Depending on the application, it is often necessary to drill a small drain hole usually called a weep hole. This hole is located at the bottom when gases are measured, to allow the condensate to pass in order to prevent its building up at the orifice plate. When the fluid is a liquid, this hole is located at the top that gases can pass and gas pockets cannot build up.
An alternative to offsetting or re-shaping the bore hole of an orifice plate is to simply drill a small hole near the edge of the plate , flush with the inside diameter of the pipe, allowing undesired substances to pass through the plate rather than collect on the upstream side. If such a hole is oriented upward to pass vapor bubbles, it is called a vent hole. If the hole is oriented downward to pass liquid droplets or solids, it is called a drain hole. Vent and drain holes are useful when the concentration of these undesirable substance is not significant enough to warrant either an eccentric or segmental orifice.
The addition of a vent or drain hole should have negligible impact on the performance of an orifice plate due to its small size relative to the main bore. If the quantity of undesirable material in the flow stream (bubbles, droplets or solids) is excessive, an eccentric or segmental orifice plate might be better choice.