The Vacuum Preconcentration System
Figure: Gas Collection, Preconcentration, and Vacuum System.
Figure: Gas Collection, Preconcentration, and Vacuum System shows the gas collection, preconcentration, and vacuum system. The heart of the vacuum system is a combination of diaphragm and turbomolecular pumps that handle two key functions of the sample handling process. First, the turbomolecular pump, backed by the diaphragm pump, is used to evacuate the spectrometer cell to an ultimate pressure of less than 1 mtorr. As described above, this low pressure is needed to support an operational pressure on the order of 10 mtorr. Second, the diaphragm pump is used by itself to pull air through the sample inlet and into a sorbent tube, where the trace constituents of interest are extracted. The conflicting requirements of system size (1 ft3) and vacuum performance (<1 mtorr) severely limited the available options for vacuum pump components.
The specific pumps selected for the sensor demonstration study were the Pfeiffer TPD 011 turbomolecular pump (208 in3) and the Pfeiffer MVP 006 diaphragm pump (116 in3). Tests of these pumps indicated that their performance exceeded our requirements. The ultimate pressure in the cell was measured to be better than 0.1 mtorr. The rate at which the air sample was flowed through the sorbent tube was measured to be 5.2 L/min, though the addition of a 2μm filter at the sample inlet reduced the rate to 1.2 L/min. Standard, off-the-shelf electronically controlled valves and pressure gauges (Pirani, baratron) also were included in the design for the demonstration system.
In the future, development of a miniaturized, portable sensor system will require a vacuum system that uses significantly smaller pumps and consumes less power. The Pfeiffer pumps are among the smallest on the market, yet they provide orders of magnitude greater performance (i.e., lower ultimate pressure) than is needed for a fieldable system. An ideal solution would be to incorporate a single, miniaturized mechanical pump, but none is available on the open market that meets the performance, size, and power requirements of this system.