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Level Measurement. Intermediate Apprentice Week 3. 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:

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level measurement
Level Measurement

Intermediate Apprentice

Week 3

level measurement2
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
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
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

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 methods
  • Load Cells
  • Capacitance
  • Conductivity
selection criteria
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
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)
resistance tape
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
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)

magnetic level sensors
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
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
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

Liquid Density (D)

Height (H)

Pressure PSI

General Theory for Head Measurement

The Pressure exerted by the Height of the liquid is:

P = H x Density*

If the Density of the liquid is known then

H = Density*

Pressure

*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

Water

Density (D)

Oil

Density (D)

Height (H)

Height (H)

Tank 1 PSI

Tank 2 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)

P = ? psi

P = ? psi

example16

Water

Density (D)

Oil

Density (D)

Height (H)

Height (H)

Tank 1 PSI

Tank 2 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)

P = ? psi

P = H x Density

= 30 ft x 0.0361 lbs/in3

= (30 x 12) x 0.0361

= 13 psi

example17

Water

Density (D)

Oil

Density (D)

Height (H)

Height (H)

Tank 1 PSI

Tank 2 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)

P = H x Density x SG

= 30 ft x 0.0361 lbs/in3x 1

= (30 x 12) x 0.0361

= 13 psi

P = H x Density x SG

= 30 ft x 0.0361 lbs/in3x 0.85

= (30 x 12) x 0.0361 x 0.85

= 11 psi

practical considerations when using head type instruments

Liquid Density (D)

Height (H)

Pressure PSI

Liquid Density (D)

Height (H)

Pressure PSI

Practical Considerations when using head type instruments

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

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

Water

Density (D)

Height (H)

P

P

Tank Elevations

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

bubblers

Instrument input does not matter

P

Regulated purge system

(air or nitrogen)

Bottom of tube determines reference point

Bubblers

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.

Can’t be used in closed tanks or where purging a liquid is not allowed.

closed tank applications

P (atmospheric)

P Head = h x D

P vapour

P Head = (h x D) + P vapour

Closed 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.

using a d p cell transmitter

4 – 20 mA

mA

24 VDC

To PLC or Controller

Lo side open to atmosphere

Using a d/P Cell Transmitter

The differential pressure cell is one of the most common methods of measuring level.

  • 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 transmitter23
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

mA

24 VDC

To PLC or Controller

H L

  • 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