Toxic (H2S) gas detectors calibration and repair
General
Toxic gas detectors are used in areas where personnel may be subjected to toxic gas during their normal course of duties.
H2S detectors are typically sited at 0.5 m or less from the ground or below (underneath) potential gas outlets such as pig receiver doors but should above splash zone. H2S is slightly heavier than air and tends to sink / gather near to the ground. As a rule H2S detectors are usually mounted below knee height.
Recent additions to Das Island are H2S smart detectors which have a yellow front panel with relevant information posted on it, yellow is an accepted standard color for hydrogen sulphide detectors.
The selected model for explanation in this course is a smart sensor, which is a Sulphide gas monitor with integral digital readout. It is connected to the plant indicating and shutdown equipment (F&GCP) by means of a screened, armoured cable. The unit draws its 24V(nominal) power from two of the cable conductors and output signal of 4-20 mA on the third conductor which represents the gas concentration of 0-100% ppm.
No user adjustments are required. Calibration values are stored in iternal memory. The entire electronics module is fully encapsulated in compliance with international standards. Calibration parameters are tested by the smart sensor's advanced software before the acceptance of such devices. Any errors detected are displayed on the digital readout by means of an appropriate fault code.
The measuring ranges of this model are normally 0-20 ppm, 0-50 ppm or 0-100 ppm.
Principle of operationThe sensor is a solid state, adsorption, diffusion type element. It utilises Hydrogen Sulphide's ability to strongly adsorb onto a very selective proprietary metal oxide semiconductor as the basis for measurement.
The semiconductor element (resistor) is located on a substrate which is temperature controlled by the integral sensor electronic circuitry using heater and thermistor.
When air containing Hydrogen Sulphide gas diffuses into the semiconductor resistor R (sensor) through the flame arrester it causes the electrical resistance of 'R' to decrease. This change in the resistance is monitored by the electronics, which gives a local readout and 4-20 mA signal proportional to the H2S gas concentration. When pure air enters the sensor the semiconductor resistance returns to its clean- air value(0 ppm = 4 mA).