Lecture 3: Flow, Level, Temperature and Light Measurement

1- Flow Measurement Optimum performance of many processes requires specific flow rates. The cost of many liquids and gases are based on the measured flow through a pipeline, making it necessary for accounting purposes to accurately measure and control the rate of flow. There are many methods for the measurement of liquid flow such as: Venturi tube Rotameter Turbine flowmeter

The Venturi tube The Venturi tube uses a specific reduction in tube size. When a fluid flows through a pipe with a constriction, the velocity of the fluid increases in the constriction while the pressure decreases to satisfy the principle of conservation of mechanical energy. Any gain in kinetic energy a fluid achieves due to its increased velocity is negated by a drop in pressure according to Bernoulli’s equation.

The Venturi tube The flow rate is proportional to the square root of the pressure drop. Therefore, a D/P cell is a used to measure the differential pressure before and at the constriction and then a square root extractor is used to obtain a measure of the flow.

Rotameter The rotameter is a cheap and reliable variable-area flow meter used for direct visual indication of flow rate. The rotameter has a float in a tapered vertical tube with the fluid flow pushing the float upwards. At equilibrium, the weight of the float is equal to the force on it due to the flowing fluid. The distance to which the float rises in column is proportional to the flow rate. The rotameter is purely mechanical device and so there is no need to external power supply.

The turbine flow meter The turbine rotor is mounted in the center of the pipe and rotates at a speed proportional to the rate of flow of the fluid or gas passing over the blades. The rate of revolution of the rotor can be determined by certain means. The turbine is only used with clean fluids such as gasoline. The rotating flow devices are accurate but expensive.

2- Level Measurement There are many methods for the measurement of liquid level such as: Float sensors. Ultrasonic devices. Pressure level gauge. Capacitive level gauge. Bubbler devices. Resistive tapes.

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. The float sensor is accurate and have a linear output with level height. However, if the surface of the material being monitored is turbulent, the float reading may vary excessively, some means of damping might be required, such as a stilling well. Stilling well arrangement

Ultrasonic level gauge An ultrasonic pulse is transmitted and then reflected by the liquid surface back to the receiver. The time to receive the echo pulse is proportional to the distance from the surface of the liquid. Since there is no contact with the liquid, this method can be used for solids, and corrosive and volatile liquids.

Pressure level gauge Pressure is often used as an indirect method of measuring liquid levels. The hydrostatic pressure at the bottom of the container is given by: where p is the pressure, is the density, g is the gravitational acceleration, and h is the height. Hence, the level can be calculated from the pressure and the density of the liquid.

Capacitive probes In the portion of the probe that is out of the liquid, air serves as the dielectric between the rod and outer shell (air has dielectric constant of 1). In the section of the probe immersed in the liquid, the dielectric is the liquid itself, which causes a large capacitive change (water has dielectric constant of 80). The capacitance change is directly proportional to the level of the liquid and can be measured using an ac bridge.

Bubbler devices Bubbler devices require a supply of clean air or inert gas to prevent interaction with the liquid. Gas from a pressure regulator is forced through a tube via a flow regulator, and the open end of the tube is close to the bottom of the tank. The pressure required to force the liquid out of the tube is equal to the pressure at the end of the tube due to the liquid, which is given by

Resistive tapes 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. The sensor can be used in corrosive liquids.

3- Temperature Measurement Temperature is without doubt the most widely measured variable. In chemical reactions, temperature control is of major importance, since chemical reactions are temperature-dependent. All physical parameters are temperature-dependent, making it necessary in most cases to measure temperature along with the physical parameter, so that temperature corrections can be made to achieve accurate parameter measurements.

Temperature sensors There are many properties that change with temperature and can be used as basis for temperature sensors. These are some of the commonly used temperature sensors: Bimetallic strips Resistor-temperature detectors (RTD) Thermistors; Thermocouples; Pyrometer; and Semiconductors.

Bimetallic strips Many materials expand when heated. This forms the basis for the bimetallic strip A bimetallic strip consists of two different metal strips of the same length bonded together. These devices operate on the principle that different metals have different coefficients of expansion. It can be used to open or close electric circuits, as in the simple thermostat commonly used with domestic heating systems. They are robust, relatively cheap, but are fairly slow.

Resistance temperature detector (RTD) The resistance of most metals increases in a reasonably linear way with temperature according to the following equation: where RT is the resistance at a temperature T °C, R0 the resistance at 0°C and α is termed the temperature coefficient of resistance and is constant for the metal. Resistance temperature detectors (RTDs) are resistive elements made from metals such as platinum, nickel or copper alloys. Resistance devices are normally measured using a Wheatstone bridge, or supplied from a constant current source. Care should be taken to prevent the electrical current from heating the device and causing erroneous readings.

Example What is the resistance of a platinum resistor at 480°C, if its resistance at 0°C is 110Ω? The temperature coefficient of resistance for the platinum is 0.00385 /°C.

Thermistors Thermistors are semiconductor material that typically has a high negative temperature coefficient of resistance. They have high sensitivity, which can be up to a 10% change per degree Celsius, making it the most sensitive temperature element available. However, they have very nonlinear characteristics which make the device difficult to use as an accurate measuring device without compensation, but its sensitivity and low cost makes it useful in many applications. The typical response time is from 0.5 to 5 seconds. When in use, care has to be taken to minimize the effects of internal heating.

Thermocouple When any conductor is subjected to a thermal gradient, it will generate a voltage. This is known as the thermoelectric effect or Seebeck effect. This effect is the basis for the thermocouple device. The thermocouple, as the name suggests, involves two different metals.

Thermocouple One of the ends of the two metals are joined together and put at temperature Th (the hot junction). The other end of both metals are put at a temperature Tc (the cold junction). The potential difference is measured between these two ends. This potential difference depends on the two metals used and the temperature difference between the two junctions.

Thermocouple The Figure shows how the e.m.f varies with temperature for a number of commonly used pairs of metals (usually given reference letters). Usually, the cold junction is held at 0 °C. Thermocouples are generally mounted in a sheath to give them mechanical and chemical protection. The response time of an unsheathed thermocouple is very fast. With a sheath this may be increased to as much as a few seconds if a large sheath is used.

IC temperature sensors LM35: the output is a linear function of temperature, of 10 mV/°C when the supply voltage is 5 V. DS18B20: is a digital thermometer provides 9 to 12-bit (configurable) temperature readings which indicate the temperature of the device.

4- Light Measurement The detection and conversion of light intensity into electrical signals can be done using photocells such as: Photo-resistors: devices change their resistance with light intensity. As the light intensity increases, their resistance decreases. Examples of these materials are selenium, zirconium oxide, and cadmium sulfide. Photo-transistor: Light generates hole-electron pairs, which cause base current in phototransistors.