Pressure-drop flow sensor

A different variety of pressure-drop flow sensor relies on the Ktot effect instead of Ohm s law. In one design of this type of device, two piessuie-sensiiig trqK are placed within the gas stream, one... 

All of these pressure transducers offer frequency response characteristics acceptable for most or all respiratory applications. In situations requiring the measurement of a single pressure (for example, the pressure at the mouth during breathing), one of the two input ports to the transducer is left open to atmospheric pressure. In other situations requiring the measurement of a pressure differential (for example, measuring the output of a pressure-drop flow sensor), the two ports of the transducer are attached to the two taps of the sensor. 

Pressure drops have been measured in a Corning glass H EX reactor with pressure sensors located on reactive and utiHty Hnes and estimated for different fluids (water, glucose solutions) at various flow rates, from 2 to 101 h, and various temperature levels (from 20 to 50°C). The results are presented in Figure 12.6. 

In addition to absolute pressure measurements, pressure sensors can be used to determine flow rates when combined with a well-defined pressure drop over a microfluidic channel. Integration of optical waveguide structures provides opportunities for monitoring of segmented gas-liquid or liquid-liquid flows in multichannel microreactors for multiphase reactions, including channels inside the device not accessible by conventional microscopy imaging (Fig. 2c) (de Mas et al. 2005). Temperature sensors are readily incorporated in the form of thin film resistors or simply by attaching thin thermocouples (Losey et al. 2001). 

Area 300 is controlled using a distributed control system (DCS). The DCS monitors and controls all aspects of the SCWO process, including the ignition system, the reactor pressure, the pressure drop across the transpiring wall, the reactor axial temperature profile, the effluent system, and the evaporation/crystallization system. Each of these control functions is accomplished using a network of pressure, flow, temperature, and analytical sensors linked to control valves through DCS control loops. The measurements of reactor pressure and the pressure differential across the reactor liner are especially important since they determine when shutdowns are needed. Reactor pressure and temperature measurements are important because they can indicate unstable operation that causes incomplete reaction. 

A. FLOW. Orifice plates are by far the most eommon type of flow-rate sensor. The pressure drop across the orifice varies with the square of the flow in turbulent flow, so iqeasuring the difierential pressure gives a signal that can be related... 

When a flow sensor is installed for accurate accounting measurements of the absolute flow rate, many precautions must be taken, such as providing a long section of straight pipe before the orifice plate. For control purposes, however, one may not need to know the absolute value of the flow but only the changes in flow rate. Therefore pressure drops over pieces of equipment, around elbows or over sections of pipe can sometimes be used to get a rough indication of flow rate changes. 

Figure 7.4c shows a AP transmitter used with an orifice plate as a flow transmitter. The pressure drop over the orifice plate (the sensor) is converted into a control signal. Suppose the orifice plate is sized to give a pressure drop of 100 in H2O at a process flow rate of 2000 kg/h. The AP transmitter converts inches of HjO into milliamperes, and its gain is 16 mA/100 in HiO. However, we really want flow rate, not orifice-plate pressure drop. Since AP is proportional to the square of the flow rate, there is a nonlinear relationship between flow rate F and the transmitter output signal ... 

The most important sensors for control of the drying process are those for inlet-air and exhaust-air temperature and the sensor for air flow measurement, located in the ah transport system. Other sensors for the spray agglomeration process include those for atomization air pressure and volume, pressure drops (across... 

Other sensors which are described in Volume 1 (Sections 6.3.7-6.3.9) are the variable area meter, the notch or weir, the hot wire anemometer, the electromagnetic flowmeter and the positive displacement meter. Some of these flowmeters are relatively less suitable for producing signals which can be transmitted to the control room for display (e.g. weir, rotameter) and others are used in more specialist or limited applications (e.g. magnetic flowmeter, hot wire anemometer). The major characteristics of different types of flow sensor are summarised in Table 6.1. Brief descriptions follow of the principles underlying the more important types of flowmeter not described in Volume 1. In many instances such flow sensors are taking the place of those more traditional meters which rely upon pressure drop measurement. This is for reasons of versatility, energy conservation and convenience. 

