Level Measurement

Level Measurement
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 material Control: continuous, batch, blending, and mixing control stabilize flow to the next process Alarming: hi/lo limits, safety shut down Data Logging: material quantities for inventory and billing purposes and where regulatory requirements are necessary

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)

Methods Direct or Indirect (inferential)

Hydrostatic Head Float Load Cells Magnetic Level Gauge Capacitance Transmitters Magnetostrictive Ultrasonic Microwave Laser Radar Guided Wave Radar Dip Stick Vibration

Direct 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 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 methods Load Cells Capacitance Conductivity

Selection Criteria
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?

Dip Stick
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

Resistance Tape
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.

Sight Glass
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)

Magnetic Level Sensors

Used where the sight glass level gauge can not be. 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

Floats
Float rides the surface level to provide the measurement. Many different styles are available.

Liquid density does not affect measurement

Hydrostatic Head Level Sensors

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.

General Theory for Head Measurement

The Pressure exerted by the Height of the liquid is:

Liquid Density (D)

Height (H)

P = H x Density*
Pressure PSI

If the Density of the liquid is known then

H=

Pressure Density*

*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.

Example
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)

Height (H)

Water Density (D)

Height (H)

Oil Density (D)

Tank 1
PSI

Tank 2
PSI

P = ? psi

P = ? psi

Example
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)

Height (H)

Water Density (D)

Height (H)

Oil Density (D)

Tank 1
PSI

Tank 2
PSI

P = H x Density = 30 ft x 0.0361 lbs/in3 = (30 x 12) x 0.0361 = 13 psi

P = ? psi

Example
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)

Height (H)

Water Density (D)

Height (H)

Oil Density (D)

Tank 1
PSI

Tank 2
PSI

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 using head type instruments

The reference point of the tank vs instrument input must be considered.
Liquid Density (D)
Height (H) Height (H)

Liquid Density (D) Pressure

PSI

Pressure PSI

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

Tank Elevations
Vertical rises and drops contribute to the overall height and therefore head pressure. Horizontal runs have no effect.
Water Density (D)

Height (H)

Bubblers
P

Instrument input does not matter

Regulated purge system (air or nitrogen)

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

Cant 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.

Closed Tank Applications

P (atmospheric)

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.
P Head = h x D

P vapour

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 vapour

Using a d/P Cell Transmitter

The differential pressure cell is one of the most common methods of measuring level.
4 20 mA 24 VDC mA To PLC or Controller

Lo side open to atmosphere

Open Tank Measurement

Lo side of the d/P cell is left open to atmosphere. Hi side measures the hydrostatic head pressure which is proportional to the height of the liquid and its density.

Using a d/P Cell Transmitter

In a closed tank, the Low side of the d/P cell is connected to the top of the tank and will cancel the effects of the vapour pressure above the surface.
4 20 mA 24 VDC mA To PLC or Controller

Closed Tank Measurement

Lo side of the d/P cell measures the vapour pressure above the surface. Hi side measures the hydrostatic head pressure which is proportional to the height of the liquid and its density + vapour pressure

Displacers
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.

Displacer 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

Conductivity Level Measurement

Point Level Measurement

Continuous Level Measurement

DC Excitation voltages may create problems with some liquids (polarization) AC excitation voltages can be used but some frequencies are not suitable for all liquids. Advantages and disadvantages Low Cost Conductive, non-coating liquids only Insulating coatings can cause problems

Capacitance Level Detection (RF Level 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-liquids

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

UltraSonic Level Measurement

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 echos

Radar Level Sensors (Microwave)

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.

Guided wave Radar Sensor

A generated pulse of electromagnetic energy travels down the probe. Upon reaching the liquid surface the pulse is reflected.

Load Cells
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.

Summary
Hydrostatic Head Float Load Cells Magnetic Level Gauge Capacitance Conductivity Ultrasonic Radar Guided Wave Radar Dip Stick

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 measurement Detect if the level is at a predetermined point Generally used to detect either a lowlevel 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.