Mechanical instmments

The Autovibron is the automated version of the Rheovibron (284). It can be purchased in automated form, or the automation system can be added later. Automation improves accuracy and productivity and eliminates the problems and operator dependence of results often associated with the manual instmment. Automation enables the Rheovibron to compete with the other commercial dynamic mechanical instmments. 

There may exist a mechanical instmment for measuring the softness of the apple material or you may simply use a taste panel to score each apple on a scale from 1 to 10. What the multivariate regression software program then will do for you is basically to produce a model that you can use to calculate the softness of each apple (Y-variable) from the measured impedance data (X-variables) or from any data derived from the measured data, such as Cole parameters. 

Much more information can be obtained by examining the mechanical properties of a viscoelastic material over an extensive temperature range. A convenient nondestmctive method is the measurement of torsional modulus. A number of instmments are available (13—18). More details on use and interpretation of these measurements may be found in references 8 and 19—25. An increase in modulus value means an increase in polymer hardness or stiffness. The various regions of elastic behavior are shown in Figure 1. Curve A of Figure 1 is that of a soft polymer, curve B of a hard polymer. To a close approximation both are transpositions of each other on the temperature scale. A copolymer curve would fall between those of the homopolymers, with the displacement depending on the amount of hard monomer in the copolymer (26—28). 

Two methods are used to measure pH electrometric and chemical indicator (1 7). The most common is electrometric and uses the commercial pH meter with a glass electrode. This procedure is based on the measurement of the difference between the pH of an unknown or test solution and that of a standard solution. The instmment measures the emf developed between the glass electrode and a reference electrode of constant potential. The difference in emf when the electrodes are removed from the standard solution and placed in the test solution is converted to a difference in pH. Electrodes based on metal—metal oxides, eg, antimony—antimony oxide (see Antimony AND ANTIMONY ALLOYS Antimony COMPOUNDS), have also found use as pH sensors (8), especially for industrial appHcations where superior mechanical stabiUty is needed (see Sensors). However, because of the presence of the metallic element, these electrodes suffer from interferences by oxidation—reduction systems in the test solution. 

Figure 13 shows a typical iastallation of a differential pressure instmment for closed tanks. Connections from the instmments are made to taps ia the vessel at minimum and maximum levels. Between the instmment and the maximum level tap is a constant reference leg. This leg is filled with Hquid until its head is equivalent to the head of the Hquid ia the vessel at maximum level. The reference leg must remain constant, with no formation of vapor under varying ambient conditions. On some appHcations it may be necessary to fiH the reference leg with a Hquid, such as water or a light oil, that remains stable. If the Hquid used ia the reference leg has a higher specific gravity than the Hquid ia the tank, the resulting difference ia head must be corrected for ia the iastmment. Most differential pressure measuriag instmments are equipped mechanically to suppress this difference. 

The use of "fixed" automation, automation designed to perform a specific task, is already widespread ia the analytical laboratory as exemplified by autosamplers and microprocessors for sample processiag and instmment control (see also Automated instrumentation) (1). The laboratory robot origiaated ia devices coastmcted to perform specific and generally repetitive mechanical tasks ia the laboratory. Examples of automatioa employing robotics iaclude automatic titrators, sample preparatioa devices, and autoanalyzers. These devices have a place within the quality control (qv) laboratory, because they can be optimized for a specific repetitive task. AppHcation of fixed automation within the analytical research function, however, is limited. These devices can only perform the specific tasks for which they were designed (2). 

Gc/k instmments are all of the gc/fdr variety that utilize an interferometer and requke a Fourier transform of the signal as opposed to employing a monochrometer and mechanical scanning. At least 3 scan/second are needed across the capillary gc peaks which are from 3 to 6 seconds wide. 

The mechanical parts of computer-controlled viscometers can be simple and should not become obsolete for many years. The complexity is in the software, and this is where changes will have to be made to keep the instmment up to date. Some manufacturers are offering new, updated software free for a period of several years after the instmment is purchased. 

In a similar fashion. Thermally Stimulated Current spectrometry (TSC) makes use of an appHed d-c potential that acts as the stress to orient dipoles. The temperature is then lowered to trap these dipoles, and small electrical currents are measured during heating as the dipoles relax. The resulting relaxation maps have been related to G and G" curves obtained by dynamic mechanical analysis (244—246). This technique, long carried out only in laboratory-built instmments, is available as a commercial TSC spectrometer from Thermold Partners L.P., formerly Solomat Instmments (247). 

