Float type Hydrostatic differential pressure gage type ( Capacitance type Ultrasonic type Radiation technique..ppt
- 1. Level, humidity and pH are three important process parameters and their measurement find wide application in chemical and manufacturing industries. In this chapter we would provide a brief overview of the different techniques adopted for measurement of liquid level and humidity.
- 2. Level Measurement There are several instances where we need to monitor the liquid level in vessels. In some cases the problem is simple, we need to monitor the water level of a tank; a simple float type mechanism will suffice. But in some cases, the vessel may be sealed and the liquid a combustible one; as a result, the monitoring process becomes more complex. Depending upon the complexity of the situation,
- 3. there are different methods for measuring the liquid level, as can be summarized as follows: (a) Float type (b) Hydrostatic differential pressure gage type (c) Capacitance type (d) Ultrasonic type (e) Radiation technique. Some of the techniques are elaborated in this section.
- 4. Hydrostatic Differential Pressure type The hydrostatic pressure developed at the bottom of a tank is given by: p = h ρ g where h is the height of the liquid level and ρ is the density of the liquid. So by putting two pressure tapings, one at the bottom and the other at the top of the tank, we can measure the differential pressure, which can be calibrated in terms of the liquid level. Such a schematic arrangement is shown in Fig. 1 . The drum level of a boiler is normally measured using this basic principle. However proper care should be taken in the measurement compensate for variation of density of water with temperature and pressure.
- 6. Capacitance type This type of sensors are widely used for chemical and petrochemical industries; and can be used for a wide range of temperature (-40 to 200 o C) and pressure variation (25 to 60 kg/cm 2 ). It uses a coaxial type cylinder, and the capacitance is measured between the inner rod and the outer cylinder, as shown in Fig. 2. The total capacitance between the two terminals is the sum of (i) capacitance of the insulating bushing, (ii) capacitance due to air and liquid vapour and (iii) capacitance due to the liquid. If the total capacitance measured when the tank is empty is expressed as C 1 , then the capacitance or the liquid level of h can be expressed as: 2 πε 0 ( ε 1 − ε 2 ) h C t = C 1 + ln ( r 2 / r 1 ) where, ε 1 is the relative permittivity of the liquid and ε 2 is the relative permittivity of the air and liquid vapour (≈ 1 ) . Hence a linear relationship can be obtained with the liquid level.The advantage of capacitance type sensor is that permittivity of the liquid is less sensitive to variation of temperature and can be easily compensated.
- 8. Ultrasonic type Ultrasonic method can be effectively used for measurement of liquid level in a sealed tank. An ultrasonic transmitter/receiver pair is mounted at the bottom of the tank. Ultrasonic wave can pass through the liquid, but gets reflected at the liquid-air interface, as shown in Fig The time taken to receive the pulse is measured, that can be related with the liquid level. For accurate measurement, variation of speed of sound with the liquid density (and temperature) should be properly compensated.
- 10. Humidity Measurement Humidity measurement finds wide applications in different process industries. Moisture in the atmosphere must be controlled below a certain level in many manufacturing processes, e.g., semiconductor devices, optical fibres etc. Humidity inside an incubator must be controlled at a very precision level. Textiles, papers and cereals must be dried to a standard storage condition in order to prevent the quality deterioration. The humidity can be expressed in different ways: (a) absolute humidity, (b) relative humidity and (c) dew point. Humidity can be measured in different ways. Some of the techniques are explained below.
- 11. Conductance/Capacitance method of measurement Many solids absorb moisture and their values of the conductance or capacitance change with the degree of moisture absorption. Moisture content in granules changes the capacitance between two electrodes placed inside. By measuring the capacitance variation, the moisture content in the granules can be measured. Similarly, moisture content in paper and textiles change their resistance. A schematic arrangement for measurement of moisture content in paper or textilesusing Resistance Bridge is shown in Fig.
- 13. Measurement of pH Measuring Electrode The measuring electrode is made of thin sodium ion selective glass. A potential is developed across the two surfaces of this glass bulb, when dipped in aqueous solution. This potential is sensitive to the H + ion concentration, having a sensitivity of 59.2 mv/pH at 25 0 C. Fig. 7 shows the basic schematic of a measuring probe. The buffer solution inside the glass bulb has a constant H + ion concentration and provides electrical connection to the lead wire.
- 14. Reference Electrode The basic purpose of a reference electrode is to provide continuity to the electrical circuit, since the potential across a single half cell cannot be measured. With both the measuring and reference cells dipped in the same solution, the potential is measured across the two lead wires. A reference electrode should satisfy the following basic requirements: The potential developed should be independent of H + ion concentration. (i) The potential developed should be independent of temperature (ii) The potential developed should not change with time. Considering all these requirements, two types of reference electrodes are commonly used: (i) Calomel (Mercury-Mercurous Chloride) and (ii) Silver-Silver Chloride. The construction of a Calomel reference electrode is shown in Fig. 8. The electrical connection is maintained through the salt bridge.