Detection and Diagnosis of Common Faults of Gas Chromatograph

There are many types of gas chromatographs with different properties. Gas chromatograph mainly consists of two systems - gas circuit system and electric circuit system. The gas circuit system mainly includes pressure gauge, purifier, pressure maintaining valve, flow maintaining valve, rotameter, six-way injection valve, sample injector, chromatographic column and detector. The electric circuit system includes stabilized voltage power supplies for various power consuming parts, temperature control device, amplifier circuit, automatic sample injection and collection device, data processor, recorder, workstation data acquisition card and other electronic devices.

To analyze and diagnose faults of the chromatograph, we must firstly get familiar with the working process of gas chromatograph, and the two major systems – gas circuit system and electric circuit system, especially the structure and function of the parts and components of both systems. The chromatograph has a variety of faults, and a certain fault may be caused by diverse reasons. Therefore, part check, namely, exclusive method, must be adopted to narrow the fault scope. Faults of gas circuit system mainly include leakage, poor gas quality and unsteady gas pressure and flow of a variety of gases (especially the carrier gas). For example, if the baseline always drifts down, that is, the "electrical level" value gradually decreases to negative, it is most likely caused by carrier gas leakage. Therefore, various connection parts should be examined to confirm if there is leakage. If there is no leakage while the baseline still drifts, the failure may lie in electrical system. Analyzers are able to find out and remove faults of gas circuit of the chromatograph; however, it is not easy for them to remedy the faults of electric circuit unless they have certain knowledge about electronic circuits and understand the host wiring diagram and the electrical schematic diagram of each system (especially the wiring diagram). These diagrams clearly indicate relations between control units and controlled objects, and specifically mark the number and destination of each connector lead wire. It is very convenient to check the electric circuit and identify faults according to the diagrams. The faults of chromatograph’s electric circuit system generally lie in the temperature control system and the detection amplification system; certainly, power supply for each system is not excluded. The major loop of the temperature control system (including column temperature control, detector temperature control and sample injector temperature control) is composed of thyristor and heater strip. The heating power changes with the variation of conduction angle of thyristor; as a result, temperature changes (constant or inconstant). It is auxiliary loop (or temperature control circuit) that controls the changes of thyristor conduction angle; the auxiliary loop includes platinum resistor (thermosensitive element) and linear integrated circuits.

We can see from the above mentioned that if faults lie in the temperature control system, firstly, check whether the thyristor fails, heating wire fails (broken or short circuit), or platinum resistor fails (broken or short circuit) or has the problem of bad contact. Secondly, check other electronic components of the auxiliary loop. Common faults of the amplification system include that ion signal lines are affected with damp or disconnected, high resistance switch (i.e. sensitivity selection) is affected with damp, and integrated operational amplifier (such as AD515JH and OP07) has worsened performance. The troubleshooting of chromatograph should consider both local parts and the entirety. Every consequence has a reason. Make clear the direction of circuits, gradually exclude reasons that may cause the result, and narrow the fault scope. For example, if the baseline flutters constantly or has loud noise, it is advised to disconnect the amplifier’s signal input line firstly to observe the baseline. If the baseline returns to normal, then the fault must lie in the gas circuit or temperature control unit instead of in the amplifier and processor (or recorder); otherwise, the fault must lie in the amplifier and processor (or recorder). This exclusive method is very practical in failure checking.