In the early days of instrumentation, most instruments were totally pneumatic. Exceptions were from the electrically powered recorders for the various inputs used mainly for registration of process variables.
Some of the recorders were for accepting pneumatic signals and registered this on running paper rolls. The transmitters were mostly of an output range of a pneumatic signal range of 3 to 15 psig.
Practically all control instruments were ranged in the same way. There were simple instruments with one input like pressure, temperature, level and so on. But for control systems, the various pneumatic signals had to be computed and then outputted in a standard range of 3 to 15 psig.
If for instance, for large control valves, a higher signal range was required, a so-called pneumatic booster instrument was used.
Depending on the pressure range of the booster, a higher supply pressure was required. Pneumatic control elements had the facility to operate with PID control. The Proportional, Integration and Derivative values could be set by moving levers in the controller.
For the first time, two pneumatic signals could be fed into a controller and the output would then be sent to a controlling instrument like a valve.
Later, and in particular in the steam boiler systems for generator drives, the controllers were equipped to receive up to five variables, i.e. temperature, pressure, level, generator load and generator speed. Famous instruments of this type of control were the "Sorteberg" pneumatic control relay. This was a very sophisticated piece of pneumatic control equipment and widely used in the steam turbine industries.
Initially, electric control instruments were scarce and very expensive. But in due course a number of companies started to produce various pneumatic-to-electric, and also electric-to-pneumatic instruments.
The standard range of most of the electrical transducer instruments was 4 to 20 mA. This range of 4 to 20 mA signals was in those days the standard for control systems.
In the oil refining industries there were so called side stream analysers to determine the values of the various components in the stream. They usually gave their output signals in a voltage range of 0 to 10 Volt DC, which is sometimes created from a 4 to 20 mA signal via a resistor.
In the modern instrumentation industries, most of the signals produced are voltage and current values. For control purposes those signals are then operated on in electronic control components. Those control units can be as sophisticated as, for example, electronic plant computers.
To fault find in these type of systems requires instrument technicians with a good knowledge of electronics. This requires the technicians to be well trained and educated in this sort of equipment.
The Institute of Instrumentation, Control and Automation (IICA) can help and point anyone interested in the right direction to obtain this knowledge.
By John Woortman