Fast response

A major advantage of the TOF mass spectrometer is its fast response time and its applicability to ionization methods that produce ions in pulses. As discussed earlier, because all ions follow the same path, all ions need to leave the ion source at the same time if there is to be no overlap between m/z values at the detector. In turn, if ions are produced continuously as in a typical electron ionization source, then samples of these ions must be utihzed in pulses by switching the ion extraction field on and off very quickly (Figure 26.4). 

Since the microchannel plate collector records the arrival times of all ions, the resolution depends on the resolution of the TOP instrument and on the response time of the microchannel plate. A microchannel plate with a pore size of 10 pm or less has a very fast response time of less than 2 nsec. The TOP instrument with microchannel plate detector is capable of unit mass resolution beyond m/z 3000. 

After acceleration through an electric field, ions pass (drift) along a straight length of analyzer under vacuum and reach a detector after a time that depends on the square root of their m/z values. The mass spectrum is a record of the abundances of ions and the times (converted to m/z) they have taken to traverse the analyzer. TOP mass spectrometry is valuable for its fast response time, especially for substances of high mass that have been ionized or selected in pulses. 

Because of its small size and portabiHty, the hot-wire anemometer is ideally suited to measure gas velocities either continuously or on a troubleshooting basis in systems where excess pressure drop cannot be tolerated. Furnaces, smokestacks, electrostatic precipitators, and air ducts are typical areas of appHcation. Its fast response to velocity or temperature fluctuations in the surrounding gas makes it particularly useful in studying the turbulence characteristics and rapidity of mixing in gas streams. The constant current mode of operation has a wide frequency response and relatively lower noise level, provided a sufficiently small wire can be used. Where a more mgged wire is required, the constant temperature mode is employed because of its insensitivity to sensor heat capacity. In Hquids, hot-film sensors are employed instead of wires. The sensor consists of a thin metallic film mounted on the surface of a thermally and electrically insulated probe. 

The helium leak detector is a common laboratory device for locating minute leaks in vacuum systems and other gas-tight devices. It is attached to the vacuum system under test a helium stream is played on the suspected leak and any leakage gas is passed into a mass spectrometer focused for the helium-4 peak. The lack of nearby mass peaks simplifies the spectrometer design the low atmospheric background of helium yields high sensitivity helium s inertness ensures safety and its high diffusivity and low adsorption make for fast response. 

Instrumental Interface. Gc/fdr instmmentation has developed around two different types of interfacing. The most common is the on-the-fly or flow cell interface in which gc effluent is dkected into a gold-coated cell or light pipe where the sample is subjected to infrared radiation (see Infrared and raman spectroscopy). Infrared transparent windows, usually made of potassium bromide, are fastened to the ends of the flow cell and the radiation is then dkected to a detector having a very fast response-time. In this light pipe type of interface, infrared spectra are generated by ratioing reference scans obtained when only carrier gas is in the cell to sample scans when a gc peak appears. 

The low detection limit, high sensitivity, and fast response times of chemoreceptor-based biosensors result primarily from the extremely high binding constants of the receptor R for the target substrate S. The receptor—substrate binding may be described... 

The performance of the catalytic converter is affected by the conditions of air/fuel control provided by the fuel metering system. A slowly responding fuel metering system can dramatically decrease the conversion efficiency of the converter compared to a fast response multipoint fuel injection system. 

Strain-gauge pressure transducers are manufactured in many forms for measuring gauge, absolute, and differential pressures and vacuum. Full-scale ranges from 25.4 mm of water to 10,134 MPa are available. Strain gauges bonded direc tly to a diaphragm pressure-sensitive element usually have an extremely fast response time and are suitable for high-frequency dynamic-pressure measurements. 

The half-pipe jacket is used when high jacket pressures are required. The flow pattern of a liquid heat-transfer fluid can be controlled and designed for effective heat transfer. The dimple jacket offers structural advantages and is the most economical for high jacket pressures. The low volumetric capacity produces a fast response to temperature changes. 

This is an alternative to FOC and can provide a very fast response. The choice of a static drive, whether through a simple V7/control, field-oriented phasor control or direct torque control with open or closed-loop control and feedback schemes, would depend upon the size of the machine, the range of speed control (whether required to operate at very low speeds, 5% and below), the accuracy of speed control and the speed of correction (response time). The manufacturers of such drives will be the best guide for the most appropriate and economical drive for a particular application or process line. 

However, power systems that cater to almost fixed loads at a time and whose variations occur only at specific times of the day may not reejuire it fast response. In such cases, it is possible to provide manual switching methods which will give enough time between two switchings. Manual switching, how ever, has certain shortcomings, due to the human factor such as its accuracy and diligence, as noted above. The recommended practice is therefore to select fast reactive controls as noted below. 

Systems sueh as the EDS use a dual mieroproeessor system. As ean be seen in Figure 4-93, eritieal parameters of memory, input/output, alarm/trips, and host eomputer eommunieation are handled in the main proeessor. To maintain fast response time, this proeessor is kept free of less eritieal tasks sueh as operator interfaeing, trending, and arehiv-ing. A seeond proeessor, an administrative CPU, is used for these less eritieal yet important tasks. 

Another form current-mode control is called hysteretic current-mode control. Here both the peak and the valley currents are controlled. This is obviously better for continuous-mode forward for boost converters. It is somewhat complicated to set-up, but it does offer very fast response times. It is not a very common method of control and its frequency varies. 

Hydraulie aetuators are employed in sueh areas as the aerospaee industry beeause they possess a good power to weight ratio and have a fast response. 

Finally, the speed of response of the detector sensor and the associated electronics once played an important part in optimum column design. The speed of response, or the overall time constant of the detector and associated electronics, would be particularly important in the analysis of simple mixtures where the analysis time can be extremely short and the elution of each peak extremely rapid. Fortunately, modern LC detector sensors have a very fast response and the associated electronic circuits very small time constants and, thus, the overall time constant of the detector system does not significantly influence column design in contemporary instruments. The instrument constraints are summarized in Table 2... 

The solvent consumption appears to be in conflict with the corresponding optimum flow rates. Substances with high (a) values have a very high optimum flow rate (over 11 per min. for (a=1.2) and the column diameter is over 6 mm which would indicate a very large solvent consumption. However, because the separation is simple, a very rapid separation is achieved with analysis times of less than a second. As a consequence, only a few ml of solvent is necessary to complete the analysis. The apparatus, however, must be designed with an exceedingly fast response and very special sample valves would be necessary. In contrast, a very... 

Continuous flow devices have undergone careful development, and mixing chambers are very efficient. Mixing is essentially complete in about 1 ms, and half-lives as short as 1 ms may be measured. An interesting advantage of the continuous flow method, less important now than earlier, is that the analytical method need not have a fast response, since the concentrations are at steady state. Of course, the slower the detection method, the greater the volumes of reactant solutions that will be consumed. In 1923 several liters of solution were required, but now reactions can be studied with 10-100 mL. 

Although thermistors with a positive temperature characteristic are available, the negative characteristic types are more common. Devices which are physically small have low power-handling capabilities and some form of output amplification is usually needed. They do, however, have a fast response. Larger units are available in which amplification can be dispensed with but the response is much slower. These are used for indication (rather than control) purposes. 

Radiant High-intensity Consist of high-temperature Fast response time. Must be mounted 0.5 to 6kW... 

Actuator type Electro-hydraulic for fast response and accurate control. 

A.J. Hceger, D. Moses, M. Sinclair, Semiconducting polymers fast response non-linear optical materials, Synth. Met., 1986. 15, 95. 

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