Traditional instruments are still the most common form of information device in the control room. In modern control rooms, however, more and more information is being transferred to VDUs. Traditional instruments may be divided into the follow­ing subgroups with regard to their areas of use:

1. Instruments with associated control devices:

Control regulation instruments—The instrument is read, and if necessary the operator adjusts the machine.

Instruments for setting up—Instruments used for making changes in the running conditions.

Instruments for following (tracking)—Instruments usually used in different types of vehicles (e. g., cars, planes).

Instruments for indication—These show ‘entrance’, ‘way ahead closed’, ‘backward’, etc.

2. Instruments without control devices:

Instruments for quantitative readings.

Instruments for qualitative readings.

Instruments for check readings—Instruments used for detecting and report­ing a deviation from a normal value.

Instruments for comparison—Used for checking that two machines are producing the same value.

Of the instruments above, the most common in industry are those for checking read­ings, for controlling regulation, and for setting up. The introduction of computer systems for process control, however, has meant that qualitative readings are becom­ing more common.