Definition of Measurement

Measurement is the acquisition

of information about a state or phenomenon (object of measurement) in the world around us.

Why measuring?

The purpose of measurements is to regulate, control, or alter the surrounding world, directly or indirectly.The results of this regulating control can then be tested and compared to the desired results and any further corrections can be made.

Level sensing devices can be divided into four categories:

(1)direct sensing, in which the actual level is monitored;(2)indirect sensing, in which a property of the liquid,such as pressure, is sensed to determine the liquid level; (3) single point measurement, in which it is only necessary to detect the presence or absence of a liquid at a specific level; and (4) free-flowing solid level sensing.

Classify as Direct or Indirect

• Dipstick• Sightglass• Float Sensors• Ultrasonic • Sonic Devices

• Displacers• Capacitive probes• Bubbler Devices• Resistive Tapes• Load Cells

• Dipsticks can also be used to measure the quantity of liquid in an inaccessible space, by inserting and removing the stick and then checking the extent of it covered by the liquid. The most familiar example is the oil level dipstick found on most internal combustion engines.

Dipstick

Dipstick

Dipsticks

INDIRECT LEVEL SENSING

• A commonly used method of indirectly measuring a liquid level is to measure the hydrostatic pressure. The level can be extrapolated from the pressure and the specific weight of the liquid. The level of liquid can be measured using displacers, capacitive probes, bubblers, resistive tapes, or by weight measurements.

Differential Pressure

Pressure is often used as an indirect method of measuring liquid levels. Pressureincreases as the depth increases in a fluid. The pressure is given by:

p = γh where p is the pressure, γ is the specific weight, and h is the depth.

Displacers

This device uses the change in buoyant force on an object to measure the changes in liquid level.Displacers are simple, reliable, and accurate, but are affected by the (temperature-dependent) specific weight of the liquid.

Advantages•Work well with clean liquids and are accurate and adaptable to wide variations in fluid densities.

Disadvantages•Displacers are relatively expensive when placed in external cages.

Practical Notes•Displacers should only be used for relatively non-viscous, clean fluids. Best used for short spans. Spans of up to 40 feet are possible.

• The buoyant force on the cylindrical displacer is given by:

Where: γ specific weight of the liquid, d is float diameter, and L is the length of the displacer submerged in the liquid.

Weight on force sensor = Weight of displacer (W) − F

Capacitive Probes

This device can be used in liquids and free-flowing solids for continuous level measurement. Materials placed between the plates of a capacitor increase the capacitance by a factor (μ), known as the dielectric constant of the material. For instance, air has a dielectric constant of 1, and water has a dielectric constant of 80.

• When two capacitor plates are partially immersed in a nonconductive liquid, the capacitance (Cd) is given by:

where: Ca is the capacitance with no liquid, μ is the dielectric constant of the liquid between the plates, r is the height of the plates, and d is the depth or level of the liquid between the plates.

The depth or liquid level is given by:

The dielectric constants of some common liquids:

Bubbler Devices

•This simple level measurement has a dip tube installed with the open end close to the bottom of the process vessel. A flow of gas (usually air) passes through the tube and when air bubbles escape from the open end, the air pressure in the tube corresponds to the hydraulic head of the liquid in the vessel. The air pressure in the bubble pipe varies proportionally with the change in head pressure.

Resistive Tapes

•It can be used to measure liquid levels. A resistive element is placed in close proximity to a conductive strip in an easily compressible nonconductive sheath. The pressure of the liquid pushes the conductive strip against the resistive element, shorting out a length of the resistive element that is proportional to the depth of the liquid.

Load Cells

• It can be used to measure the weight of a tank and its contents. The weight of the container is subtracted from the total reading, leaving the weight of the contents of the container.Knowing the cross-sectional area of the tank and the specific weight of the material, the volume and/or depth of the contents can be calculated.

• The weight of a container can be used to calculate the level of the material in the container, the volume (V) of the material in the container is given by:

• where r is the radius of the container, and d is the depth of the material. The weight of material (W) in a container is given by: