The makers of our test meters recommend turning the vacuum level up one to one and a half inches of mercury. The test meters will drop the vacuum level approximately this amount because of the increased friction from additional hose length and turns inside the meter. You increase milking time. You break the letdown pattern. These can also give an inaccurate somatic cell count because the SCC attaches to the butterfat.
Low vacuum makes the test meters measure on the low side.
In these economically hard times it becomes even more important to meet with your equipment representative, and to be prepared to change the vacuum on test day .