Flow measuring sensors

Yu Dun, Hsieh H Y and Zemel J N 1993 MicroChannel pyroelectric anemometers for gas flow measurements Sensors Actuators 39 29-35 Pfahler J, Harley J C, Bau H H and Zemel J N 1990 Liquid transport in micron and submicron channels Sensors Actuators A 21-23 431 Wilding P, Pfahler J, Bau H H, Zemel J N and Kricka L J 1994 Manipulation and flow of biological fluids in straight channels micromachined in silicon Clin. Chem. 40 43-7... 

What happens if the steam pressure is allowed to build up too fast in the drum Program the microcomputer so that it admits a small amount of steam at first, then gradually opens the steam control valve during a five-minute interval while watching the pressure in the drum. After drum pressure reaches specified limits, program the microcomputer to activate the timer. When the time is complete, the control valve in the steam line will be almost closed. Water will then be admitted through the three-way valve to start the cooling cycle. A water flow measurement sensor must be added to the system to notify the microcomputer that water flow has been established before steam is cut off completely. 

Flow. The principal types of flow rate sensors are differential pressure, electromagnetic, vortex, and turbine. Of these, the first is the most popular. Orifice plates and Venturi-type flow tubes are the most popular differential pressure flow rate sensors. In these, the pressure differential measured across the sensor is proportional to the square of the volumetric flow rate. 

The proper installation of both orifice plates and Venturi-type flow tubes requires a length of straight pipe upstream and downstream of the sensor, ie, a meter mn. The pressure taps and connections for the differential pressure transmitter should be located so as to prevent the accumulation of vapor when measuring a Hquid and the accumulation of Hquid when measuring a vapor. For example, for a Hquid flow measurement in a horizontal pipe, the taps are located in the horizontal plane so that the differential pressure transmitter is either close-coupled or connected through downward sloping connections to allow any trapped vapor to escape. For a vapor measurement in a horizontal pipe, the taps should be located on the top of the pipe and have upward sloping connections to allow trapped Hquid to drain. 

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. 

Test iastrumentation has been touched on, but a few additional comments are appropriate at this point. The code provides guidance test arrangements and instrumentation. It includes details on sensor point location as well as pressure tap construction. Flow measurement is defined in detail. 

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. 

Pulsed hot wire anemometer A device used for gas flow measurement, similar to the hot grid anemometer, in which measurement, are made by pulses of hot air at a downstream sensor. 

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 temperature of the screw was measured by several investigators [29-32]. The measurements were performed by mounting thermocouples in an axial hole bored in the center of the screw or by protruding the thermocouples into the melt flow. The sensor signals were then transmitted to a chart recorder using an electrical rotary union. The technology available at the time of these measurements limited the number of sensors in the screw and the quality of the data. 

Wastewater generation can be reduced by general good housekeeping procedures such as substituting dry cleanup methods for water washdowns of equipment and floors. This is especially applicable for situations where liquid or solid materials have been spilled. Flow measuring devices and pH sensors with automatic alarms to detect process upsets are two of many ways to effect reductions in water use. Prompt repair and replacement of faulty equipment can also reduce wastewater losses. 

One other, very descriptive classification of flow-through sensors is based on the location of the active microzone and its relationship to the detector. Thus, the microzone can be connected (Figs 2.6. A and 2.6.B) or integrated (Fig. 2.6.C) with the measuring instrument. Sensors of the former type use optical or electric connections and are in fact probe sensors incorporated into flow-cells of continuous analytical systems they can be of two types depending on whether the active microzone is located at the probe end (e.g. see [17]) or is built into the flow-cell (e.g. see [18]) — in this latter case. 

Figure 2.6 — Classification of flow-through sensors according to the location of the active microzone relative to the measuring instrument (A,B) connected (C) built-in. (Reproduced from [1] with permission of the Royal Society of Chemistry).

Thermal and mass flow-through sensors rely on differential measurements owing to the low selectivity of these types of detection. They use two flow-cells arranged in series (Fig. 2.9.B) or parallel (Fig. 2.9.C), each containing a sensitive microelement (a piezoelectric crystal or a thermistor). One of the cells houses the sensitive microzone, whereas the other is empty or accommodates an inert support containing no immobilized reagent (e.g. see [35]). 

On the other hand, its should be emphasized that such basic analytical properties as precision, sensitivity and selectivity are influenced by the kinetic connotations of the sensor. Measurement repeatability and reproducibility depend largely on constancy of the hydrodynamic properties of the continuous system used and on whether or not the chemical and separation processes involved reach complete equilibrium (otherwise, measurements made under unstable conditions may result in substantial errors). Reaction rate measurements boost selectivity as they provide differential (incremental) rather than absolute values, so any interferences from the sample matrix are considerably reduced. Because flow-through sensors enable simultaneous concentration and detection, they can be used to develop kinetic methodologies based on the slope of the initial portion of the transient signal, thereby indirectly increasing the sensitivity without the need for the large sample volumes typically used by classical preconcentration methods. 

Flow-through sensors integrating detection and a chemical or biochemical reaction rely on immobilization in the probe proper or the flow-cell (or a special housing included in it) of a species intended to take part in or catalyse the reaction by which the analyte, viz. the catalyst or reagent, is measured, according to which the sensors described in this Chapter are divided into two broad categories. 

The flow-through sensors described in this Section comply essentially with the definition of biosensor. This word, like every term used to designate devices of scientific and popular note, has been the object of a number of definitions of both generic and specific scope. In a broad sense, a biosensor is any instrument or technique that measures biomolecules. In stricter terms, Rechnitz defines a biosensor as "a device that incorporates a biochemical or biological component as a molecular recognition element and yields an analytical signal in response to biomolecules" [10]. In between these two... 

Most chemical flow-through sensors based on piezoelectric phenomena (measurements of gases or liquids) are of the regenerable type. 

Figure 3.38 — Integrated flow-through sensors. (A) With electrochemical generation of the luminescent reagent. The flow stream path follows the line between the analyte inlet and the outlet to waste. (B) With immobilization of a phosphor (length, 3 cm internal diameter, 2 mm) 1 immobilized phosphor 2 CFG 3 quartz wool plug 4 KEL-F caps 5 hand-tightened screw 6 stainless steel capillaries. (C) Sensor based on reflectance measurements. The sensor membrane is fixed on a Plexiglas disc. Reflectance spectra are measured from the rear side. (Reproduced from [267] and [269] with permission of the American Chemical Society and Elsevier Science Publishers, respectively).

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