Sensors and Transmitters

The pneumatic or electrical signals depict only the analog measured variable. The direct measured variable is not always the desired result, however. Thus, for example, one obtains from flow measurements with diaphragms a differential pressure, not the mass flow, q , which for compressible flow in an orifice is given by Eq. (32), where Ap = pi — pi is the pressure drop between the taps upstream and downstream respectively, pj is the upstream density, Q is an empirical discharge coefficient, Ai is the area of the orifice, Y is a dimensionless expansion factor, and p = 2/ 1 is the ratio of the orifice to the upstream pipe diameter (see Section 12.2.5) [10]. 

As can be seen from Eq. (32), the mass flow is proportional to the product y/P Ap. As density and flow can be subject to variations during the process, the downstream pressure will change and so will the density, and the two measured values are coupled together. Additionally, density is also a function of temperature so this must also be measured and a correction added. A smart transducer provides functions beyond those necessary for generating a correct representation of a sensed or controlled quantity. A smart transducer can be used in this situation to correct the flow for pressure and temperature. 

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To obtain the benefits of this seal, it is necessary to install a gauge indicating the actual seal chamber pressure. Sensors and transmitters can be used to monitor and act on a pressure change. One of the two seals can fail without product loss or fugitive emissions. This seal would be connected to a pumping unit seen later in this chapter. 

Figure 1.1. Schematic diagram of instrumentation associated with a fermentor. The steam sterilization system and all sensors and transmitters are omitted for clarity. Solid lines represent process streams. Hairlines represent information flow.

Once we know what to control, we need to find a way to measure the quantity. If the transducer (sensor and transmitter packaged together) is placed far downstream or is too well insulated and the response is slow, the measurement transfer function may appear as... 

Figure 5.18 Schematic representation of the Dexcom sensor and transmitter mounted on the skin. Copyright 2008 Ahhott. Used with permission.

The sensor comes to the patient in a sterile package that contains a sensor support mount for adhering the sensor and transmitter to the skin and a sensor insertion device. The mount is applied to the skin in a similar manner to apply an... 

Sensor type and location, as well transmitter characteristics, noise, and sampled data issues, also can affect loop performance. Most continuous measurement sensors and transmitters have relatively fast dynamics and a noise filter, which can be approximated by a... 

A controller (or control device) that receives the input signal from the sensor and transmitter, and compares the value of the process variable with a target value (the set point) and sends out an output signal to adjust the variable as needed. 

A controlled process variable is usually a flow rate, pressure, or temperature signal from a sensor and transmitter that is used to provide the input to a controller. A controlled variable is controlled to a setpoint. 

Measurement devices (sensors and transmitters) and actuation equipment (for example, control valves) are used to measure process variables and implement the calculated control actions. These devices are interfaced to the control system, usually digital control equipment such as a digital computer. Clearly, the... 

The dynamic behavior of a temperature sensor and transmitter can be described by the FOPTD transfer function... 

Figure 2.12 shows a selection of thermocouples, RTDs, and temperature accessories, such as thermal wells, that are typically available from instrument suppliers (in this case Emerson Process Management). Figure 2.13 shows a picture of a typical temperature sensor and transmitter assembly. 

Figure 2.13 Temperature sensor and transmitter assembly (reproduced by permission of Emerson Process Management)...

Loop two or more elements where a signal passes between them for measurement and/or control, eg, sensor -i- transmitter -i- control + valve device two or more elements combined for a discrete state condition, eg, block valve -i- solenoid valve -i- actuator -I- limit switches... 

A factor which previously limited installation of automatic corrosion monitoring systems was the cost of cabling between sensors and control room instrumentation-this was particularly relevant to the electrical resistance (ER) systems. Developments to overcome this have included transmitter units at the probe location providing the standard 4-20 mA output (allowing use of standard cable) for onward transmission to data systems or the use of radio linkage which has been successfully used for other process-plant instrumentation. 

PW 9610 Dissolved Oxygen Sensor and PW 9600 Dissolved Oxygen Transmitter, 7600.32.9000.11, Philips, Eindhoven. 

The inclusion of radiative heat transfer effects can be accommodated by the stagnant layer model. However, this can only be done if a priori we can prescribe or calculate these effects. The complications of radiative heat transfer in flames is illustrated in Figure 9.12. This illustration is only schematic and does not represent the spectral and continuum effects fully. A more complete overview on radiative heat transfer in flame can be found in Tien, Lee and Stretton [12]. In Figure 9.12, the heat fluxes are presented as incident (to a sensor at T,, ) and absorbed (at TV) at the surface. Any attempt to discriminate further for the radiant heating would prove tedious and pedantic. It should be clear from heat transfer principles that we have effects of surface and gas phase radiative emittance, reflectance, absorptance and transmittance. These are complicated by the spectral character of the radiation, the soot and combustion product temperature and concentration distributions, and the decomposition of the surface. Reasonable approximations that serve to simplify are ... 

We will consider all the components of this temperature control loop in more detail later in this book. For now we need only appreciate the fact that the automatic control of some variable in a process requires the installation of a sensor, a transmitter, a controller, and a final control element (usually a control valve). Most of this book is aimed at learning how to decide what type of controller should be used and how it should be tuned, i.e., how should the adjustable tuning parameters in the controller be set so that we do a good job of controlling temperature. 

Acoustic chemometrics has its greatest benefits in cases where haditional sensors and measurement techniques, such as flow, temperature and pressure transmitters cannot be used. In many processes it is preferable to use noninvasive sensors because invasive sensors may cause disturbances, for example fouling and clogging inside the process equipment such as pipelines, reactors cyclones, etc. In this chapter we concentrate mainly on new industrial applications for acoustic chemomehics, and only discuss the necessary elements of the more technical aspects of the enabling technology below - details can be found in the extensive background literature [3-5],... 

Smart fibers can make smart sutures—threads used by surgeons to sew up incisions—such as suture that ties itself when heated And Sensatex Incorporated makes a smart shirt that automatically monitors the wearer s vital signs. Special sensors and conducting fibers embedded in the shirt measure heart rate, respiration, and body temperature, and a tiny controller relays this information by transmitter to a medical station. 

A measuring instrument consists basically of a sensor and a transducer. The sensor transmits a signal x to the transducer every second and the transducer responds as a first-order system with a time constant of Ss and a steady-state gain of 2 units. The output yt of the transducer drives a transmitter which also approximates to first-order behaviour with a time constant of 4s and a steady-state gain of 5. 

Radar level transmitters and gauges use electromagnetic waves, typically in the microwave bands to make a continuous liquid and some solid level measurements. The radar sensor is mounted on the top of the vessel and is aimed down, perpendicular to the liquid surface. Most tank-farm gauges are operated on the FMCW principle (Figure 3.121). Other gauges and transmitters, particularly the lowest-cost units, are operated on the pulse principle. Both principles are fundamentally based on the time of flight from the sensor to the level of the surface to be measured. In the FMCW method, this time of flight is tracked on a carrier wave in the pulse method, it is the echo return. 

In the automation and optimization of solar, wind, and wave energy systems, linear or angular position measurement plays an important role. There is a difference between absolute position sensors and sensors that detect only displacement. Position measurement requires a sensor and a transmitter. Mounting of position-sensing equipment in most applications must be custom-made. In position sensing, the linearity is only about 1%, but that is acceptable, because repeatability matters more. 

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