The Measuring Device

To classify measuring devices is not a trivial task, since there are so many different types and the principles they are based on are vastly different. A general classification is difficult to make, since it depends strongly on the criterion one is using. A possible classification of sensors could be based on the measuring principle they are using, for example  

Even this classification is hard to complete, since there are always sensors that do not fall in one of the above categories. 

Another classification one could make is by looking at the medium the sensors are used for, for example gases, liquids, slurries, powders, etc. 

Another way of looking at sensors or measuring devices is from an industrial point of view which sensors are very coimnon and used mostly in the process industries. The five types that are most widely used are measurements for  

The last category could include devices for measirring density, viscosity, pH, oxygen in stack gases, taste, octane niunber, particle size distribution, etc. 

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This first part of the standard deals with the fundamental demands that are placed on the measuring devices to be used and on the procedures to be implemented toward a definition of the relevant parameters of the imaging system /6/. 

If very many measurements are made of the same variable a , they will not all give the same result indeed, if the measuring device is sufficiently sensitive, the surprising fact emerges that no two measurements are exactly the same. Many measurements of the same variable give a distribution of results Xi clustered about their arithmetic mean p. In practical work, the assumption is almost always made that the distribution is random and that the distribution is Gaussian (see below). 

There are five locations ia use for the taps used to couple the differential to the measurement device. These locations are depicted ia Figure 7. 

The process and instrumentation (P I) diagram provides a graphical representation of the control configuration for the process. The P I diagrams illustrate the measurement devices that provide inputs to the control strategy, the actuators that will implement the results of the control calculations, and the function blocks that provide the control logic. 

Manufacturers of measurement devices always state the accuracy of the instrument. However, these statements always specify specific or reference conditions at which the measurement device will perform with the stated accuracy, with temperature and pressure most often appearing in the reference conditions. When the measurement device is apphedat other conditions, the accuracy is affected. Manufacturers usually also provide some statements on how accuracy is affected when the conditions of use deviate from the referenced conditions in the statement of accuracy. Although appropriate cahbration procedures can minimize some of these effects, rarely can they be totally eliminated. It is easily possible for such effects to cause a measurement device with a stated accuracy of 0.25 percent of span at reference conditions to ultimately provide measured values with accuracies of 1 percent or less. Microprocessor-based measurement devices usually provide better accuracy than the traditional electronic measurement devices. 

For regulatory control, repeatability is of major interest. The basic-objective of regulatory control is to maintain uniform process operation. Suppose that on two different occasions, it is desired that the temperature in a vessel be 80°C. The regulatoiy control system takes appropriate actions to bring the measured variable to 80°C. The difference between the process conditions at these two times is determined by the repeatability of the measurement device. 

Dynamics of Process Measurements Especially where the measurement device is incorporated into a closed loop control configuration, dynamics are important. The dynamic characteristics depend on the nature of the measurement device, and also on the nature of components associated with the measurement device (for example, thermowells and sample conditioning equipment). The term mea-.sui ement system designates the measurement device and its associated components. 

Time constants. Where there is a capacity and a throughput, the measurement device will exhibit a time constant. For example, any temperature measurement device has a thermal capacity (mass times heat capacity) and a heat flow term (heat transfer coefficient and area). Both the temperature measurement device and its associated thermowell will exhibit behavior typical of time constants. 

While the manufacturers of measurement devices can supply some information on the dynamic characteristics of their devices, interpretation is often difficult. Measurement device dynamics are quoted on varying bases, such as rise time, time to 63 percent response, settling time, and so on. Even where the time to 63 percent response is quoted, it might not be safe to assume that the measurement device exhibits first-order behavior. 

Where the manufacturer of the measurement device does not supply the associated eqiiipment (thermowells, sample conditioning equipment, and the like), the user must incorporate the characteristics of these components to obtain the dynamics of the measurement system. 

Reliability. Data available from the manufacturers can be expressed in various ways and at various reference conditions. Often, previous experience with the measurement device within the purchaser s organization is weighted most heavily. 

Electiical classification. Article 500 of the National Electric-Code provides for the classification of the hazardous nature of the process area in which the measurement device will be installed. If the measurement device is not inherently compatible with this classification, suitable enclosures must be purchased and included in the installation costs. 

