Instrument inertia

A major limitation in controlled stress rheometers is instrument inertia. It is very similar to the transducer inertia described with eq. 8.2.1. The torque imposed on the drag cup must overcome reluctance torque, air bearing friction, and rotor inertia as well as the sample viscosity. The inertia portion dominates with low viscosity samples (Kreiger, 1990). Figure 8.2.11a illustrates the problem. Stress was commanded to increase linearly from 0 to... 

Errors introduced in transient viscosity measurements (broken lines) as a result of instrument inertia can be eliminated by an active control loop (—). (a) Stress ramp 0-W seconds for a 5 mPa S Newtonian standard. Adapted from Franck (1992). (b) Sinusoidal oscillations on a 100 mPa-s standard. 

For the DHR-3, an apparent maximum in the go-curve was observed at 23 rad/s. This maximum is believed to be caused by instrument inertia effects. When using the DHR-3 in the correlation acquisition mode, the transient data that is Fourier-transformed by the software is the raw torque data. For a CMT-rheometer the raw torque Mr is the sum of the sample torque Ms and an additional contribution caused by instrument inertia M/ as shown in Eq. 6 [11]. 

Usually this type of anemometer does not provide information on the flow direction. Vice versa, the. sensors are made as independent of the flow direction as possible—omnidirectional. This is an advantage for free-space ventilation measurements, as the flow direction varies constantly and a direction-sensitive anemometer would be difficult to use. Naturally, no sensor is fully omnidirectional, but satisfactory constructions are available. Due to the high sensor thermal inertia, this type of anemometer is unsuitable for high-frequency flow fluctuation measurement. They can be used to monitor low-frequency turbulence up to a given cut-off frequency, which depends on the dynamic properties of the instrument. 

The differential equations Eqs. (10) and (29)3, which represent the heat transfer in a heat-flow calorimeter, indicate explicitly that the data obtained with calorimeters of this type are related to the kinetics of the thermal phenomenon under investigation. A thermogram is the representation, as a function of time, of the heat evolution in the calorimeter cell, but this representation is distorted by the thermal inertia of the calorimeter (48). It could be concluded from this observation that in order to improve heat-flow calorimeters, one should construct instruments, with a small... 

The development of the theory of heat-flow calorimetry (Section VI) has demonstrated that the response of a calorimeter of this type is, because of the thermal inertia of the instrument, a distorted representation of the time-dependence of the evolution of heat produced, in the calorimeter cell, by the phenomenon under investigation. This is evidently the basic feature of heat-flow calorimetry. It is therefore particularly important to profit from this characteristic and to correct the calorimetric data in order to gain information on the thermokinetics of the process taking place in a heat-flow calorimeter. 

Isoperibolic instruments have been developed to estimate enthalpies of reaction and to obtain kinetic data for decomposition by using an isothermal, scanning, or quasi-adiabatic mode with compensation for thermal inertia of the sample vessel. The principles of these measuring techniques are discussed in other sections. 

Current concentrations of GHG have already caused the mean global temperature to increase by 0.76 °C in the period from 1850 to 2005 owing to the inertia of the climate system this will lead to at least a further half-degree warming over the next few decades. Eleven of the twelve years from 1995 to 2006 rank among the 12 warmest years in the instrumental record of global surface temperature (since 1850). 

The working principle of the thermocouple was discovered (1823) by Seebeck who observed that if wires of two different metals were joined to form a continuous circuit, a current flowed in the circuit when the two junctions were at different temperatures. In order to make a measurement, one junction (the reference junction) is maintained at a constant temperature (typically at 0°C) and the electromotive force produced when the other junction is at the test temperature is measured, or recorded, by a suitable instrument (or used as the input of a controller ). In order to choose the right kind of thermocouple among the many types available, the temperature range to be studied must be considered, as well as several requirements regarding sensitivity, calibration stability, chemical, thermal, mechanical inertia, etc. 

Due to their inertia and lack of immunogenicity, PFCs are analyzed by instrumental analytical techniques. Such existing methods have been included in this chapter due to the emerging relevancy of PFCs in food safety (Table 1). 

