Liquid Level Measurement Techniques

• This chapter explains the units for level measurement and the most common methods for using level standards.
• The topics in this chapter will cover the formulas used in level measurements, the difference between direct and indirect level measuring devices, the difference between continuous and single-point measurements, various types of instruments available for level measurements, and applications of the various types of level sensing devices.
• Most industrial processes use liquids as well as free flowing solids, which are stored in containers ready for use.
• Knowing the levels and remaining volumes of these materials is imperative for replenishment to avoid the cost of large volume storage.
• This chapter will also discuss the measurement of the level of liquids and free flowing solids in containers.
• The detector normally senses the interface between a liquid and a gas, a solid and a gas, a solid and a liquid, or possibly the interface between two liquids.
• Liquid level sensing falls into two categories: single-point sensing and continuous level monitoring.
• Single-point sensing detects the actual level of the material when it reaches a predetermined level, so action can be taken to prevent overflowing or to refill the container.
• Continuous level monitoring measures the level of the liquid on an uninterrupted basis allowing the constant monitoring of material levels. If the cross-sectional area of the container is known, the volume can be calculated.
• Level measurements can be direct or indirect, like using a float technique or measuring pressure and calculating the liquid level.
• Free flowing solids are dry powders, crystals, rice, or grain.

Level Formulas

• Pressure is often used as an indirect method of measuring liquid levels, as it increases with depth in a fluid.
• Pressure is defined as: Ap = γ Δh, where Ap is the change in pressure, γ is the specific weight, and Ah is the depth.
• Units must be consistent, e.g., pounds and feet, or newtons and meters.
• Buoyancy is also an indirect method used to measure liquid levels, determining level using the buoyancy of an object partially immersed in a liquid.
• The equation to workout buoyancy B or upward force on a body in a liquid is: B = γ x area x d, where area is the cross-sectional area of the object and d is the immersed depth of the object.
• Liquid level can be calculated from the weight of a body in a liquid using the buoyancy principle.
• The formula is: d = (WA-WL) / (γ x area), where WL, is the weight of a body in a liquid which is equal to its weight in air (WA - B)
• A container's weight can be used to calculate the level of the material in the container. For a container, the volume V is given by V = area × depth = πr² × d, 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 W = γV

Capacitive Probes

• Capacitive probes are suitable for level measurement in nonconductive liquids and free-flowing solids.
• When placed between the plates of a capacitor, many materials increase the capacitance by a factor u, known as the dielectric constant of the material.
• Air has a dielectric constant of 1, while water has a dielectric constant of 80.

Capacitive Plates

• Two capacitor plates partially immersed in a nonconductive liquid can demonstrate the relation between liquid volume, cross-sectional area, liquid depth, plate capacitance, and a known dielectric constant.
• The capacitance (Cd) is: Cd = Ca(μr/d) + Ca, 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.
• Dielectric constants vary with temperature, so temperature correction may be needed.
• Liquid level can be expressed as: d =(Cd-Ca)r / μ΄α

Level Sensing Devices

• Level sensing devices fall into two categories: direct sensing and indirect sensing
• Direct sensing instruments monitor the actual level.
• Indirect instruments sense a property of the liquid, such as pressure, to determine the level.

Dielectric Constant Table

• Table 6.1 provided constants for common liquids with varying temperatures.
• Water @ 20°C = 80
• Water @ 0°C = 88
• Glycerol @ 25°C = 42.5
• Glycerol @ 0°C = 47.2
• Acetone @ 25°C = 20.7
• Alcohol (Ethyl) @ 25°C = 24.7
• Gasoline @ 20°C = 2.0
• Kerosene @ 20°C = 1.8

Direct Level Sensing

• Sight glass is the simplest method for direct visual reading, it is generally mounted vertically close to the container, allowing the liquid level to be observed directly.
• In closed containers with pressurized or hazardous liquids: the ends of the glass connect to the top and bottom of the tank.
• In open containers with inert liquids like water: the tank and sight glass can both be open to the atmosphere.
• The top of the sight glass must have the same pressure conditions as the top of the liquid; dissimilar pressures will result in differing level measurements.
• In excessively long sight glasses: a second inert liquid with higher density than the liquid in the container can work, making allowances for the density difference.
• Magnetic floats in the sight glass can be monitored using a magnetic sensor like a Hall effect device.

Floats

• Floats use an angular arm or pulley and consist of a float material that is less dense than the liquid.
• A float sensor advantage: independence of the density of the liquid or solid (when agitation is sometimes used).
• A float sensor disadvantage: susceptibility to turbulence if the surface is turbulent, this can be amended by using damping in system.
• A ball float attached to an arm and measured for angle is commonly used to monitor fuel levels in automobiles.
• The ball float's disadvantage lies in its non-linearity which can be corrected with potentiometer or linearized for industrial use.
• Visual scales can be replaced by a potentiometer to obtain a linear electrical output with level.

