Level and Pressure Measurement

Christopher Foot

Patrick Lawlor

Level Measurement

Why is it important?

Three Mile Island

Level Measurement Assess the Situation

Can the level sensor be inserted into the tank or should it be completely external?

Should the sensor detect the level continuously or will a point sensor be adequate?

Can the sensor come in contact with the process fluid or must it be located in the vapor space?

Is direct measurement of the level needed or is indirect detection of hydrostatic head (which responds to changes in both level and density) acceptable?

Is tank depressurization or process shut-down acceptable when sensor removal or maintenance is required?

Level Measurement

5 Main TypesRadarDifferential PressureAdmittance-probeUltrasonicNuclear

Radar Level Measurement

Consists of Transmitter, Antenna, receiver/signal processor, operator interface.

Electromagnetic wave sent downward toward surface of process fluid, reflected back and collected

Level of tank based on difference of time from emission to collection to EM wave.

Radar Level Measurement Advantages

Can detect the liquid level a layer of light dust or airy foam.

Least expensive of 5 common types Automated calibration.

Disadvantages If foam layer is too thick, level of foam will be

transmitted, not process fluid Interference from rat-holing, angled process material

surfaces, solid deposition on antenna

Differential Pressure Level Measurement

Also know as Hydrostatic Tank Gauging

Based on density of fluid and pressure changes

P = h (SG)

Differential Pressure Level Measurement

Advantages Can be used in either open top or pressurized vessels Simple theory – easy to apply

Disadvantages Bubbles in system (affecting SG) can cause errors in

measurement Error associated with pressure measurement passed

along to level measurement

Admittance-probe Level Measurement

Operate in the low MHz range measuring admittance of an AC current circuit that varies with level.

Based on Dielectric Constant of the material

Often used as level switches

Admittance-probe Level Measurement

Advantages Good for level measurement of solids and other non-

reflective materials Sensitivity easily adjusted by varying probe

parameters Disadvantages

Materials with high conductivity can cause a short within the probe

Temperature has a large effect on the system Likelihood of false indications high with fluids

Ultrasonic Level Measurement

Also known as SONAR

Similar to radar – issues an electronic pulse (sound wave) and measures time until echo received

Ultrasonic Level Measurement

Advantages Instruments include temperature compensation, data

filters and self-calibration options Can be used to detect the interface level between 2

liquids

Disadvantages Cannot be used with agitated liquids Signal will be absorbed by foam, dust, mist, humidity

Nuclear Level Measurements

Uses Radiation absorption to directly measure level using point source radiation and strip detection

Highest absorption indicates lowest level in vessel

Nuclear Level Measurements

Advantages Can be used to measure either level or density of a

process material Radiation source can be very long-lasting, often

outliving the process Can be installed outside the process avoiding process

interruptions as radioactivity can “see” through walls Disadvantages

Radiation safety is very involved and can inhibit the process

Measurements can be skewed by density

Pressure Measurement

Why is it important?

Pressure Measurement

Asses the situationWhat is the range of pressures to be

measured? Is pressure dynamic or static?What is more important: absolute accuracy or

good repeatability?How much overpressure protection is

required?

Types of Measurement

MechanicalU-tube manometer, Bourdon tube, Diaphragm

and Bellows Electrical

Strain Gauge, Capacitive sensor, Potentiometric, Resonant Wire, Piezoelectric, Magnetic, Optical

Mechanical

Mechanical pressure measurement devices are large and cumbersome.

Not suited for automated control loops typical in industry.

Mechanical devices: U-tube Manometer Bourdon tube Diaphragm and Bellows element

U-tube Manometer

Measures difference in pressure between two points in a pipe.

Typical in laboratories.

Bourdon Type

Flexible element used as sensor. Pressure changes cause change in element

position. Element connected to pointer to reference

pressure.

Diaphragm and Bellows Element

Similar concept to Bourdon type. Widely used because they require less space

and can be made from materials that resist corrosion.

Electrical

Have become more common with increased reliance on computerized control systems.

Strain Gauge Measures deflection of elastic diaphragm due to

pressure difference across diaphragm. Widely used in industry. Used for small pressure ranges. Measurements tend to drift.

Capacitive Sensor Measures changes in capacitance of electrically

charged electrodes from movement of metal diaphragm due to pressure difference across diaphragm.

Capacitive Sensor, cont.

Can be operated in balanced or unbalanced mode.Balanced always has capacitance of zero.

Measures pressure indirectly by measuring drift in capacitor arms.

Unbalanced measures ratio between output voltage and excitation voltage.

Widely used in industry. Large rangeability.

Resonant Wire

Wire is oscillated at resonant frequency by oscillator circuit.

Pressure changes cause change in wire tension which changes oscillatory frequency.

Generates digital signal. Very precise, used for low differential

pressure measurements. Sensitive to temperature variation and

has non-linear output

Piezoelectric

Measures the charge developed across quartz crystal due to change in pressure.

Charge decays rapidly making unsuitable for static pressure measurements.

Sensors are very rugged. Pressure can be applied longitudinally or transversally.

Used to measure dynamic pressure changes associated with explosions and pulsations .

Magnetic

Measures induced current caused by movement of magnetic components from pressure changes.

Used in applications where high resolution in small range is desired due to very high output signals.

Sensitive to stray magnetic fields and temperature changes.

Optical Detects effects of minute motions due to process

pressure changes through partial blocking of an LED.

Immune to temperature effects. Excellent stability and long-duration capability.

Questions?

References

http://www.omega.com/literature/transactions/volume3/pressure.html

http://www.omega.com/literature/transactions/volume4/

http://www.mme.wsu.edu/~me305/va.web/Exp16/U-Tube/utube_schematic.htm