Guide to Level Measurement

Best book to study for level Measurement.

http://mydigimag.rrd.com/publication/?i=182123

How to Select a Pressure Switch?

Following requirements will help you in selection of the right pressure switch for your application 

Set points

First decide how many set points do you require. If you require one set point you may use fixed differential pressure switch or for two set points use adjustable differential pressure switch.

Fluids

You should know which fluid you are going to control. Before using any fluid you should ensure that the switch material is compatible with the process fluid or not.

Pressure Range

Pressure range of a switch is a critical factor and the best practice is to make pressure settings in the middle of 80 percent of the range of switch.
The pressure applied should never exceed the max range of the switch.

Surges

A pressure switch will face surges and the frequency at which they occur depends upon the application if the surges are frequent in your system or there are high pressure surges you may need to have a device with high pressure range.

Switch Enclosure

What type of enclosure do you need?

Most common enclosure type

IP66, IP65

Output

Pressure switches comes in different output configuration.

Electrical Connection

Pressure switches has different output configurations so they have different electrical connection configurations too.
What you need to know for selection of pressure switch is which electrical connection you require in your application.

Pressure Connection

What type of pressure connection do you want.

System Response Time

If in your application the systems response time is critical you may select a switch with a volumetric displacement.