Presently a commercially available two stage vacuum system comprising a membrane (Pfeiffer MVP 006-4) and a turbo pump (Pfeiffer HiPace 10) in combination with a pressure sensor (Leybold Vacuum Ionivac ITR 90) establish a pressure of about 0.1 Pa in the system. Three electric valves are used to control the gas flow into the capillary system and for the bypasses. The use of macro devices simplifies the handling of the experimental setup and also the electronic control. Pressure drops for plasma and sample gases are accomplished by an appropriate combination of capillaries with different diameters and lengths as described in Sect. 4. 

The heat flow rate (Q) of a gaseous fuel is calculated as the product of its volumetric flow rate at standard conditions (V0) and its calorific value (CV). The Wobbe index (WI) measures the ratio between the net CV and the square root of specific gravity (SG). With orifice-type flow sensors, the advantage of detecting the WI is that it eliminates the need to separately measure the specific gravity this is because the product of the WI and orifice pressure drop results in a constant times the heat flow rate (KxQ), without requiring a separate measurement of SG. 

Measurement and control of low-flow rates are a requirement in such applications as fuel cells, purging, bioreactors, leak testing, and controlling the reference gas flow in chromatographs or in plasma-emission spectrometers. The most traditional and least expensive low-flow sensor is the variable-area flowmeter. It has a high rangeability (10 1) and requires little pressure drop. Due to its relatively low accuracy, it is limited to purge and leak-detection applications. 

The variable-area flowmeter is a head-type flow sensor, but it does not measure the pressure drop across a fixed orifice instead, the pressure drop is held relatively constant, and the orifice area is varied to match the flow (Figure 3.98). In gravity-type variable-area flowmeters, increase in flow lifts the float, piston, or vane, and it is the weight of these flow elements that has... 

Flow. Nonintrusive sensors that can be maintained at the process temperature are ideally suited to measure the flow rate of feed and product streams. Magnetic flow meters are suitable and inexpensive choice for aqueous streams. Organic streams with low dielectric constants require a vibrating tube mass flow meter to satisfy these criteria. Although commonly installed, flow meters that operate by inducing a pressure drop proportional to the flow rate present restrictions for solids accumulation that may alter the calibration. An alternative approach is to monitor the rotational speed of a positive displacement pump. Accuracy of this method is subject to wear and tolerances in the pump. 

Vortex meters are intrusive because they rely on disturbing the flow regime by placing an object in the fluid stream to produce an oscillatory motion downstream. The object can take many shapes but often a thin wire is used, as shown in Figme 6.11, which minimizes the pressure drop. The oscillatory motion is referred to as a vortex and may be detected by piezoelectric transducers, or magnetic or optical sensors. The number of vortices present is proportional to the volumetric flow rate. 

The operation of the Porter mass-flow controller is a follows. The sensing system utilizes a by-pass tube with a heater situated at the center. Precision temperature sensors are placed equidistant up stream and down stream of the heater. A proprietary baffle system in the main conduit creates a pressure drop that causes a fixed proportion of the flow to be diverted through the sensor tube. At zero flow both sensors are at the same temperature. When there is flow, the down stream sensor is heated producing a differential temperature across the sensors. As the temperature of the gas will be proportional to the product of mass flowing and its specific heat, the differential temperature will also be... 

Heat is usually applied in various amounts and in different locations, whether in a metal plasticating barrel (extrusion, injection molding, etc.) or in a metal mold/die (compression, injection, thermoforming, extrusion, etc.). With barrels a thermocouple is usually embedded in the metal to send a signal to a temperature controller. In turn, it controls the electric power output device regulating the power to the heater bands in different zones of the barrel. The placement of the thermocouple temperature sensor is extremely important. The heat flow in any medium sets up a temperature gradient in that medium, just as the flow of water in a pipe sets up a pressure drop, and the flow of electricity in a wire causes a voltage drop. 

A fluidic pressure sensor measures the pressure drop due to the flow of fluid. Velocimetry is a technique for measuring the velocity profile of fluids by means of particles which are viscously dragged by the bulk of the fluid. Under laminar flow with no slip at the wall boundaries, a parabolic velocity distribution exists, and a good flow sensor integrates the parabolic velocity profile over the cross section of the flow. 

A low-cost, low-power-consuming micromachined flow and pressure sensing device has also been reported. This sensor can be used for monitoring pressure and flow rate in clean fluids without particles and without the tendency to coat the channel. The pressure is measured with capacitive or piezoresistive pressure sensors and the flow rate is computed from the pressure drop over a well-defined, hydraulic resistance. Although this device is yet to be used for process analysis in industrial stream, it will gain more use in this area in the future. 

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