Introduction to Liquid Level Measurement in Instrumentation Engineering
Liquid level measurement is a critical aspect of instrumentation engineering, aimed at determining the height or depth of a liquid in a container or process vessel. Accurate liquid level measurement is essential in various industries, including chemical processing, oil and gas, water treatment, and pharmaceuticals, where precise control and monitoring of liquid levels are vital for efficient and safe operations.
There are several methods employed in liquid level measurement, each with its own principles and applications. One commonly used technique is hydrostatic pressure measurement, which relies on the principle that the pressure exerted by a liquid is proportional to its height. This method involves the use of pressure sensors or transmitters to measure the pressure at the bottom of the container, which is then converted into a liquid-level reading. Another approach is based on the principle of buoyancy, where the level of a liquid is determined by the force exerted on a float or displacer suspended in the liquid. The position of the float or displacer is detected using mechanical or electronic sensors, providing an indication of the liquid level.
Measurement of Level
Basically, there are two categories of level-sensing devices. They are direct sensing, in which case the actual level is monitored, and indirect sensing where a property of the liquid such as pressure is sensed to determine the liquid level.
Pressure is often used as an indirect method of measuring liquid levels. Pressure increases as the depth increases in a fluid. The pressure is given by
∆p = γ∆h
- where ∆p = change in pressure
- γ= specific weight
- ∆h = depth
Buoyancy is an indirect method used to measure liquid levels.
B = γ× area × d
Capacitive probes can be used in nonconductive liquids and free-flowing solids for level measurement. Many materials, when placed between the plates of a capacitor, increase the capacitance by a factor μcalled the dielectric constant of the material. For instance, the air has a dielectric constant of 1 and water 80.
Ca = capacitance with no liquid
μ= dielectric constant of the liquid between the plates
r = height of the plates
d = depth or level of the liquid between the plates
Level Sensing Devices: There are two categories of level sensing devices. They are direct sensing, in which case the actual level is monitored, and indirect sensing where a property of the liquid such as pressure is sensed to determine the liquid level.
Direct level sensing
- Sight glass: Open end/differential or gauge is the simplest method for direct visual reading.
- Floats: Angular arm or pulley two types of simple float sensors. The float material is less dense than the density of the liquid and floats up and down on top of the material being measured.
- A displacer: With force sensing is device uses the change in the buoyant force on an object to measure the changes in liquid level. The displacers must have a higher specific weight than that of the liquid level being measured and have to be calibrated for the specific weight of the liquid.
- Probes: For measuring liquid levels fall into three categories, i.e., conductive, capacitive, and ultrasonic.
- Conductive probes: They are used for single-point measurements in liquids that are conductive and non-volatile as a spark can occur.
- Capacitive probes: They are used in liquids that are nonconductive and have a high μ and can be used for continuous level monitoring.
- Ultrasonic: They can be used for single-point or continuous-level measurement of a liquid or a solid. A single ultrasonic transmitter and receiver can be arranged with a gap to give a single-point measurement.
Indirect level sensing
- The most commonly used method of indirectly measuring a liquid level is to measure the hydrostatic pressure at the bottom of the container.
- The depth of liquid can also be measured using Bubblers, Radiation, Resistive tapes, and Weight measurements.
- Load cells can be used to measure the weight of a tank and its contents.
- Paddle wheels driven by electric motors can be used for sensing the level of solids in the form of power, grains, or granules. When the material reaches and covers the paddle wheel, the torque needed to turn the motor greatly increases.
Measurement of Humidity
Humidity is a measure of the relative amount of water vapour present in the air or gas. Relative humidity (Φ) is the percentage of water vapour by weight present in a given volume of air or gas compared to the weight of water vapour present in the same volume of air or gas saturated with water vapour, at the same temperature and pressure.
Humidity measuring devices:
- Hygrometers Devices that indirectly measure humidity by sensing changes in physical or electrical properties in materials due to their moisture content are called hygrometers. Materials such as hair, skin, membranes, and thin strips of wood change their length as they absorb water. The change in length is directly related to the humidity. Such devices are used to measure relative humidity from 20 to 90 per cent, with accuracies of about ±5 per cent. Their operating temperature range is limited to less than 70°C.
- The laminate hygrometer is made by attaching thin strips of wood to thin metal strips forming a laminate.
- A hair hygrometer is the simplest and oldest type of hygrometer. It is made using hair, Human hair lengthens by 3 per cent when the humidity changes from 0 to 100 percent, The hair hygrometer has an accuracy of about 5 per cent for the humidity range of 20 to 90 percent over the temperature range 5 to 40°C.
- Resistive hygrometers or resistive humidity sensors consist of two electrodes with interdigitated fingers on an insulating substrate, The electrodes are coated with a hygroscopic material (one that absorbs water such as lithium chloride).