A fully automated microscale indentor known as the Nano Indentor is available from Nano Instmments (257—259). Used with the Berkovich diamond indentor, this system has load and displacement resolutions of 0.3 N and 0.16 nm, respectively. Multiple indentations can be made on one specimen with spatial accuracy of better than 200 nm using a computer controlled sample manipulation table. This allows spatial mapping of mechanical properties. Hardness and elastic modulus are typically measured (259,260) but time-dependent phenomena such as creep and adhesive strength can also be monitored. 

Dynamic techniques are used to determine storage and loss moduli, G and G respectively, and the loss tangent, tan 5. Some instmments are sensitive enough for the study of Hquids and can be used to measure the dynamic viscosity T 7 Measurements are made as a function of temperature, time, or frequency, and results can be used to determine transitions and chemical reactions as well as the properties noted above. Dynamic mechanical techniques for sohds can be grouped into three main areas free vibration, resonance-forced vibrations, and nonresonance-forced vibrations. Dynamic techniques have been described in detail (242,251,255,266,269—279). A number of instmments are Hsted in Table 8. Related ASTM standards are Hsted in Table 9. 

Another resonant frequency instmment is the TA Instmments dynamic mechanical analy2er (DMA). A bar-like specimen is clamped between two pivoted arms and sinusoidally oscillated at its resonant frequency with an ampHtude selected by the operator. An amount of energy equal to that dissipated by the specimen is added on each cycle to maintain a constant ampHtude. The flexural modulus, E is calculated from the resonant frequency, and the makeup energy represents a damping function, which can be related to the loss modulus, E". A newer version of this instmment, the TA Instmments 983 DMA, can also make measurements at fixed frequencies as weU as creep and stress—relaxation measurements. 

Pig. 45. TA Instmments DMA 2980 dynamic mechanical analy2er. Courtesy of TA Instmments, Inc. 

Rheometric Scientific markets several devices designed for characterizing viscoelastic fluids. These instmments measure the response of a Hquid to sinusoidal oscillatory motion to determine dynamic viscosity as well as storage and loss moduH. The Rheometric Scientific line includes a fluids spectrometer (RFS-II), a dynamic spectrometer (RDS-7700 series II), and a mechanical spectrometer (RMS-800). The fluids spectrometer is designed for fairly low viscosity materials. The dynamic spectrometer can be used to test soHds, melts, and Hquids at frequencies from 10 to 500 rad/s and as a function of strain ampHtude and temperature. It is a stripped down version of the extremely versatile mechanical spectrometer, which is both a dynamic viscometer and a dynamic mechanical testing device. The RMS-800 can carry out measurements under rotational shear, oscillatory shear, torsional motion, and tension compression, as well as normal stress measurements. Step strain, creep, and creep recovery modes are also available. It is used on a wide range of materials, including adhesives, pastes, mbber, and plastics. 

The Weissenbetg Rheogoniometer is well suited to research on homogeneous viscoelastic fluids and elastic melts. For oscillatory shear a second motor-drive mechanism is added. This allows the use of 60 frequencies in the range of 7.6 x 10 to 40 Hz at ampHtudes between 2 x 10 and 3 X 10 rad. An electronic circuit improves the precision of oscillatory measurements, particularly at frequencies neat the natural resonance frequency of the instmment itself (298). 

Interferometry is difficult in the uv because of much greater demands on optical alignment and mechanical stabiUty imposed by the shorter wavelength of the radiation (148). In principle any fts interferometer can be operated in the uv when the proper choice of source, beam spHtter, and detector is made, but in practice good performance at wavelengths much shorter than the visible has proved difficult to obtain. Some manufacturers have claimed operating limits of 185 nm, and Fourier transform laboratory instmments have reached 140 nm (145). 

Future development of SAM-based analytical technology requires expansion of the size and shape selectivity of template stmctures, as well as introduction of advanced chemical and optical gating mechanisms. An important contribution of SAMs is in miniaturization of analytical instmmentation. This use may in turn have considerable importance in the biomedical analytical area, where miniature analytical probes will be introduced into the body and target-specific organs or even cell clusters. Advances in high resolution spatial patterning of SAMs open the way for such technologies (268,352). 

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