Electrical sensing meter

Flow meters have traditionally been classified as either electrical or mechanical depending on the nature of the output signal, power requirements, or both. However, improvement in electrical transducer technology has blurred the distinction between these categories. Many flow meters previously classified as mechanical are now used with electrical transducers. Some common examples are the electrical shaft encoders on positive displacement meters, the electrical (strain) sensing of differential pressure, and the ultrasonic sensing of weir or flume levels. 

Turbine flow meters are composed of some form of rotary device such as a helical rotor, Pelton wheel or a vane mounted in the flow stream. The fluid passing the rotor causes the rotor to turn at an angular velocity which is proportional to the flow velocity and hence the volumetric flowrate through the meter. The rotary motion of the rotor is sensed by some form of pick-up device that produces an electrical pulse output. The frequency of this signal is proportional to the flowrate and the total count of pulses is proportional to the total volume of liquid passed through the meter. 

Differential pressure transmitters (or DP cells) are widely used in conjunction with any sensor that produces a measurement in the form of a pressure differential (e.g. orifice plate, venturi meter, flow nozzle, etc.). This pressure differential is converted by the DP cell into a signal suitable for transmission to a local controller and/or to the control room. DP cells are often required to sense small differences between large pressures and to interface with difficult process fluids. Devices are available that provide pneumatic, electrical or mechanical outputs. 

Figure 18-1 A pH meter gives the pH of the solution directly. When the electrode is dipped into a solution, the meter displays the pH. The pH meter is based on the glass electrode. This sensing device generates a voltage that is proportional to the pH of the solution in which the electrode is placed. The instrument has an electrical circuit to amplify the voltage from the electrode and a meter that relates the voltage to the pH of the solution. Before being used, a pH meter must be calibrated with a series of solutions of known pH.

Thermal mass flow meter. The meter s probe is inserted into a duct of known dimensions. Flowing gases pass through the sensing element at the bottom of the probe. The sensing element consists of two rods. The heat transfer between these rods is correlated to flow. An electrical transmitter and digital display is located in the head of this instrument. 

Differential amplifiers are useful because biopotentials generated within the body vary over the body surface, but line-coupled noise does not For example, subtraction of the heart electric potential at two points on the chest surface will produce a res ting potential since the local biopotential amplitudes and wave shapes at each electrode are different. ivironniental electric fields from the power line are more remote and couple such that they are present unifonnly over the body. This is partly due to the distributed nature of capacitive coupling. It is also because die low 50- to 60-Hz line frequencies have electric field wavelengths so long (hundreds of meters) that a person s body can be considered to be, in some sense, an antenna in the uniform near field of an electric field source. 

These can be used for corrosive, dangerous liquids that can attack the sensor if placed directly in contact. A magnetic field is applied to the metering tube, which generates an electrical signal (potential difference) proportional to the flow velocity perpendicular to it. The potential difference is sensed by the electrodes fitted perpendicular to the flow and the applied magnetic field and converted to readings on the indicator. 

Near infrared detectors These are usually photoconductive cells which detect infrared radiation in the range 0.8 3.0 p. The sensing element is a semiconductor (germanium, lead sulphide, or lead tellurlde). Upon illumination with radiation of appropriate wavelength, the electrons of the semiconductor are raised to conduction bands. Tills causes a drop in electrical resistance. Consequently, if a small voltage is applied, a large Increase in current can be noted. The resistance of the system is such that the current may be amplified and finally indicated on a meter is recorded. 

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