Optical Scientific Inc
OFS 2000CW User’s Guide
P/N 1910-905
Rev. 03/28/13
21
3.4 Purge Air
The OFS 2000CW, being an optical measuring device,
needs to “see” clearly. The function of purge air is to keep
the interior atmosphere of the mounting nozzle clear, the
light path unobstructed, and provide an air barrier to slow
the rate of deposition on the window. (In cases of high flue
gas temperatures the purge can also prevent hot gases
from entering the nozzle.)
There are two methods of supplying purge air: Passive
and Active
Passive Purge
In applications where the internal pressure is
approximately negative 3 inches (WC) or more, and unchanging, then the purge fittings can be removed and
the natural inflow of outside air through the open holes themselves may be used to keep the light path clear.
NOTE:
This is no longer recommended as best practice. Passive purge can be effective and has been adapted by
users wishing to economize on extra equipment. It has been found to work adequately, but due to the
incidence of problems occurring from misapplication, and/or lack of control, it's use is not encouraged. The use
of Active Purge, as described herein, is recommended for optimum results.
Active Purge
It is recommended the user apply instrument grade
1
compressed air (nitrogen, or other available clean gas) to
the purge fittings. This purge air should be controlled through a rotameter. (See Figure 3.7) The nominal
volume to start with is 1 CFM per fitting. There are two fittings per spool piece and four fittings per OFS
system. So: 4 CFM supplied altogether; with 2 CFM going to the Transmitter side, and 2 CFM going to the
Receiver side.
The flange adapters each have two purge holes threaded for 1/8” NPT and are supplied with fittings sized for
¼” tubing. These must be oriented so that the two holes are at right angles (90°) to the direction of the flow.
This is very important:
the introduction of purge air flow aligned along the axis of stack flow will result in false
velocity reports
.
1
According to ISO 8573.1:
ISO Class 1-6-1 Industrial Grade Air: Efficient removal of solid particulates and oil. ISO Class 6 PDP or a 50%
(or less) Rh will be maintained.
Figure 3.6 Purge Air Detail
Figure 3.7 Rotameter
(flow meter)
Figure 3.8 "Natural" Air Purge
Figure 3.9 Compressed Air Purge
