Sensor Measurements

Process-variable feedback for the controller is achieved by one of two methods. The process variable can (I) be measured and transmitted to the controller by using a separate measurement transmitter with a 0.2-I.0-bar (3-15-psi pneumatic output, or (2) be sensed directly by the controller, which contains the measurement sensor within its enclosure. Controllers with integral sensing elements are available that sense pressure, differential pressure, temperature, and level. Some controller designs have the set point adjustment knob in the controller, making set point adjustment a local and manual operation. Other types receive a set point from a remotely located pneumatic source, such as a manual air set regulator or another controller, to achieve set point adjustment. There are versions of the pneumatic controller that support the useful one-, two-, and three-mode combinations of proportional, integral, and derivative actions. Other options include auto/manual transfer stations, antireset windup circuitry, on/off control, and process-variable and set point indicators. 

A regulator is a compact device that maintains the process variable at a specific value in spite of disturbances in load flow. It combines the functions of the measurement sensor, controher, and final control element into one self-contained device. Regulators are available to control pressure, differential pressure, temperature, flow, hquid level, and other basic process variables. They are used to control the differential across a filter press, heat exchanger, or orifice plate. Regulators are used for monitoring pressure variables for redundancy, flow check, and liquid surge relief. 

Fermenter An industrial microbiological reactor in which the addition of nutrients, removal of products, and insertion of measuring sensors and control devices are maintained while accessories like heating, aeration, agitation, and sterilization systems are provided. 

Contact temperature measurement is based on a sensor or a probe, which is in direct contact with the fluid or material. A basic factor to understand is that in using the contact measurement principle, the result of measurement is the temperature of the measurement sensor itself. In unfavorable situations, the sensor temperature is not necessarily close to the fluid or material temperature, which is the point of interest. The reason for this is that the sensor usually has a heat transfer connection with other surrounding temperatures by radiation, conduction, or convection, or a combination of these. As a consequence, heat flow to or from the sensor will influence the sensor temperature. The sensor temperature will stabilize to a level different from the measured medium temperature. The expressions radiation error and conduction error relate to the mode of heat transfer involved. Careful planning of the measurements will assist in avoiding these errors. 

Pressure. Pressure so defined is sometimes called absolute pressure. The differential pressure is the difference between two absolute pressures. The most common types of pressure-measuring sensors are silicon pressure sensors, mechanical strain gauges, and electromechanical transducers. 

The development of rapid, accurate, and noninvasive online measurement sensors and instraments is a high-priority goal in the commercialization of biotechnology (Figure 3.4). Some of these instraments will build on analytical methods now used in catalysis and other surface sciences, such as... 

From experimentally measured sensor signals as functions of distance X we estimated the coefficients of diffusion for protium and deuterium. At T = 345 K they are equal to 1.56-10 and I.OO-IO" m/s, respectively. 

Uncertainty in Process Measurements. Sensor measurements are always subject to noise, calibration error, and temporary signal loss, as well as various faults that may not be immediately detected. Therefore, data preprocessing will often be required to overcome the inherent limitations of... 

Baldini F., Bechi P., Bracci S., Cosi F., Pucciani F., In vivo optical-fibre pH sensor for gastro-oesophageal measurements, Sensors Actuators 1995 B29 164-168. 

Kunz R.E., Totally integrated optical measuring sensors, Proc. SPIE, Vol. 1587 98-113. 

Netto E.J., Peterson J.I., McShane M., Hampshire V., A fiber-optic broad-range pH sensor system for gastric measurements Sensors and Actuat.B 1995 29 157. 

Grant S.A., Glass S.R., A sol-gel based fiber optic sensor for local blood pH measurements, Sensors andActuat. B 1997 45 35. 

Hoefer, U. Steiner, K. Wagner, E. Contact and sheet resistances of Sn02 thin films from transmission line model measurements. Sensors and Actuators B (1995), p. 59-63. 

An optional high-pressure crucible (Fig. 8 1) may be used for chemical reactions in solutions with an inherent vapor pressure of up to 100 atmospheres. The crucible is equipped with a centering pin for exact positioning on the measuring sensor. 

J. R. Astles and W. G. Miller, Reversible fiber-optic immunosensor measurements, Sensors and... 

Although some research institutes started already in the middle of the seventies to measure sensorically odour emissions, it was not until 1980 before 4 different olfactometers were compared with each other for the first time. Great differences were found ranging from a factor of 3 to 40. A relationship between the differing compounds tested and the factor number was also noticed. The differences between the two TNO olfactometers almost disappeared after setting both on the same flow rate of 16 1/min. (7). 

The third block in Fig. 2.1 shows the various possible sensing modes. The basic operation mode of a micromachined metal-oxide sensor is the measurement of the resistance or impedance [69] of the sensitive layer at constant temperature. A well-known problem of metal-oxide-based sensors is their lack of selectivity. Additional information on the interaction of analyte and sensitive layer may lead to better gas discrimination. Micromachined sensors exhibit a low thermal time constant, which can be used to advantage by applying temperature-modulation techniques. The gas/oxide interaction characteristics and dynamics are observable in the measured sensor resistance. Various temperature modulation methods have been explored. The first method relies on a train of rectangular temperature pulses at variable temperature step heights [70-72]. This method was further developed to find optimized modulation curves [73]. Sinusoidal temperature modulation also has been applied, and the data were evaluated by Fourier transformation [75]. Another idea included the simultaneous measurement of the resistive and calorimetric microhotplate response by additionally monitoring the change in the heater resistance upon gas exposure [74-76]. 

In order to implement the control methodologies indicated, one needs proper measurements (sensors and diagnostics), controllers, and actuators. Extensive research and development are carried out to realize the most appropriate sensors and actuators for various applications. Diagnostics developed serve dual purpose to physically measure the various combustion parameters and interpret the results as quickly as possible, preferably in situ, so that the mechanisms involved can be understood and to validate the numerical computer codes so that... 

Figure 4.1 Schematic diagram of a general-purpose CSTR recycle system for in situ investigations of liquid-phase homogeneous catalyzed reactions. The blocks represent in-line instruments and their signals, namely, (i) sets of scalar valued measurements (sensors), (ii) sets of vector valued measurements (ID spectroscopies) and (iii) sets of matrix valued measurements (2D spectroscopies). The recycle time for the system is given by t.

Marxen K, Vanselow KH, Lippemeier S, Hintze R, Ruser A, Hansen U-P. Determination of DPPH Radical Oxidation Caused by Methanolic Extracts of Some Microalgal Species by Linear Regression Analysis of Spectrophotometric Measurements. Sensors. 2007 7, 2080-2095. 

All parts of the plant, such as venting hlter, pressure-measuring sensors, temperature-measuring sensors, and their lead-throughs, within the sterilization area are also sterilized. 

Figure 2. Gas-flow diagram for measuring sensor characteristics.

Emission measuring sensors also need an internal calibration source. In the case of absorption sensors, the external source most often served as the standard. In the case of emission, a reference source is usually incorporated in the instrument to certify instrument performance and provide an absolute reference. 

Sensing of hydrocarbons with tin oxide sensors possible reaction path as revealed by consumption measurements. Sensors and Actuators B-Chemical, 89 (3), 232-6. 

Following this overall review of level measurement sensors and applications, the individual detector designs will be described. 

---