The gas sample should be reasonably dry. If the presence of water vapour is or is not expected
it is recommended that the instrument is continuously switched 'ON' to prevent condensation.
The flow rate of the gas sample affects the response time (to change) of the measurement -
the measurement being on average the mix of the sample flowing through the cell. In practice
a sample flow rate of nominally 500 ml/min
1
provides satisfactory performance for any
instrument. The instrument is fitted with a flow sensor and the flow rate is indicated on the
display. There is a potentiometer and throttle fitted internally that allows this flow rate to be
reset if necessary.
Generally the sample gas will be pumped to achieve the required flow rate. An internal pump is
fitted in the instrument and can be switched on or off via the key labelled ‘pump’ on the front
panel. It is recommended that the pump is switched 'OFF' if the sample lines are being
attended to or the condition of the sample gas is suspect (for example it contains water).
The sample rate is factory pre-set for sample gas at ambient pressure.
The installation of pressurised cylinders must include suitable pressure regulators and carefully
controlled to avoid damage to pump or flow sensor due to over-pressure.
The instrument must be allowed to 'temperature stabilise' after first being switched on. The
display will show a thermometer symbol until the associated bench has reached operating
temperature. The instrument's enclosure plays a role in temperature control as well as
providing protection and the enclosure (top and front) should be securely in place before use.
Unless the instrument is only used occasionally it is advisable to leave the instrument switched
on, particularly over-night when reduced temperatures may result in condensation. In standby
conditions the sample pump should be switched off'.
1.6.3 The Oxygen Sensor in Use
As oxygen does not absorb infrared (it is diatomic), it's measurement is via a chemical oxygen
sensor. This operates in a similar way to a battery in that a voltage is generated between two
plates immersed in electrolyte. The sample gas is passed over a membrane which allows
oxygen molecules to pass through it into the electrolyte, stimulating the reaction, and resulting
in a voltage proportional to the oxygen concentration.
Typically the sensor is calibrated by adjusting for zero reading with nitrogen applied, and for
span using a cylinder containing 20.9% O
2
, though fresh air can be used for a quick check.
Both span and zero adjustments are made via front panel controls.
The sensor fitted has a long operational life and is very stable. If the output falls (i.e. the gas
reading is low), the sensor should be replaced. Sensor lifetime depends primarily conditions of
use (e.g. average sample oxygen concentration, sample flow rate, temperature), and so this
part is excluded from the instrument warranty. For typical applications, ADC recommends
routine replacement of the sensor every two years as part of the maintenance programme.
ADC (Sales & Service) Ltd. or their authorised local agents can supply replacement sensors with
the correct fittings and electrical characteristics. Fitting alternative parts may cause problems
and will invalidate the instrument warranty.
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