Two types of instruments for control loops

When selecting an instrument for a given control loop you need, first, to know how these devices actually work.

See the post Does it matter to know how an instrument works? for further reference.

However, in a general way you also need to know general classifications of some instruments available in the market. Here is an important classification you may start with.

Analog vs electronic instruments

Instruments according to its technoloy

• analog/mechanical instruments
• electronic instruments

Of course there are other options we could name as a category not included in oir list but these can be better considered as sub-categories of those major categories. Here is a short explanations of this.

Analog/mechanical instruments

These are devices that are purely mechanical and are tipically old (or mature) technologies. Examples of these instruments are:

• 
  the Bourdon manometer,
• the bulb thermometer,
• the glass level meter, 
• etc.

Some features of these instruments may also be listed

• do not need electricity or batteries to work,
• since these are mature technologies, there are many low cost options,
• depend on a human for reading measurements,
• have poor precision since engraved/printed scalings have some limitations,
• closed control loops are not always possible with these instruments.

Electronic instruments

These type of instruments have some advantages over analog/mechanical technologies. Examples of these are

• pressure transducers,
• thermocouples,
• infrared thermometers,
• pH meters,
• ultrasonic flow meters,
• etc.

As you may notice from the above list, technologies such as: optical, magnetic, ultrasonic, etc. are all placed under the electronic instruments category.

These instruments provide in fact improvements of the analog/mechanical options. Some of its features are

• are preferred for closed loop possibilities,
• are more precised than analog/mechanical options,
• need power input or batteries,
• do not depend on a human for measurements (its measurements are automated),
• installation may not be easy,

Any question? Write in the comments and I shall try to help.

Other stuff of interest

• LE01 - AC and DC voltage measurement and continuity test
• LE 02 - Start and stop push button installation 24V DC
• LE 03 - Turn on/off an 24V DC pilot light with a push button
• LE 04 - Latch contact with encapsulated relay for turning on/off an AC bulb light
• What is a process variable?
• What are the most important process variables?
• Time dependence of process variables
• A list of process variables

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Ildebrando.