- Capacitive HygrometerThe dielectric constant of certain thin polymer films changes linearly with humidity so that the capacitance between two plates using the polymer as the dielectric is directly proportional to humidity.
- Piezoelectric or Sorption hygrometers use two piezoelectric crystal oscillators; one is used as a reference and is enclosed in a dry atmosphere, and the other is exposed to the humidity to be measured.
- PsychrometersA psychrometer uses the latent heat of vaporization to determine relative humidity. If the temperature of the air is measured with a dry bulb thermometer and a wet bulb thermometer, the two temperatures can be used with a psychometric chart to obtain the relative humidity, water vapour pressure, heat content, and weight of water vapour in the air.
Measurement of Density/Specific Gravity
Density and Specific Gravity:
The density ρ of a material is defined as the mass per unit volume. Units of density are pounds (slug) per cubic foot [lb (slug)/ft3] or kilogram per cubic meter(kg/m3).
Specific weight γis defined as the weight per unit volume of a material, i.e., pounds per cubic foot (lb/ft3) or newton per cubic meter (N/m3).
Density/Specific measuring devices
- Hydrometers are the simplest device for measuring the specific weight or density of a liquid. The device consists of a graduated glass tube, with a weight at one end, which causes the device to float in an upright position. The specific weight or density can then be read directly from the graduations on the tube.
- Thermohydrometer is a combination of hydrometer and thermometer, so that both the specific weight/density and temperature can be recorded and the specific weight/density corrected from lookup tables for temperature variations to improve the accuracy of the readings.
- Induction hydrometers are used to convert the specific weight or density of a liquid into an electrical signal. As the density/specific weight of the liquid changes, the buoyant force on the displacement device changes.
- Vibration sensors are an alternate method of measuring the density of a fluid. The amplitude of the vibration decreases as the specific weight or density of the fluid increases, so by measuring the vibration amplitude the specific weight/density can be calculated.
Measurement of pH Value
- pH is a measure of the relative amount of hydrogen and hydroxide ions in an aqueous solution. In any collection of water molecules a very small number will have dissociated to form hydrogen (H+) and hydroxide (OH-) ions:
H2O = H+ + OH -
- pH is another way of expressing the hydrogen ion concentration. pH is defined as follows:
pH = -log [H+] (2) so, if the hydrogen ion concentration is 1.0 x 10-4 moles/litre, the pH is 4.00.
- At 25°C fewer than 2x10-7% of the water molecules have dissociated. In terms of molar concentrations, water at 25°C contains 1x10-7 moles per litre of hydrogen ions and the same concentration of hydroxide ions.
- The determination of pH is one of the most common process chemical measurements, the determination of ORP is not nearly as common as pH, certain industries make valuable use of the measurement.
- For an indication of acid, alkali, or neutral water, litmus paper is used; it turns pink when acidic, blue when alkaline, and stays white if neutral.
- A pH sensor normally consists of a sensing electrode and a reference electrode immersed in the test solution which forms an electrolytic cell.
- One electrode contains a saturated potassium chloride (alkaline) solution to act as a reference; the electrode is electrically connected to the test solution via the liquid junction.
Measurement of Viscosity
Viscosity μ in a fluid is the resistance to its change of shape, which is due to molecular attraction in the liquid that resists any change due to flow or motion.
Viscosity measuring instruments
- Viscometers or viscosimeters are used to measure the resistance to the motion of liquids and gases.
- Several different types of instruments have been designed to measure viscosity, such as the inline falling-cylinder viscometer, the drag-type viscometer, and the Saybolt universal viscometer.
- The falling-cylinder device measures the rate of descent of a cylinder in a liquid and correlates the rate of descent to the viscosity of the liquid.
- A rotating disc viscometer is a drag-type device. The device consists of two concentric cylinders and the space between the two cylinders is filled with the liquid being measured.
Practical Considerations and Challenges in Liquid Level and Viscosity Measurement
Practical considerations and challenges in liquid level and viscosity measurement are important factors to consider in various industrial processes, laboratory experiments, and quality control applications. When it comes to liquid level measurement, several challenges arise due to the nature of liquids and the specific requirements of each application. One common challenge is the selection of the appropriate level measurement technology that suits the characteristics of the liquid being measured.
Different liquids may require different measurement techniques, such as using float-based devices, pressure sensors, ultrasonic sensors, or capacitive sensors. The compatibility of the measurement technology with the liquid's properties, such as density, temperature, and corrosiveness, must be taken into account to ensure accurate and reliable measurements. Additionally, factors like liquid turbulence, foam formation, and surface conditions can affect the precision of level measurements, requiring proper calibration and installation techniques.
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