Measurement instrument selection

Instrumentation normally is denoted by a circle in which the variable being measured or controlled is denoted by an appropriate letter symbol inside the circle. When the control device is to be located remotely, the circle is divided in half with a horizontal line. Table 1.3 gives various instrumentation symbols and corresponding letter codes. The specific operating details and selection criteria for various process instrumentation are not discussed in this book. The reader is referred to earlier works by Cheremisinoff [1,2] for discussions on essential control and measurement instrumentation. 

Quality assurance of radiopharmaceutical preparation and use is obviously a very important topic because of its direct impact on patient diagnosis, treatment and health (see, e.g. Abreu 1996). Reference materials play only a small - but nevertheless important -role in this process, mainly in the area of calibration of radioactivity-measuring instruments. The materials of interest are all pure chemical containing calibrated activities of selected radionuclides used commonly in nuclear medicine (e.g. Co, Ga, I,... 

In addition to these instruments, an interface was required. The interface serves to count the pulses coming from the autoburet. The time it takes to deliver a preset number of microliters (1,2,5,10...) is measured accurately selection of a higher number of microliters eliminates the effect of mechanical imperfections in the operation of the buret and gives less fluctuating measurements, but also less frequent ones. By the time a CMT measurement has been completed, there is a recording of pH and also of the potential of the corroding metal vs. a separate reference... 

The first factor, especially important with ion-selective microelectrodes, can be eliminated by a suitable modification of the measuring instrument, notably by the use of a coaxial microelectrode (see [167] and section 4.2). If an inter-ferent is present in the solution at a concentration at which it does not affect the ISE potential, factors 4 and 6 are not operative. Penetration of the deter-minand into the membrane, factor 5, is very important for the response times of ISEs with ionophores in their membranes, provided that no hydrophobic anion is present in the membrane solution, as has been theoretically treated by Morf et aL [114]. As shown in section 3.3, the presence of a hydrophobic anion stabilizes the conditions in the membrane, with a marked effect on the shortening of the response time [93]. 

The temperature coefficient of the ISE potential has received relatively little attention. As follows from (3.1.7), the constant term (the ISE standard potential), the determinand and interferent activity coefficients and the selectivity coefficient, liquid-junction potentials and, of course, also the RTIZfF coefficient, depend on the temperature [118]. When the internal reference electrode and the reference electrode in the test solution are identical, the interferent activity sufficiently low and the liquid-j unction potentials negligible, then the constant term depends on the determinand activity in the electrode internal solution alone and thus the temperature coefficient of the measured EMV depends only on the temperature coefficient of the determinand activity coefficient and on the/ 77z,F coefficient. Measuring instruments are usually... 

ISEs are well suited for flow measurements because the instrumentation and signal handling are simple, the measurement is almost independent of the liquid flow-rate, the linear dynamic range is broad, the temperature dependence is not very pronounced and the measurement is selective (the selectivity is, however, a drawback in applications to chromatography). The experimental conditions are readily adjusted and often only consist of ionic strength and pH maintenance. ISEs with solid membranes usually exhibit better performance than liquid membrane electrodes and gas probes, because their response is faster and they are mechanically stronger. The most difficult problem is passivation of the electrodes in some media, for example, biological fluids or surface and waste waters. 

The sensitivity of a method (not to be confused with selectivity or limit of detection) is how much the indication of the measuring instrument increases... 

The pH can be measured instrumentally using a pH meter, a potentiometer, and an ion-selective electrode. The pH meter should be calibrated using buffers obtained from commercial sources. All pH meters come with model-specific instructions for calibration. The following protocol gives a basic overview of the calibration procedure and subsequent measurement for all pH meters. Review the instructions of the specific model in your laboratory to insure proper calibration. 

Dynamic Measurements. A Vibron direct reading viscoelastometer (Toyo Measuring Instruments Co., Ltd., Tokyo, Japan) was used to determine viscoelastic properties. This instrument and its operation have been described in detail by Takayanagi and Yoshino (II, 14). All polymers and blends were examined at a standard frequency of 110 cps. In a few selected examples measurements were also made at 3.5 cps. 

The chemical composition can be measured by traditional wet and instrumental methods of analysis. Physical surface area is measured using the N2 adsorption method at liquid nitrogen temperature (BET method). Pore size is measured by Hg porosimetry for pores with diameters larger than about 3.0 nm (30 A) or for smaller pores by N2 adsorp-tion/desorption. Active catalytic surface area is measured by selective chemisorption techniques or by x-ray diffraction (XRD) line broadening. The morphology of the carrier is viewed by electron microscopy or its crystal structure by XRD. The active component can also be measured by XRD but there are certain limitations once its particle size is smaller than about 3.5 nm (35 A). For small crystallites transmission electron microscopy (TEM) is most often used. The location of active components or poisons within the catalyst is determined by electron microprobe. Surface contamination is observed directly by x-ray photoelectron spectroscopy (XPS). 

An important class of electronic measuring instruments is designed to retrieve weak voltage signals from accompanying noise. ° Since the frequency spectrum of white noise is very wide, typically from 0.1 Hz to several MHz with a l//intensity distribution, much of it can be eliminated with the use of a frequency-selective amplifier that passes... 

This book is the most comprehensive text on particle size measurement published to date and expresses the experience of the author gained in over fifty years of research and consulting in particle technology. Previous editions have found wide use as teaching and reference texts. For those not conversant with particle size analysis terminology, techniques and instruments, the book contains basis information from which instrument selection can be made. For those familiar with the field, it provides an update of new instrumentation - particularly on-line or in - process instruments -upon which the control of particle processes is based. 

Scheme 2. Since it is not easy to modify the evaluation software in commercially available PCS measuring instruments, one can always select the hydrodynamic or Stokes radius (equivalent spherical radius) rather than using the roundabout approach. It is possible to eliminate several parameters by setting both terms, Stokes formula and its new version, equal.

These are automatic instruments which have as many channels (crystals and counters) as there are spectral lines to be measured. In each channel a crystal and counter are fixed at the correct angular positions to measure a selected spectral line, such as line from element A. A number of channels are arranged in a circle around a centrally located x-ray tube (Fig. 15-6). All the analyzing channels receive the same fluorescent radiation from the sample, while one nondispersive control channel receives fluorescent radiation directly from a standard. The control channel serves to monitor the output of the x-ray tube. Some instruments have as many as 30 channels. 

The instrument or group of instruments selected for a particular application depend on several factors. Most important, of course, is the type of information sought. Other factors include cost, portability, and reliability under the conditions of operation. Process and stack gas monitoring po.se particuUmly difficult demands because of extreme conditions of temperature and humidity. In the case of measurement systems designed for routine monitoring, the maintenance required is an important factor. 

When properly shielded and connected to the appropriate apparatus, ion-selective microelectrodes should show the response and selectivity similar to conventional size electrodes. They should be calibrated in a solution of similar composition to that of the sample solution to be analyzed. The response time of the electrode itself should be comparable to that of conventional electrodes. However, with these electrodes the diffusion of the ions through the stagnant layer near the ion-selective membrane has a smaller effect on the transient response than the time constant of the measuring circuitry. Measuring instruments often contain a variable capacitor to adjust the delay caused by the capacitance of the input stage. 

To measure physical properties reliably, the following must occur The measuring instruments must be carefully selected and the instrument must be properly installed and operated. Factors to be considered in the selection process include the range of property to be measured, accuracy and reproducibility of the readings desired, ease of installation and servicing, and both capital and expected maintenance costs. 

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