Differential measurements

Fig. 11. Capacitive pressure sensor in the differential measurement configuration. Courtesy of Rosemount, Inc.

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. 

Measurement of Performance The amount of useful work that any fluid-transport device performs is the product of (1) the mass rate of fluid flowthrough it ana (2) the total pressure differential measured immediately before and after the device, usually expressed in the height of column of fluid equivalent under adiabatic conditions. The first of these quantities is normally referred to as capacity, and the second is known as head. 

Detection in HPLC by ultraviolet absorption is commonplace and does not merit special mention here (see Section F.l below). There appear to be only few quantitative procedures for sulphones which are based on measurements of UV absorption. The final stage in a determination of the acaricide Sulphenone by Shuman29 was differential measurement of the absorption at 230,240 and 250 nm in isooctane solution. Kashiwa and... 

We see here the gain brought by the use of light the small value of h is partially counterweighted by L/Xopt- If T 1 km and Xopt 10 m we gain a factor 10 and the challenge is now to measure phase shifts of the order of 10 rad. We may also note at this point that we perform a differential measurement as we measure the phase difference between waves that followed different paths. 

To improve the selectivity of chronoamperometric in vivo analysis, a differential measurement ta hnique has been employed Instead of a single potential pulse, the potential is alternately pulsed to two different potentials giving rise to the name double chronoamperometry. This waveform is shown in Fig. 15 B. Because the current contributions of individual electroactive components add linearly to produce the observed current output, the difference in current response at the two potentials is the current due to only those compounds which are oxidized at the higher potential and not oxidized at the lower potential. This system provides two responses, the current due to easily oxidized compounds and the current due to harder to oxidize compounds. This gives greater selectivity than the direct chronoamperometric method. 

The determination of rate change of the logarithm of the neutron level, as in the source range, is accomplished by the differentiator. The differentiator measures reactor period or startup rate. Startup rate in the intermediate range is more stable because the neutron level signal is subject to less sudden large variations. For this reason, intermediate-range startup rate is often used as an input to the reactor protection system. 

Generally, dependent on the sensor design and working conditions, the optimum separation distance or optimum density of the immobilized ssDNA should be found in order to achieve a maximal hybridization signal. A twice increase in the hybridization signal can be obtained when combining a cation- and anion-sensitive FED in a differential measuring set-up. 

Rather sophisticated applications of Mossbauer spectroscopy have been developed for measurements of lifetimes. Adler et al. [37] determined the relaxation times for LS -HS fluctuation in a SCO compound by analysing the line shape of the Mossbauer spectra using a relaxation theory proposed by Blume [38]. A delayed coincidence technique was used to construct a special Mossbauer spectrometer for time-differential measurements as discussed in Chap. 19. 

In practice, the true heating rates (dT/dt)ca and (dT/dt)cb are assumed to be equal to the programmed scan rate j3, and the true heat flow rate difference (heat flow rate difference, Ao, which reflects the intrinsic thermal asymmetry of the differential measuring system ... 

Differential measurements are counts of the number of emissions In a unit time. 

A significant increase in the measuring precision and accuracy can sometimes be attained by differential measurement with two ISEs. Two principal approaches can be taken ... 

A technique utilizing the advantages of both differential measurement and titration is null-point potentiometry [37, 38, 83]. The same arrangement is used as described under (Z ), but concentration c is not constant and is varied until Cj = c = c. Then,... 

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]). 

Figure 4.20.A shows a more recent cell reported by Cobben et al. It consists of three Perspex blocks, of which two (A) are identical and the third (B) different. Part A is a Perspex block (1) furnished with two pairs of resilient hooks (3) for electrical contact. With the aid of a spring, the hooks press at the surface of the sensor contact pads (4), the back side of which rests on the Perspex siuface, so the sensor gate is positioned in the centre of the block, which is marked by an engraved cross as in the above-described wall-jet cell. Part B is a prismatic Perspex block (2) (85 x 24 x 10 mm ) into which a Z-shaped flow channel of 0.5 mm diameter is drilled. Each of the wedges of the Z reaches the outside of the block. The Z-shaped flow-cell thus built has a zero dead volume. As a result, the solution volume held between the two CHEMFETs is very small (3 pL). The cell is sealed by gently pushing block A to B with a lever. The inherent plasticity of the PVC membrane ensures water-tight closure of the cell. The closeness between the two electrodes enables differential measurements with no interference from the liquid junction potential. The differential signal provided by a potassium-selective and a sodium-selective CHEMFET exhibits a Nemstian behaviour and is selective towards potassium in the presence of a (fixed) excess concentration of sodium. The combined use of a highly lead-selective CHEMFET and a potassium-selective CHEMFET in this type of cell also provides excellent results.

Fig. 2a,b. Principle of integral and differential measurement of reaction kinetics a concentration-time course b rate-time course. Symbols indicate initial and final concentrations in parallel integral measurements. Corresponding curves use same dashes... 

Differential measurement of cortisol and cortisone in human saliva. (161)... 

Ultra-sensitive differential measurement of cortisol and > cortisone in biological samples using fluorescent ester derivatives in normal phase HPLC. (162)... 

It is the differential measurement of the solubility of a product in two solvents (with O figuring octanol and W figuring water). When log P is very high, the chemical compound under study reveals much more soluble in octanol than in water. Therefore, the chemical compound has a much more lipophilic character. Conversely, when log P is low, the chemical substance is more hydrophilic. When log P equals zero, the chemical is equally split in two solvent phases. 

Double beam spectrophotometers allow differential measurements to be made between the sample and the analytical blank. They are preferable to single beam instruments for measurements in problematic solutions. For high performance instruments, the bandwidth can be as low as 0.01 nm. 

In its ultimate form, AIDECS is intended to be an inspection device which will provide a high resolution, three-dimensional scan profile of an entire expl charge in an artillery shell. It is designed to perform a differential measurement which, with an appropriately small inspection volume element, will not only identify the presence of discontinuities in the expl (such as voids, cracks, annular rings, base separations and inclusions), but is also to provide data about their size, three-dimensional location and orientation... 

The earliest design of the glucose electrode (which is generally applicable to any oxidase and uses oxygen as the ultimate electron acceptor) is based on the differential measurement of oxygen deficiency at the oxygen electrode, caused by the... 

Subtraction of reference Linear drift subtraction Normalization Averaging Linearization Differential measurements Baseline correction Relative signals Signal quality redundancy Transform functions, e.g., antilog... 

Ratio measurements in analytical chemistry will often relate values in different units for the numerator and denominator. The most commonly used ratios between SI units are summarized in matrix form in Table 1 (adapted from [5]). Confusion may result when values are stated in different SI quantities. The pharmaceutical industry, for example, is careful to distinguish values in mg/g from values in mmol/g. It is important to retain the two units used in expressing a ratio such as mol/g. Differential measurements are often made, obtaining a ratio of ratios for which the numerator and denominator are generally expressed in multiples of identical pairs of units, e.g. (g/L)/(g/L). For such a ratio of ratios no great harm is done by stating, for instance the con-... 

Figure 10.10 Coefficients for sharp-edged orifice with pressure differential measured either at the flanges or at the vena contracta. (Calculated from data in Refs. f24] and [25].)...

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