Physical access. Subsequent to installation, maintenance personnel must have physical access to the measurement device for maintenance and cahbration. If additional structural facilities are required, they must be included in the installation costs. 

Calibration Cahbration entails the adjustment of a measurement device so that the value from the measurement device agrees with the value from a standard. The International Standards Organization (ISO) has developed a number of standards specifically directed to cahbration of measurement devices. Furthermore, compliance with the ISO 9000 standards requires that the working standard used to cahbrate a measurement device must be traceable to an internationally recognized standard such as those maintained by the National Institute of Standards and Technology (NIST). 

Within most companies, the responsibility for cahbrating measurement devices is delegated to a specific department. Often, this department may also be responsible for maintaining the measurement device. The specific cahbration procedures depend on the type of measurement device. The frequency of calibration is normally predetermined, but earlier action may be dic tated if the values from the measurement device become suspect. 

Cahbration of some measurement devices involves comparing the measured value with the value from the working standard. Pressure and differential pressure transmitters are calibrated in this manner. Calibration of analyzers normally involves using the measurement device to analyze a specially prepared sample whose composition is known. These and similar approaches can he applied to most measurement devices. 

Modern control systems permit the measurement device, the control unit, and the final actuator to be physically separated by several hundred meters, if necessary. This requires the transmission of the measured variable from the measurement device to the control unit, and the transmission of the controller output from the control unit to the final ac tuator. 

Pulse Inputs Where the sensor within the measurement device is digital in nature, analog-to-digital conversion can be avoided. For rotational devices, the rotational element can be outfitted with a shaft encoder that generates a known number of pulses per revolution. The digital system can process such inputs in either of the following ways ... 

Measurement Devices and Actuators Often referred to as level 0, this layer couples the control and information systems to the process. The measurement devices provide information on the cur-... 

An interlock is a protec tive response initiated on the detection of a process hazard. The interlock system consists of the measurement devices, logic solvers, and final control elements that recognize the hazard and initiate an appropriate response. Most interlocks consist of one or more logic conditions that detect out-of-hmit process conditions and respond by driving the final control elements to the safe states. For example, one must specify that a valve fails open or fails closed. 

These tests must encompass the complete interlock system, from the measurement devices through the final control elements. Merely simulating inputs and checking the outputs is not sufficient. The tests must duplicate the process conditions and operating environments as closely as possible. The measurement devices and final control elements are exposed to process and ambient conditions and thus are usually the most hkely to fail. Valves that remain in the same position for extended periods of time may stick in that position and not operate when needed. The easiest component to test is the logic however, this is the least hkely to fail. 

Where and to the extent that the availability of technical data pertaining to the measurement devices is a specified requirement, the standard requires such data to be made available, when required fay the customer or customer s representative, for verification that it is functionally adequate. 

Linearity is the difference in the bias values through the expected operating range of the measuring device. 

This requirement hides an important provision. It not only applies to inspection, measuring, and test equipment but to the measurements that are performed with that equipment. Anywhere you intend performing product verification or monitoring processes you need to ensure that the environmental conditions are suitable. By environmental conditions is meant the temperature, pressure, humidity, vibration, lighting, cleanliness, dust, acoustic noise, etc. of the area in which such measurements are carried out. To avoid having to specify the conditions each time, you need to establish the ambient conditions and write this into your procedures. If anything other than ambient conditions prevail, you may need to assess whether the measuring devices will perform adequately in these conditions. If you need to discriminate between types of equipment, the ones most suitable should be specified in the verification procedures. 

As with the reverse dial indicator method, the measuring device used for rim-and-face alignment is also a dial indicator. The fixture has two runout indicators mounted on a common arm as opposed to reverse-dial fixtures, which have two runout indicators mounted on two separate arms. 

More accurate measurement of air flow can be achieved with nozzles or orifice plates. In such cases, the measuring device imposes a considerable resistance to the air flow, so that a compensating fan is required. This method is not applicable to an installed system and is used mainly as a development tool for factory-built packages, or for fan testing. Details of these test methods will be found in BS.1042, BS.2852, and ASHRAE 16-83. 

The uncertainty depends on the nature of the measuring device. Eight mL of liquid can be measured with less uncertainty in the 10-mL graduated cylinder than in the 100-mL graduated cylinder. 

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