Volume Verification for Colorimetric Detector Tube Pumps
A detector tube pump should be periodically checked for leaks as part of any well-run detector tube-monitoring program. A leaking pump will cause detector tube readings to understate the concentration level of the target gas, which can make an unsafe area appear safe. All brands and types of detector tube pumps have a field leak check procedure in the operating instructions that allows a semi-quantitative leak check to be run in about one minute. These tests are so simple to run and so important that they are usually recommended daily. A field leak check typically consists of placing an unbroken tube in the pump, activating a pump stroke and watching for some telltale motion that would indicate unwanted air intrusion.
A more accurate performance test using a bubble meter will indicate quantitatively the volume of air drawn in a single pump stroke. This test can also indicate a slow leak that might not be indicated by a field leak check. It is a simple procedure that can be run with minimal equipment. We recommend that a volume verification using a soap bubble meter be conducted annually or after any repair or maintenance work.
A soap bubble is initiated by raising the soap solution container to momentarily touch the rim of the bubble meter opening. The bubble is zeroed by drawing slightly on the pump handle until the bubble is positioned at the zero mark. Then the pump handle is re-set and a full stroke is drawn. A good working pump will displace the bubble to the 100 cc mark within +/- 5 cc. (Note that +/- 5 cc is an operational specification for the pump. The manufacturing specification for the pump is +/- 2 cc or +/-2%).
It should be noted that a dry bubble meter will require some priming in order to get a bubble to rise the entire height of the meter. This is done by wetting the inner wall of the meter with a series of soap bubbles. Each new bubble will rise a bit higher before breaking until a bubble can survive the full distance. This can be done using the detector tube pump as per the test procedure, but a motorized pump (e.g. a personal monitoring pump) will save time in this step.
It should also be noted that using the same detector tube test after test and allowing it to set over time without capping will cause the silica gel substrate inside the tube to saturate with moisture and swell. This will add resistance to the airflow, and the system will draw continually slower as the moisture absorption persists. It is wise to replace the tube occasionally. Further, a detector tube should not be used for a field measurement once it has been used for this test.
A kit, part number 830-1010-01 is available from Sensidyne to conduct this volume verification test. It includes the bubble meter, a stand and clamp, soap solution and the necessary tubing. The kit will work with Sensidyne pumps AP-1S and AP-20S as well as any other detector tube pump that is of 100 cc per stroke volume or less.
Volume verifications run at least annually are an important part of a good detector tube sampling program. A quantitative test like a bubble meter provides has the added advantage of producing data that can be included in a maintenance log to verify that a pump is operating as intended.