Measurement of various Quantities (Liquid Level and Viscosity) Study Notes For Instrumentation Engineering

By Rahul Singh|Updated : August 23rd, 2017

In this article, you will find the study notes on Measurement of Various Quantities which will cover the topics such as measurement of   liquid level , pH and viscosity measurement.

1. 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 non conductive liquids and free flowing solids for level measurement. Many materials, when placed between the plates of a capacitor, increase the capacitance by a factor μ called the dielectric constant of the material. For instance, 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 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 by 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.

2. Measurement of Humidity

Humidity is a measure of the relative amount of water vapor present in the air or a gas.Relative humidity (Φ) is the percentage of water vapor by weight present in a given volume of air or gas compared to the weight of water vapor present in the same volume of air or gas saturated with water vapor, 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 percent, with accuracies of about ±5 percent. Their operating temperature range is limited to less than 70°C.
  • Laminate hygrometer is made by attaching thin strips of wood to thin metal strips forming a laminate.
  • Hair hygrometer is the simplest and oldest type of hygrometer. It is made using hair, Human hair lengthens by 3 percent when the humidity changes from 0 to 100 percent,The hair hygrometer has an accuracy of about 5 percent for the humidity range 20 to 90 percent over the temperature range 5 to 40°C.
  • Resistive hygrometer or resistive humidity sensors consist of two electrodes with interdigitated fingers on an insulating substrate,The electrodes are coated with a hydroscopic material (one that absorbs water such as lithium chloride).
  • Capacitive hygrometer The 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.
  • Psychrometers A psychrometer uses the latent heat of vaporization to determine the relative humidity. If the temperature of 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 vapor pressure, heat content, and weight of water vapor in the air.

3. Measurement of Density/Specific Gravity

Density and Specific Gravity:

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 γ is 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 that by measuring the vibration amplitude the specific weight/density can be calculated.

4. 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/liter, 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 liter 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 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.

5. 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 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.
  • 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.


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