10
Measuring sensors
The device uses pre-calibrated gas sensors from the long-life Flex-Sensor series for the measurement of oxygen (O
2)
and carbon monoxide
CO (hydrogen-compensated H
2
), nitrogen oxide (NO), nitrogen dioxide (NO
2
), sulphur dioxide (SO
2
), and hydrogen sulfide (H
2
S). Oxygen
(O
2
%) measurements are performed using an electrochemical sensor that acts as a battery; this sensor is subject to a loss of sensitivity
over time.
Measurements of toxic gases (CO, SO
2
, NO, NO
2,
H
2
S) are carried out using electrochemical sensors that are not subject to natural
deterioration because they are inherently devoid of oxidation processes.
The measuring sensors are electrochemical in nature and consist of an anode, a cathode, and an electrolytic solution that depends on the
type of gas to be analysed. The gas enters the sensor through a selective diffusion membrane and generates an electrical current
proportional to the absorbed gas. This current is measured, converted to a digital reading, the temperature is compensated, before being
processed by the microprocessor and shown on the display.
The gas must not be at a pressure that could damage or destroy the sensors. The maximum pressure is ± 100 mbar. The response
times of the measuring sensors used in the analyser are:
•
O
2
= 20 secs. at 90% of the measured value
•
CO(H
2
) = 50 secs. at 90% of the measured value
•
CO = 50 secs. at 90% of the measured value
•
NO = 40 secs. at 90% of the measured value
•
NO
2
= 50 secs. at 90% of the measured value
•
SO
2
= 50 secs. at 90% of the measured value
•
H
2
S
= 50 secs. at 90% of the measured value
It is recommended to wait 5 minutes (but not less than 3 minutes) to obtain reliable analytical data.
Toxic gas sensors, if exposed to concentrations greater than 50% of their measuring field for more than 10 consecutive minutes, may
exhibit a ±2% drift and an increase in the reset times.
Should this be the case, before switching off the analyser, it is advisable to wait until the measured value is less than 20 ppm by drawing
in clean air. If there is an automatic autozero solenoid valve, the unit automatically performs a purging cycle and stops when the sensor
returns to a value close to zero.
Sauermann certifies the accuracy of the measurements only under cover of a calibration certificate issued by its laboratories or by another
approved laboratory.
Dilution of CO
One of the characteristics of electrochemical sensors for CO measurement is the need to have a very long auto-calibration if they have been
in contact with a very high gas concentration (higher than the highest range) for a long time.
The CO sensor is therefore protected in the instrument by an automatic dilution system that extends the measuring range of the sensor
without overloading it.
The dilution system makes it possible to constantly maintain the effective CO sensor and ensure it is ready to operate correctly, even at very
high CO concentrations.
The dilution system also extends the measuring range of the CO sensors:
•
Up to 100,000 ppm for the CO 8000 ppm full range sensor
•
Up to 250,000 ppm for the CO 20,000 ppm full range sensor
This makes it possible to manage the use of the sensor and to continue measuring samples without having to interrupt the measurement.
Pressure sensor, piezoresistive sensor, temperature compensation
The analyser is equipped with a piezoresistive internal sensor and temperature compensation for measuring the draught (vacuum) of the
chimney, for leak testing of pipes, and potentially for other measurements (gas pressure in the network, pressure loss through filters, etc.).
The measuring range is from -10.00 mbar to +200.00 mbar.
Any potential drift of the sensor is zero due to the instrument's autozero system, however, this should NOT be done with the measuring
probe inserted in the chimney.
Any pressure applied to the sensor above ±300 mbar can cause permanent deformation of the membrane,
causing irreversible damage to the sensor itself.
Types of fuels
The analyser is supplied with a pre-memory that holds technical data characteristics relating to the most well-known fuel types. Using the
optional PC configuration program, it is possible to add fuels and corresponding coefficients so that up to 32 fuels can be defined in
addition to those that are already predefined. For further details, see Appendix B.
Gas extraction pump
The pump inside the instrument is a diaphragm pump, powered by the instrument, which provides an optimal flow rate of the analysed
gas sample. An internal sensor measuring the flow and thus allowing:
•
the pump flow rate to be kept constant
•
the efficiency of the pump to be checked
•
the degree of clogging of the filters to be checked
