This spare test lid is for the 6102 Micro-Bath, because, sometimes you need a lid with holes drilled in it and sometimes you need a lid that is completely sealed. For calibration, it is convenient to drill a pattern of holes into a test lid to keep the bath sealed during calibration for optimal stability, and to ensure probes are properly spaced from one another and go into the bath vertically. However, you also want to have a lid that prevents spilling when you are carrying the bath from the lab or shop to the test site, so we recommend keeping a spare lid to use during testing. That way you can have ideal test conditions and a clean work environment.