Pressure measurement Flashcards
S.I units of pressure
N/m^2
the difference between measured pressure and a perfect vacuum
absolute pressure
usually the difference between measured pressure and local atmospheric pressure
Gauge pressure
difference between two measured pressure
differential pressure
Old fashioned pressure measurement device which is a U-shaped tube with the ends exposed to different pressures. Liquid column will move until its weight is in equilibrium with the differential pressure
Manometer
Pressure difference equation
density * gravitational acceleration * height
- Has a oval cros section
- Increase in pressure in inside of tube
- Measures gauge pressure
- Tube tries to attain a circular cross section: straightening, movement is magnified by using a curved tube, causes pointer to move and display pressure (small stresses so tube won’t bend permanently)
Bourdon Tube
Advantages of mechanical pressure gauges
- display only, transmitters or switching devices
- Instruments available for low to very high pressures
- Very good visual display
- Manual devices are inexpensive
- Accurate
- Certificate available
Disadvantages of mechanical pressure gauges
- Bulky
- Transmitters types are very bulky and relatively expensive
- Over pressure damages instrument
- Non hygienic: threaded connection, process fluid in tube, difficult CIP/SIP
- flexible membrane to separate fluid from measuring element
- deflection of diaphragm is translated into a pressure measurement
- ## Typically 316 stainless steel: range of materials and liners can be selected for compatibility with process fluid
Diaphragm element gauges
- These have circular shaped corrugated membranes. Typically supplied with clamp connections. Process fluid only contacts the surface of the diaphragm
- The deflection on the diaphragm is a measure of the pressure and is indicated on the dial
- The deflection of the diaphragm exerts a pressure on the fill fluid in the bourdon tube behind the diaphragm
Diaphragm element gauges
Diaphragm valves are … and suitable for CIP/SIP.
hygienic
- Good if the measurement is required in an awkward location - use of a longer capillary allows the gauge to be located away from the measurement point
- Hear can cause fill fluid to expand: need to use temperature compatible fluids
Diaphragm element gauges
Diaphragm element gauges advantages
- Hygienic (3A & EHEDG certificates)
- Suitable CIP/SIP
- Suitable for aggressive media
- Suitable for adhesive,crystallising and corrosive media
- Suitable for highly viscous media
- Suitable for environmentally hazardous, toxic media
- Range of contact materials (SS, monel, hastelloy)
convert pressure directly to electrical signal and are a more modern type of gauge.
these gauges are usually supplied with flush mounted hygienic connections (CIP/SIP).
pressure cells
gauge filled with liquid such as silicone oil or glycerine
filled gauge
gauge with no filling
dry gauge
Advantages of filled gauges
- protects against high dynamic pressure loads
- dampens vibrations
- prevents internal moisture
- reduces condensation on the dial
- protects against adverse hot & cold temperature extremes
- more reliable and durable
Must be chemical compatible with process fluid and non txic in food applications. Special fill fluids are also available for oxidising media such as oxygen and chlorine
fill fluids
Pressure cells disadvantage
tend to have a small display if fitted at al
Pressure cells advantages
- have hygienic connections
- compact
- accurate
- can function as transmitters or switches
- incorporating overload protection
- dry sensor
- pressure deflects ceramic diaphragm
- this causes a change in capacitance : capacitance inversely proportional to distance
- hygienic
- high chemical stability due to resilient ceramic
- high mechanical stability
- suitable for vacuums
- Up to 40 bar
Ceramic diaphragm capacitive pressure sensor
- pressure deflects diaphragm
- fill fluid transfer pressure to a resistance bridge
- pressure causes change in voltage
- available with and without diaphragm seals
- capillary extender to move sensor away from high temperatures
- very high pressure capability - 700 bar
Metallic pressure sensor
opens or closes an electrical contact when a certain pressure is reached
pressure switches
Two options for differential pressure measurement
- two sensor modules wit ha single transmitter
- single differential pressure module
Protection against overpressure
- pressure relief
- bursting discs/panels
Circular discs fitted between two flanges
Expensive but very accurate
Non fragmenting
Need replacing after an over pressure event
bursting discs
typically comprised of a polyamide membrane with an insulated electrically conductive path mounted between supporting rings. It is replaced along with the rupture disc
Burst Tel
Magnetic switch that is activated by the fluid flow from the burst disc. It is not destroyed by over pressure and therefore does not need to be replaced
Flow Tel
What presssure Bourdon tubes measure
gauge pressure
Why are control signals sent as currents?
Overcomes issues of resistance in wires
How are valves usually actuated?
Using pressurised air