Displacer

• Measurements with the change in buoyant force on an object to measure the changes in liquid level.
• Displacers must have a higher specific weight than the liquid being measured, and must be calibrated for that specific weight.
• A force or strain gauge measures the excess weight of the displacer.

Displacer Formula

• Buoyant force F on a cylindrical displacer is yπd²L / 4, where y= specific weight of the liquid, d = float diameter, L = length of the displacer submerged in the liquid.
• The weight as seen by the force sensor has the formula of: Weight on force sensor = weight of displacer - F

Probes

• Three categories for measuring liquid: conductive, capacitive, and ultrasonic.
• Conductive probes are suitable for single-point measurements in liquids that are conductive and nonvolatile.
• Two or more probes can indicate set levels, and if the liquid is in a metal container, the container can serve as the common probe.
• Contact between the liquid and two probes causes a voltage between them causing a current to flow indicating the set level has been reached.
• Probes are generally used to indicate when liquid levels are low (to start filling pumps) and when liquid levels are high (to automatically cease filling).
• Capacitive: suitable for liquids that are nonconductive and have a high dielectric constant.
• The capacitance is measured using a bridge between an inner rod and outer shell with air as the dielectric for the out of liquid section and the material is the dielectric constant of the immersed section.
• Continuous level monitoring changes are proportionally related to the level of the liquid.
• The dielectric constant of the liquid should be known for best results; if unknown temperature measurements are advised to correct results.

Ultrasonics

• Ultrasonics have a versatile quality and can be selected for single point or continuous measurement of liquid or solid substance.
• Single point: an ultrasonic transmitter and receiver.
• Continuous measurement: ultrasonic waves from the transmitter are reflected by the surface of the liquid to the receiver.
• Time taken to reach the receiver measured to determine the distance from the transmitter and receiver unit surface, from which the liquid level can be calculated knowing the liquid flow velocity.
• Liquids are generally placed at the bottom of the container, and the depth of a liquid is measured by in measuring signal reflected to surface liquid to the the receiver.

Indirect Level Sensing

• The most common method of indirectly measuring the liquid level measures the hydrostatic pressure at the bottom of the container.
• Depth is extrapolated based on pressure using the formula to calculate specific weight.
• Using this equation, the pressure may be calculated.
• A dial pressure gauge can be calibrated in liquid depth terms.
• Bubblers, radiation, resistive tapes, and weight measurements may additionally measure liquid depth.
• Bubbler setups use clean air/gas supplies and gas force to measure the liquid.
• Equation: the bubbles equal to the pressure at the end of the tube due to liquid which can be specified by the depth of fluid * liquid specific weight.
• Advantage: This is specifically advantageous for corrosive liquids since the measuring tube can be composed of corrosion resistant material.
• Radiation method for hazardous, very hot, or those requiring no means of sensor installation.

Resistive Tapes

• 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 resistive element against the conducive strip, shorting a length proportional to the liquid depth.
• Sensors are used in liquid and slurries, although it can be inaccurate, cheap, and susceptible to humidity.

Load Cells

• Used to measure the weight of tank and contents.
• The container weight is subtracted from the gross, leaving the net content weight.
• Calculated in conjunction with cross sectional area of container plus the specific weight of liquid/material.
• Suitable for continuous weight measurement.
• Load sensors and force include strain gauge technique and a piezoelectric technique (for force/weigh measurements.)
• Strain gauge attached to beam is used to measure the stress in the beam with piezoelectric measures for high range compression.

Paddle Wheels

• Electric powered wheel that can measure powder, grains, or granules levels.
• Require significant amounts of torque when in contact, with torque amount required as good indicator towards material depth.
• Agitation is required to level particles.

Application Considerations

• Factors: such as the liquid pressure, temperature, volatility, corrosiveness, accuracy needed, particulates, types of measurement process (single, continuous, direct, indirect), and fluids.
• For fluid levels, floats are the suitable choice because they are unaffected by particulates and the presence of slurries.
• Measurement methods: flat panel floats are often used to measure fluid levels, while cylindrical vessels are also implemented.
• Float displacement of liquid has follows the equation: Float weight = buoyant force = Υπά²h / 4 where Υ₁ is the specific weight of the liquid, d is diameter, h is the immersion depth of the float

Device Characteristics

• Displacers should not measure liquid depths completely submerged and must have a known weight.
• Corrections for density made by monitoring liquid temperature must be set.
• Corrections should include device placement and dielectric constant.

Pressure Gauge Types

• Gauge considerations; the gauge can indicate damage or excessive force placed upon the container, as well as corrosion type that can damage container.
• Differential models best chosen for liquid in under considerable volume.
• Bubbler models needs supply of gas and selected gas must not react with liquid.

Ultrasonic

• Useful in pressure containers up to 2MPa and temperature constraints of 100C
• Should have a trained person to operate radioactive source and handle storage.

This lesson covers various liquid level measurement techniques, including sight glasses, float sensors, and displacer-type sensors. It explores their applications, advantages, and limitations in different industrial contexts. Includes practical considerations for accurate and reliable level monitoring.