Many different techniques are available for flow measurement and for recording of respiratory functions or flow parameters in particular (e.g. [115,116]). However, not all methods are appropriate for measurement of inhalation flows, either because they have low frequency responses or they influence the shape of the inspiratory flow curve by a large volume or by the inertia of the measuring instrument (e.g. rotameters). They may also interfere with the aerosol cloud from the inhalation device during drug deposition studies. 

A related technique that is suitable for measurement of aerosols at lower mass loadings is the aerodynamic particle sizer (3, 10). In this instrument the aerosol is rapidly accelerated through a small nozzle. Because of their inertia, particles of different aerodynamic sizes are accelerated to different velocities, and the smallest particles reach the highest speeds. The particle velocity is measured at the outlet of the nozzle. From the measurements of velocities of individual particles, particle size distributions can be determined. The instrument provides excellent size resolution for particles larger than about 0.8 xm in diameter, although sampling difficulties limit its usefulness above 10 xm. 

The constants Kp, Kt, and Kd are settings of the instrument. When the controller is hooked up to the process, the settings appropriate to a desired quality of control depend on the inertia (capacitance) and various response times of the system, and they can be determined by field tests. The method of Ziegler and Nichols used in Example 3.1 is based on step response of a damped system and provides at least approximate values of instrument settings which can be further fine-tuned in the field. 

The reactions in Table 1 are transfer of H, O, N, S, halogen and alkali metal atoms. They are also reactions of atoms (H, O, N, S, halogen and alkali metal atoms) with small molecules. They are exothermic and have high specific rates (low activation energies and normal steric factors). These features are desirable for the study of chemi-excitation. High heat release permits substantial excitation. Small, low moment of inertia product molecules have spectra that may be resolved with moderate power spectrographic and spectrometric instruments. High speed provides the necessary number of reaction acts per unit time. 

There are other types of adiabatic calorimeters available on the market [14, 15], such as the VSP (Vent Sizing Package) [16], PHITEC [6], and RSST (Reactive System Screening Tool). These instruments are essentially designed for vent sizing requirements [17-20] and present a lower thermal inertia than the ARC. 

While an intensity profile at the detector as a function of retardation may be acquired in a step-scan mode, two major drawbacks affect this method of interferogram acquisition. First, the mirror(s) requires stabilization times with mirror inertia and time constants of the control loop determining this parameter in achieving a given optical retardation. Second, additional hardware and control mechanisms need to be incorporated into the spectrometer, thus increasing instrument cost and complexity. In certain cases, however, the utility of a step-scan instrument justifies this additional expense. Historically, the step-scan approach was favored with slow detectors. With the advent of fast detectors and electronics, step-scan interferometry became... 

PC instruments are preferable to HF instruments for isothermal studies, and in studies in which the temperature scanning rate is high, because the very small sample and reference holders in the PC type have much smaller thermal inertia than the relatively large heating block in the furnace of the HF type (Hatakeyama and Quinn, 1994). 

Another feature of FTMA is the fact that it readily provides direct assessment of sample inertia, bending and geometry effects. This would be very difficult with single frequency instruments. 

Infrared spectroscopy has broad appHcations for sensitive molecular speciation. Infrared frequencies depend on the masses of the atoms involved in the various vibrational motions, and on the force constants and geometry of the bonds connecting them band shapes are determined by the rotational stmcture and hence by the molecular symmetry and moments of inertia. The rovibrational spectmm of a gas thus provides direct molecular stmctural information, resulting in very high specificity. The vibrational spectrum of any molecule is unique, except for those of optical isomers. Every molecule, except homonudear diatomics such as O2, N2, and the halogens, has at least one vibrational absorption in the infrared. Several texts treat infrared instrumentation and techniques (22,36—38) and their appHcations (39—42). 

Departures from laminar flow, which are attributed to inertia and/or viscoelasticity, result in turbulences, i.e., an uneven flow pattern with locally clear deviations from the flow direction. In the extreme, the flowing sample can start to circulate locally, which is known as Taylor vortices and mainly observed in concentric cylinder instruments, where the inner cylinder rotates,i.e., in cup and bob viscometers. ... 

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