. Since the gauge only measures the pressure from the most-filled tank, the gauge pressure was reading 2500 psi as is typical with full tanks, even though 2 of the tanks were in fact near empty.. The two old tanks emptied beyond a typical changeout level (we typically use 500 psi) after the system ran for some time, which would not be evident from reading the gauge. This would release any contaminant higher molecular weight gases which were pooled at the bottom of the tanks into the system, creating a "mix" of gases. This could cause the sine wave effect for a TCD, due to inconsistent thermal conductivity of the "mixed" reference gas.

    What was learned from this is that a multi-tank manifold should have all hooked up tanks changed at the same time, and not in a staggered fashion.

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