1 Industrial Instrumentation Dr. –Ing. Naveed Ramzan

1

Industrial Instrumentation

Dr. –Ing. Naveed Ramzan

Level Sensors

Level is another common process variable that is measured in many industries. The method used will vary widely depending on the nature of the industry, the process, and the application.

Inventory: -- a constant supply or storage of materialControl: -- continuous, batch, blending, and mixing control-- stabilize flow to the next processAlarming: -- hi/lo limits, safety shut downData Logging: -- material quantities for inventory and billing

purposes and where regulatory requirements are necessary

Level Measurement

What is measured?What is measured?

The measured medium can be liquid, gas or solid and stored in vessels (open/closed tanks), silos, bins and hoppers.

Units of level can be expressed in:feet (meters)gallons (liters)pounds (kilograms)cubic volume (ft3, m3)

Hydrostatic Head Float Load Cells Magnetic Level

Gauge Capacitance

Transmitters Magnetostrictive Ultrasonic Microwave Laser Radar Guided Wave Radar Dip Stick Vibration

Methods ---- Direct or Indirect (inferential)

PEC – UET Lahore Dr. Shahid Naveed 11. April 2023 / Dr. –Ing Naveed Ramzan 5

Direct MethodsDirect Methods

Direct methods sense the surface or interface of the liquid and is not affected by changes in material density (Specific Gravity)

Examples: Dip Stick Resistance Tapes Sight Glass Floats Ultrasonic

Indirect Methods (Inferential)Indirect Methods (Inferential)

Indirect methods “infer” liquid level by measuring some other physical parameter such as pressure, weight, or temperature.

Changing materials means a corrective factor must be used or recalibrating the instrument.

Examples: Hydrostatic head methodsLoad CellsCapacitanceConductivity

When determining the type of level sensor that should be used for a given application, there are a series of questions that must be answered:

Open tank or closed tank? Can the level sensor be inserted into the tank or

should it be completely external? Contact or non-contact?

Continuous measurement or point measurement? Direct or Indirect measurement? What type of material is being measured? Liquid

or Solid? Clean or Slurry?

Selection Criteria

For all liquids you will need:

The system operating temperature with max. and min. excursions?

two wide range – expensive the sensor The system operating pressure?

Check that system ‘T’ and ‘P’ do not conflict with the materials of construction?

Selection Criteria


For Solids:

Bulk density Be careful with very large silos as compaction

at the bottom can greatly change assume bulk densities

Flow characteristics?

Expected particle size distribution?

Is solid abrasive and/or corrosive and what is the moisture/solvent content?

Selection Criteria


Simple and cheap Can be used with any wet

material and not affected by density.

Can not be used with pressurized tanks

Visual indication only (electronic versions are available)

RodGauge - similar to a dipstick found in a car, it has weighted line markings to indicate depth or volume

For Liquids

Dip Stick

Another simple direct method of measuring liquids.

Can be used in pressurized tanks (as long as the glass or plastic tube can handle the pressure)

Good for applications where non-contact measurement is needed (like beverages)

Sight Glass

For Liquids

Float rides the surface level to provide the measurement. Many different styles are available. Usually used for pump control, high/low level alarms and emergency shut-off

Liquid density does not affect measurement

Floats

For Liquids

Point Level Measurement Continuous Level Measurement

Advantages and disadvantages

Low CostConductive, non-coating liquids only

Insulating coatings can cause problems

Conductivity Level Measurement

For Liquids

The pressure of the fluid in the tank causes the tape to short-circuit, thus changing the total resistance of the measuring tape. An electronic circuit measures the resistance; it's directly related to the liquid level in the tank.

Resistance Tape

For Liquids

Bubblers allow the indicator to be located anywhere.

The air pressure in the tube varies with the head pressure of the height of the liquid.

Bottom of tube determines

reference point

P

Regulated purge system

(air or nitrogen)

Instrument input does not matter

Can’t be used in closed tanks or where purging a liquid is not allowed (soap). Very popular in the paper industry because the air purge keeps the tube from plugging.

Bubblers


For Liquids

Advantages:

-- Easy installation

-- Continuous reading providing analogue or digital signal

-- No moving parts

-- Good accuracy and repeatability


reference point

P


(air or nitrogen)


Bubblers

For Liquids

Limitations:

-- Not suitable for pressurized tanks

-- Sediments may block tube or probe

-- Tanks must be freely vented


reference point

P


(air or nitrogen)


Bubblers

For Liquids

These methods infer level by measuring the hydrostatic head produced by the liquid column.

A pressure sensing element is installed at the bottom of the tank and pressure is converted to level.

Different liquid densities or closed tank applications must be accounted for.

Hydrostatic Head Level Sensors

General Theory for Head General Theory for Head MeasurementMeasurement

The Pressure exerted by the Height of the liquid is:

P = H x Density*

If the Density of the liquid is known then

H = Pressure Density*

Height (H)

Pressure PSI

Liquid Density (D)

*Note: For liquids other than water, use the density of water 0.0361 lb/in3 as a reference and multiply by the SG of the liquid.

