Probe tip filters are designed to protect the ability of the phx21 to deliver a very precise sample to the FID chamber. These flows are measured in cubic centimeters per minute (tiny). At the proper flow rate it would take ~10 hours to fill a basketball – and the pressure would be so low that you would never be able to dribble it!
The flow precision is accomplished by the use of two meticulously pressed sintered metal restrictors: one for the VOC Sample and the other for the air that we use to combust the sample. Sintering is the process of taking loosely packed metallic powder and applying pressure to the powder in such a way that the molecules of the metal begin to press together. Just before this happens, the sintered metal will form a precisely structured barrier. This barrier can control with extraordinary accuracy how many molecules of air, or VOC, are able to pass through. These restrictors are positioned just downstream of the pump. They are difficult to install; impossible to clean; and difficult to replace.
They are VERY important.
The 1 um probe tip filter is designed to capture a substantial amount of particles. Take a look at the photo on the left to get a sense of how much the probe tip filter is able to suck up and still keep running. When the filter gets “too full” the phx21 will prompt you to replace it – No problem. Conversely, if you operate the phx21 without a probe tip filter for even the shortest amount of time , then the smallest sample of particles can pass into the unit, through the pump and into one or more of the restrictors. The photo indicates how little it would take to ruin the restrictor. Keep in mind that the particles displayed in the photos are black pepper. Remember: the restrictors are impossible to clean and very difficult to replace.
Contamination can happen in the blink of an eye: while the unit is warming up, the dust in the room or from a nearby window can contaminate the unit.
This amount of contamination can pass through your sample probe and pump, then lodge in the restrictor in a matter of seconds!