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. 

Applications For Differential Pressure Gauges

What Differential Pressure Gauges do?

A Differential Pressure Gauges basically indicate the difference between two input connections.

Differential Pressure Gauge is just a visual indicator which is designed to measure and display the difference between two points. Differential Pressure Gauges have two inlet ports and each of them are connected to the pressure points. Differential pressure gauge just perform a mathematical operation of subtraction through mechanical means.

Differential Pressure Gauges are mostly used in industrial process systems and most of the times it is limited to a specific operation or it can be said that DP gauges functions are misunderstood. In fact, differential pressure gauges can provide multiple solutions to several problems.

Differential pressure is also written as DP or ∆p. Differential Pressure gauge is the difference between two applied pressures. 

For example:

Let

There are two points, Point “A” and Point “B”.

The pressure at point “A” is equal to 50psi and the pressure at point “B” is equal to 20psi. The differential pressure is 30psi.

Pressure at Point A = 50 Psi

Pressure at Point B = 20 Psi

So the Difference shown on the DP Gauge will be = 50 psi – 20 psi = 30 psi

Unit 

Pound per square inch (psi)

1 bar = 14.503773773022 psi

Some typical applications of differential pressure gauges are:

Liquid Level Monitoring

Sealed tanks have an atmospheric pressure gas blanket on the top. This blanket contain liquid. Thus the pressure of this gas blanket adds to the hydrostatic pressure which is created by water column of the content. This makes difficult for a pressure gauge to get accurate level measurement. Differential pressure gauge measures the difference in gas pressure from the total pressure and it is translated to a true liquid level reading.

Filter Monitoring

Differential pressure gauge can be used for filter monitoring. The Differential pressure gauge measures the pressure before and after the filter. As the filter gets more clogged with particles the differential pressure increases and when DP reaches up to the limit then the concerned operator knows that the filter should be changed.

Flow Monitoring

Differential pressure gauges can also be used to measure flow of a liquid in a pipe just by using an orifice plate.The differential pressure gauge measures pressure before and after the orifice. Thus the pressure drop across the orifice is later translated mechanically by the differential pressure gauge into the flow rate.  Differential pressure gauges are easy solution to measuring process flow.

Instrumentation and Control Basics

Introduction

Instrumentation is defined as the art and science of measurement and control of process variables within a production or manufacturing area.

Instrument is a device that measures or manipulates variables such as

1. Flow
2. Temperature
3. Level
4. Pressure

Instruments may be as simple as direct reading thermometers or may be complex multi-variable process analyzers. Instruments are often part of a control system in refineries, factories, and vehicles. The control of processes is one of the main branches of applied instrumentation. Instrumentation can also refer to handheld devices that measure some desired variable. Diverse handheld instrumentation is common in laboratories, but can be found in the household as well. For example, a smoke detector is a common instrument found in most of the offices and homes.

Things that we commonly measure are:

1. Temperature 
2. Pressure
3. Speed
4. Flow rate
5. Velocity and Acceleration
6. Stress and Strain
7. Level or Depth
8. Mass or Weight Density
9. Size or Volume Acidity/Alkalinity

Basic instrument system consists of three elements as shown in Figure 1:

1. Sensor or Input
2. Signal Processor
3. Receiver or Output

Figure 1

Control systems consist of a control loop and it have four elements as shown in Figure 2:

1. A measurement of the state or condition of a process
2. A controller calculating an action based on this measured value against a pre-set or desired value (set point)
3. An output signal resulting from the controller calculation which is used to manipulate the process action through some form of actuator
4. The process itself reacting to this signal, and changing its state or condition.

Figure 2

In industrial process control, the Process Variable (PV) is measured by an instrument in the field and acts as an input to an automatic controller which takes action based on the value of it. Alternatively, the PV can be an input to a data display so that the operator can use the reading to adjust the process through manual control and supervision.