Thermo Scientific Prima PRO User Manual page 109

Hazardous Area Operation
Purge Operation
A-12 Prima PRO & Sentinel PRO Mass Spectrometers User Guide
Combining this figure with the CDP flow rate shows that any such
leak will be diluted to 0.25%, which satisfies the dilution ratio
requirement. This is, however, the overall dilution ratio in the
enclosure, and close to the leak point there is region where the leak
must be considered as not fully diluted. As there are electrical items
in close proximity to the point of release, it is necessary to treat these
as potential sources of ignition, and additional precautions must be
taken.
A vacuum ejector pump, driven by a small fixed fraction of the purge
air inlet, generates a suction flow that is connected to a port on the
RMS sample tube bracket. This is arranged such that suction, the
flow is > 5 l/min, is applied between the sample tube bracket and the
pressure transducer PCB, and any leakage from the pressure
transducer is drawn into the ejector airflow and diluted.
The suction flow from the ejector is also used to assist in the initial
purging of some internal volumes within the enclosure, specifically
the turbo pump controller, the signal amplifier box, cold cathode
gauge, and RMS stepper motor.
Sentinel PRO Ex
In the Sentinel PRO Ex, the situation is different in that flow through
the sample loop (the sample containment system) is much higher
compared to the Prima PRO Ex (typically 5 l/min), and there are
connections within the loop that must be considered as potential
leaks. It would require an unacceptably large flow of purge air to
dilute any such leak to a safe level. The sample flow on a Sentinel
PRO Ex system is induced by an external sample (suction) pump,
connected to the common exhaust of the multi-stream inlet system
(RMS). As a result, the sample loop is actually maintained under
negative pressure and therefore, in normal operation, there can be no
leakage out of the loop into the purged enclosure.
In the event of a break in the sample loop, connection with the pump
will be lost and sample will no longer be drawn down the sample line
to the pump. It is necessary to consider that the sample line could be
in a zone that is pressurised and could continue to drive sample down
the line to the break in the sample loop (albeit at a much reduced
flow rate). To provide protection, the sample loop is monitored by a
pair (for redundancy) of vacuum switches that will detect either a
break in the sample loop or a failure of the sample pump. Both result
in a pressure rise.
Note:
In the event of a loss of vacuum in the sample loop, the RMS

drives immediately to the designated safe inlet, Port 1.
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