Pressure Gauge

 

Pressure definition : is defined as Force Applied Per Unit Area.

* P= F/A      (Static pressure)

   P –  Pressure , F – Force , A – Area

*  P = ρgh    (Hydrostatic pressure)

                   ρ – Density in (Kg/m³)                 g - Acceleration due to gravity (9.8 m/s²)

  h - Depth in liquid in (m)

   Pressure units :

  1 Bar    =100,000   Pa

                     =14.7        PSI   (pound /inch²)

                     = 10,200     mm H2o

                     = 29           inHg

We commonly measure the pressure of :

  Liquids , Air , Gases , Steam,…

 Pressure types

• Gauge pressure : Reference to atmospheric pressure.
•  Absolute pressure : Reference is a vacuum (0 bar or No pressure)   

                                           P(abs) = p(g)+ p(atm).

• Differential pressure :Level , Flow, Filter clogged

How could I measure Pressure?

• Pressure Gauge
• Pressure Switch
• Pressure Transmitter

 Primary Elements of pressure measurements

Based on pressure sensing elements:

• Bourdon Tube
• Diaphragm
• Capsule
• Bellow

 

Bourdon Tube

Used for the measurements of medium and high pressures.

Max. Range : 

            (0:0.6) bar       to       (0:5000) bar

• C-shaped
•  Helical
•   Spiral

 

C-shaped

      

              Range: Cover the majority of applications   (1:1360 bar) 

     Pressures is measured by Bourdon tube converted to motion that transmitted directly to the pointer.

•         Positive Direction 
•               Negative direction (Vacuum)

         Helical

      

 Range: from (2 bar to 5000 bar)

  Advantages

•        Increased accuracy and sensitivity

•        Higher over range protection 

       " For example  0-1000 PSI may be safely exposed  to 10.000 PSI"

Disadvantages

•        Very expensive

 Spiral 

            Range: low as 10 PSI available

Construction : Series of c-bourdon tubes joined end to end

• More sensitive
• More accurate
• For positive and negative ranges

Advantages of Bourdon tube pressure gauges:

1. Accurate results specially at high pressure..
2. Low cost.
3. Simple construction.
4. Safe even for high pressure.

Disadvantages of Bourdon tube pressure gauges:

1. Not for Low pressure measurements.
2. No Overload protection.
3. Not suitable for Critical media.

 Diaphragm pressure gauges

           Range : Low Pressure ranges 0 :16 mbar and 0 :40 bar

The pressure is transmitted wave shape diaphragm to a link this then transfer the pressure to the movement of pointer

 Advantages

• More accurate
• Insensitive to vibration
• Overload protection (5x ,10x) .
• For Critical media (Chemicals and Viscous media).

 Capsule pressure gauges

Diaphragm is a single sheet ,joining two diaphragm to form a capsule

Range: Low Pressure ranges can be between 0- 2.5 mbar and 0-600 mbar

                      with accuracy classes of 0.1 to 2.5

 Bellow

Measuring pressures 

up to 40 mm Hg (50 mbar)       

Working principle

•   Pressure is applied to one side of the bellows and the resulting deflection is balanced by a spring.
•   Made of high strength alloy (brass or phosphor bronze)

Advantages of the bellows sensor include

• Simplicity
• Low cost
• The ability to connect directly to a pointer
• More sensitive than bourdon tube 

Disadvantages
    It is not suitable for high pressures.

Troubleshooting of bourdon tube

4 common pressure gauge failure and safety risk

• Temperature
• Corrosion
  
• Vibration
• Overpressure

 

 How to select pressure gauge?

To properly select a pressure gauge, consider the gauge7 steps

1. Accuracy.
2. Dial size.
3. Mounting requirements.
4. Environment.
5.  Connection size.
6. Accessories.
7. Pressure range.

 

 Steps of calibration

• Test points (0 , 25% , 50% , 75% and 100% of span).
• Fix reference gauge and gauge under calibration on the comparator.
• Apply the pressure slowly and take the readings.
• Observe the gauge under calibration reading compared to reference.
• At least 30 sec waiting to observe readings
• Calculate measurement and final results based on observation.