Fixed frequency equipment

Plant and equipment that require a service on completion of a certain specified period of hours may, through experience, be catered for on a fixed frequency basis. If employed, this method avoids the need to record running hours (or, in the case of transport, mileage) on a daily or weekly basis to schedule the relevant planned maintenance. If it is essential that the maintenance of the plant and equipment be carried out on the completion of a certain number of operating hours or mileage, then this must be allowed for in the allocated work hours of the relevant trade group. 

Microwaves have wavelengths between 1 mm and 1 m and hence have similar frequencies to radar and telecommunication devices. So as not to cause interference with these systems the frequency of radiation that can be emitted by household and industrial appliances is strictly regulated, with most appliances operating at a fixed frequency of 2.45 GHz. To some extent this reduces the flexibility of such equipment. 

The basic optical setup was shown in Fig. 12 [90]. The spectra were recorded on a commercially available spectrometer equipped with an external PM setup. The photoelastic modulator modulated the polarization of the IR light at a fixed frequency. Demodulation was performed with a lock-in amplifier and a low-pass filter. After the IR beam passes through the polarizer and modulator, it is focused on the sample, then focused on an mercury-cadmium-telluride (MCT) detector cooled by liquid nitrogen. 

Dynamic Mechanical Analysis (DMA) is a technique in which the elastic and viscous response of a sample under oscillating load, are monitored against temperature, time or frequency. This technique became well known by the impressive amount of information about the structure of polymers obtained with the torsion pendulum apparatus. The torsion pendulum DMA apparatus is a so-called resonant system i.e. the measuring frequency is not constant. The modern DMA systems are nearly always fixed frequency systems operating at frequencies between about 0.01 and 100 Hz. and in a temperature region ranging from about -150°C to 300°C. A survey of the DMA technique and the available commercial equipment was given by Wunderlich [1]. 

Medium to high equipment cost Uneven heating and hot spots (in case of fixed frequency system) Specific tooling material is mandatory... 

The DuPont 983 DMA is a versatile laboratory instrument for characterising the viscoelastic and rheological behaviour of materials. This system is designed to operate in any of four modes fixed frequency, resonant frequency, stress relaxation, or creep. It is discussed in detail next as an example of the type of equipment now available. 

Most equipment used in vibration welding produces linear vibratory motion. Equipment is elassified as low-frequency (120 to 135 Hz) or high-frequency (180 to 260 Hz) and can be variable frequency or fixed frequency. Variable-frequency equipment is electrically driven, and a dial adjustment is used to tune the frequency to match the part mass. 

Mobile equipment is often preferred to fixed types. Front-end loaders, scrapers, and bulldozers are used with increasing frequency, especially on projects of short duration or when capital investment must be limited. Front-end loaders are especially advantageous because of their ability to carry material as well as to plow or bulldoze it. 

As discussed in Chapter 6, in systems with more than one unpaired electron the ESR spectrum contains features that involve electron-electron coupling parameters analogous to the nuclear hyperfine parameters. In those types of samples the advantages of double resonance are carried out by employing the use of two different microwave frequencies, one fixed and saturating, and one variable frequency that searches for transitions. This technique is known as ELDOR (electron-electron double resonance).38,40,41,44 It has been used much less than ENDOR and usually requires custom-built equipment. 

Several companies supply density equipment which was considered suitable for automatic, continuous operation with sufficient precision for calculation of polymerization conversion. These break down into three classes based on mode of operation y-ray absorption, oscillatory frequency of a sample filled tube, and mass measurement at fixed volume. Only one of these, an oscillator-based system distributed by Mettler Instrument Corp. (representing Anton Paar Ag.) has models with dead volumes small enough for laboratory scale experimentation. The other units generally also suffered from narrow density spans when the precision was sufficient for conversion studies. Table... 

Advantages of the technique are its relatively low cost combined with quickness and ease of operation. One can work with dilute solutions (indeed they are usually a prerequisite) and water is a good solvent as it does not absorb in the UVMsible range. Quartz cells are required for observation of absorptions in the UV range, otherwise glass or plastic (for aqueous solutions) can be used for visible frequencies. Flow cells are available for coupling to separation equipment and absorption can either be monitored at a fixed wavelength for detection of a specific compound or class of compounds, or, if a diode array is available, a complete spectrum can be recorded for each fraction on-line. 

It is possible to equip large extruders with fully variable drives. In this case, speed variability is often not the main intention, but rather the fact that the existing supply network does not permit a fixed-speed motor to be started, because other users on the same network would be starved by the drop in voltage. In such cases, a frequency converter offers the advantage that much less current is required for start-up. Starting currents can of course also be reduced by a starting transformer or a softstarter. 

The inductance L is fixed, while the capacitance C is the sum of several separate capacitances that of the conductance cell, Cx, that of a separate variable tank capacitor incorporated in the oscillator, Cj (if present) and stray capacitances due to leads, etc., Q. Also present in some methods is a precision air capacitor Cp graduated directly in capacitance units (generally picofarads), used when the equipment is operated in a null mode. In this mode the precision air capacitor is used to bring the frequency /to the same value in each measurement, and the change in reading of the precision air capacitor is then equal in magnitude to the change in capacitance of the cell. 

In the inertial microbalance, the mass located at the tip of an oscillating tapered quartz element is detected as a change in its vibrational frequency. The design of this equipment provides a packed bed of catalyst through which all the gas is forced to flow, and the classical methods of testing for differential operation in an ideal plug-flow fixed-bed reactor can therefore be applied. 

---