Technical Description: Inlet
Rapid Multi-Stream Sampler
C-4
Prima PRO & Sentinel PRO Mass Spectrometers User Guide
Thermo Fisher Scientific
connections are on the outside of the cabinet. All electrical,
electronic, and moving mechanical parts are internal.
In the case of the Prima PRO, the selected gas flows around a loop
contained entirely within the sample tube body and returns to the
main RMS exhaust chamber via the inner tube of the sample tube
assembly. A small fraction of the gas in the loop is sampled by the
system capillary, which is built in to the sample tube body, and flows
on to the mass spectrometer inlet probe via the transfer line. Flow
through the capillary is in the range 7 to 10 atm cm
3
/min, and the
pressure in the transfer line is approximately 1 mbar. Compared to
the quantity of gas in the sample tube, the flow rate is very high,
resulting in a rapid response to sample gas composition changes.
Temperature control of the sample tube assembly (see
later in this chapter) means that flow through the capillary is very
stable with minimal dependence on ambient temperature.
In the Sentinel PRO, all of the selected gas flows from a port on the
sample tube body to the inlet probe where the gas is sampled through
a membrane assembly. The excess gas returns to a second port on the
sample tube body and is returned to the common exhaust chamber
via the inner tube of the sample tube assembly.
In both cases, the returning gas passes through an orifice that
generates a small pressure difference. A differential pressure sensor
is used to measure this difference and thereby give an indication of
the gas flow rate. The flow measurement can be used from within
GasWorks to generate alarms that can alert to low flow conditions in
either sample or calibration gas supplies.
The seal between the stator and the sample arm (at the point where
the selected sample gas is collected) consists of a spring loaded seal.
The spring loading of the seal means that wear is not a significant
issue, and no degradation of performance is expected over a period
of several years. Furthermore, the flow of the selected gas through
the filter, sample arm, sample tube, and pressure sensing orifice
generates a slight overpressure within the sample arm. As a result,
any leakage at this seal will be out from the sample arm and into the
common exhaust. Cross contamination of the selected gas from other
streams is thus avoided.
The main drive shaft for the sample arm is sealed through the stator
by standard rotary shaft seals. These seals have integral spring
loading for wear compensation and will give many years of service.
The sample tube is sealed within the main shaft by similar (smaller)
seals.
Connection to the
Mass Spectrometer
Seals
