130-0047 Issue 8
This is particularly important for instruments fitted with L
sensors, irrespective of the positioning of flow meters etc.
Page 6 of 18
It is important to fit a pipe of at least 300mm length to the
sample vent/outlet connection to prevent back diffusion of air.
It should be noted that back diffusion can occur even when the
sample is flowing, and is due to the difference in oxygen partial
pressure between the sample and air. The pipe should be
short enough and of sufficient bore to prevent significant back
pressure when the sample is flowing. Standard 6mm or ¼"
tubing is ideal.
The sensor must not be pressurised beyond 0.25BAR gauge or less
than (-)250mm water gauge. Rapid pressure changes could
damage the sensor, and pulsation will give an erratic display.
3.6 Sensor installation
The sensors are shipped with the leads shorted and the sample
tube plugged with a rubber bung. This ensures that a sensor is
purged down to low levels and ready to be applied to the
measurement of low levels of oxygen (less than 1000ppm) with the
minimum of delay. The bung should be left in place until the system
is ready for the sensor to be fitted. If it is intended to apply the
sensor immediately to the measurement of low oxygen
concentrations then the sample should be left flowing during the
sensor change operation.
The bung should be removed from the
sensor and the sensor coupled to the holder as rapidly as
possible. THIS ASSUMES THAT THE SAMPLE THAT WILL
INEVITABLY ESCAPE WILL NOT CONSTITUTE ANY SORT OF
HAZARD. IT IS THE RESPONSIBILITY OF THE USER TO
ENSURE THIS.
To fit the sensor, first slacken the compression nut on the sensor
sample tube. Remove the rubber bung from the inlet tube, and put
the inlet tube into the compression fitting. Tighten the compression
nut -
do not over tighten,
½ to ¾ of a turn beyond finger tightness
is adequate. Finally connect the sensor signal leads to the terminal
block - see Fig. 2 and section 3.4 for details.
Page 7 of 18
WARNING -
Do not leave an L sensor exposed to oxygen
concentrations above 5% connected to the instrument for extended
periods as this will cause premature depletion of
the sensor.
Instead, leave the sensor disconnected or apply a sample of low
oxygen concentration. This applies whether the instrument is on or
off. When fitting a sensor intended for immediate use at
concentrations of 1000ppm or less it is advisable to:
A) have low level gas flowing through the system prior to fitting the
sensor.
B) unseal and fit the sensor as fast as possible.
3.7 Alarm contacts
The contacts are rated for signal applications. Where AC voltages
are to be switched, the use of suitable snubbers across the contacts
is recommended to ensure reasonable contact life. Typical values
are 0.047µF to 0.22µF with 47 ohms to 100 ohms in series with the
capacitors.
N.B. Mains rated capacitors must be used.
