A "gas guard" in a lab setting isn't a standard term. It's possible you're referring to something else, or the term is specific to a particular laboratory or application.

To help me understand what you're looking for, could you please provide more context?

For example:

* What kind of lab are you working in? (chemistry, biology, physics, etc.)

* What type of experiment are you conducting?

* Can you describe the device or setup you're referring to?

With more information, I can provide a more accurate answer.