Hydrostatic Head Level Sensors

ExampleExample

Height (H)

Tank 1 PSI

WaterDensity (D) Height

(H)

Tank 2 PSI

OilDensity (D)

A dip stick measurement of the level of these 2 tanks indicates 30 feet of liquid in both tanks. Calculate the pressure that each gauge will read if tank 1 contains water (S.G. = 1) and tank 2 contains oil (S.G. = 0.85)

P = ? psi P = ? psi

ExampleExample

Height (H)

Tank 1 PSI


(H)

Tank 2 PSI

OilDensity (D)

A dip stick measurement of the level of these 2 tanks indicates 30 feet of liquid in both tanks. Calculate the pressure that each gauge will read if tank 1 contains water (S.G. = 1) and tank 2

contains oil (S.G. = 0.85)

P = H x Density

= 30 ft x 0.0361 lbs/in3

= (30 x 12) x 0.0361

= 13 psi

P = ? psi

ExampleExample

Height (H)

Tank 1 PSI


(H)

Tank 2 PSI

OilDensity (D)

A dip stick measurement of the level of these 2 tanks indicates 30 feet of liquid in both tanks. Calculate the pressure that each gauge will read if tank 1 contains water (S.G. = 1) and tank 2

contains oil (S.G. = 0.85)

P = H x Density x SG

= 30 ft x 0.0361 lbs/in3 x 1

= (30 x 12) x 0.0361

= 13 psi

P = H x Density x SG

= 30 ft x 0.0361 lbs/in3 x 0.85

= (30 x 12) x 0.0361 x 0.85

= 11 psi

Practical Considerations when Practical Considerations when using head type instrumentsusing head type instruments

The reference point of the tank vs instrument input must be considered.

Height (H)

Pressure PSI

Liquid Density (D)

Height (H)Pressure

PSI

Liquid Density (D)

This may not be practical in some applications where the tank elevation is below grade or where a remote visual reading is required.

Hydrostatic Head Level Sensors (Cont‘d)

Tank ElevationsTank Elevations

Vertical rises and drops contribute to the overall height and therefore head pressure. Horizontal runs have no effect.

Height (H)

P

WaterDensity (D)

P


Closed Tank ApplicationsClosed Tank Applications

Open tanks are vented to atmosphere so the pressure at the bottom of the tank is only due to the head pressure of the liquid.

Closed tanks are not vented to atmosphere so the pressure at the bottom of the tank is due to the head pressure of the liquid + the vapour pressure above the

surface.

P Head = h x D

P (atmospheric)

P Head = (h x D) + P vapour

P vapour


Used where the sight glass level gauge can not be used.

Magneto-resistive types can provide an electrical output.

Liquid/liquid interface (such as water and oil) can be measured by changing the buoyancy of the magnetic float

Magnetic Level Sensor

Not the same as a float. The displacer is immersed in the tank and the

buoyant force of the liquid produces a torque which is proportional the amount of liquid level.

The output force can be

converted to provide a

proportional pneumatic or

electrical continuous

output of tank level.

Displacers

Displacer – side mounted unitDisplacer – side mounted unit

The displacer float size and diameter is selected according to the size of tank and the height of the level to be measured.

The output force can be converted to provide a proportional pneumatic or electrical continuous output of level.

Advantages and disadvantages:

Very little movement of displacer float

Affected by different liquid densities

Limited to smaller tanks sizes

Displacers

Non-Contact direct level sensor

Level is a function of the time it takes an ultrasonic pulse to hit the surface and return

Limitations include:• Surface foam absorbs signal, agitation create reflections

• High Pressure & High Temperatures affect the signal speed• Vapour and condensate create false echo’s

UltraSonic Level Measurement

Similar to ultrasonic but at a much higher frequency (6.3 GHz)

Various designs -- Frequency Modulated Continuous Wave-- Pulsed Wave-- Guided Wave

These sensors have better performance in applications where vapour, dust or uneven surfaces exist.

Radar Level Sensors (Microwave)

Tank level is determined by the weight of the quantity of material

Load Cells (strain gauge transducers) placed at the bottom of the tank measure the weight and then convert it to an electrical signal.

Load Cells

Capacitance Level Detection (RF Level Capacitance Level Detection (RF Level

Probes)Probes)

RF Capacitance level sensors are similar in operation to conductivity sensors and are typically used when non-

conductive liquids are measured.

The only variable is the dielectric of the tank material that varies in proportion to the level.

Conductive liquids like mineral water will short out the probe to the tank wall.

Good for solids

Can be used for non-Can be used for non-liquidsliquids RF capacitance

probes are used in a wide range of applications and are subjected to certain limitations such as: Electrode Coating Low sensitivity Temperature Different

materials Shielding & noise

Summary

• Level is measured by locating the boundary between two media, called

the interface• Level can be measured directly or

indirectly• Noninvasive devices are preferred

when the material is corrosive, hazardous, sterile, or at a high

temperature or pressure

Batch application using point level Batch application using point level measurementmeasurement

Detect if the level is at a predetermined point

Generally used to detect either a low-level condition or high-level condition

The output of a point level sensor is on-off, or 1 or 0 state digital signal

A batch process does NOT use a continuous level measurement device.

Creative Assignment! HAZOP?????

Discussion & Questions?

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1 Industrial Instrumentation Dr. –Ing. Naveed Ramzan