Device criteria

FDA, 1997, Magnetic Resonance Diagnostic Devices Criteria for Significant Risk Investigations, at http //www.fda.gov/cdrh/ode/magdev.html. 

Measurement Requirements. Any analysis of measurement requirements must begin with consideration of the particular accuracy, repeatabihty, and range needed. Depending on the appHcation, other measurement considerations might be the speed of system response and the pressure drop across the flow meter. For control appHcations repeatabihty may be the principal criterion conversely for critical measurements, the total installed system accuracy should be considered. This latter includes the accuracy of the flow meter and associated readout devices as well as the effects of piping, temperature, pressure, and fluid density. The accuracy of the system may also relate to the required measurement range. 

In comparing alternative collectors for a given service, a figure of merit is desirable for ranking the different devices. Since power consumption is one of the most important characteristics of a collector, the ratio of Nt to power consumption is a useful criterion. Another is the ratio of Nt to capital investment. 

In the operational sense, some filters are batch devices, whereas others are continuous. This difference provides the principal basis for classifying cake filters in the discussion that follows. The driving force by which the filter functions—hydrostatic head ( gravity ), pressure imposed by a pump or a gas blanket, or atmospheric pressure ( vacuum )— will be used as a secondary criterion. 

The duration, however, is no criterion for a current limiting type protecting device, and a protected equipment, device or component can have a short-time rating commensurate with the tripping characteristics of the protecting interrupter. Accordingly these two types of tripping characteristics are explained below. 

Protection current devices with potential control are described in Section 8.6 (see Figs. 8.5 and 8.6) information on potentiostatic internal protection is given in Section 21.4.2.1. In these installations the reference electrode is sited in the most unfavorable location in the protected object. If the protection criterion according to Eq. (2-39) is reached there, it can be assumed that the remainder of the surface of the object to be protected is cathodically protected. 

The study of transport between immiscible phases requires a device which (1) yields contacting surfaces with known contact area, (2) has a stationary interface, and (3) has known hydrodynamics. Stowe and Shaewitz [6] developed and evaluated a cell which meets these criteria. Each liquid is stirred with a rotating disk. As noted by these investigators, if the fluids are rotated in opposite directions by the rotating disks, then the torque on the interface cancels. The criterion for operation of the stirred cell with a stationary fluid interface is... 

Low transit-time dispersion with point of illumination on the photocathode, i.e., < 20 psec/mm. Linear focused photomultipliers satisfy this criterion, but side-window devices do not. This again is relevant to successful data analysis. 

Boron does not meet Glassman s criterion for vapor-phase combustion of the metal. Thus, the boron surface remains coated with a vitreous B203 layer and boron consumption becomes extremely slow consequently, boron is not burned efficiently in propulsion devices. 

The physical realizability of pulsed transfer functions uses the basic criterion that the current output of a device (digital computer) cannot depend upon future information about the input. We cannot build a gadget that can predict the future. 

For routine work at around 0°C on any form of data collection device, a simple low-temperature attachment may be constructed from a copper coil, a refrigerated ethanol bath, a headess air drier, a compressor, and a flexible hose (Marsh and Petsko, 1973). Such an apparatus costs less than 2000 to build and has essentially no operating costs. It fulfllls the most important criterion for a successful low-temperature device that it be so easy to operate as to encourage its use. Its disadvantage is that very low temperatures are not attainable without more sophisticated—and expensive—hardware. 

Finally, most formulations successful in meeting other criteria have been tested for heat distortion temperature as a final criterion for judgment. A commercial device (Tinius-Olsen) has been employed for these tests. 

The essential requirements relate to the safety in use of the device, including labelling requirements, but are principally expressed in terms of scientific and technical performance characteristics. Efficacy, as such, is not a criterion. Confirmation of conformity must include evaluation of clinical data for many devices, generated from... 

The Monte Carlo analyses are used to observe how device tolerances can affect a design. There are two analyses that can be performed. The Worst Case analysis is used to find the maximum or minimum value of a parameter given device tolerances. Device tolerances are varied to their maximum or minimum limits such that the maximum or minimum of the specified parameter is found. The Monte Carlo analysis is used to find production yield. If the Worst Case analysis shows that not all designs will pass a specific criterion, the Monte Carlo analysis can be used to estimate what percentage will pass. The Monte Carlo analysis varies device parameters within the specified tolerance. The analysis randomly picks a value for each device that has tolerance and simulates the circuit using the random values. A specified output can be observed. 

To us it is obvious that these men were reaching desperately for a way to hold onto a familiar idea, and it is equally obvious that their efforts were doomed once they had admitted the conservation of weight as an active and decisive criterion for testing chemical phenomena. These devices of Kirwan and Cavendish permitted phlogiston to retain a role, but the chemical behavior at stake could be easily described without reference to that elusive body it was not needed. Kirwan finally admitted as much when he wrote to the editor of Crells chemical journal in 1791 ... 

We must draw the following conclusion A particular scale-up criterion that is valid in a given type of apparatus for a particular process is not necessarily applicable to other processes occurring in the same device. 

It will be realized as the discussion develops that adequate data for strict comparison of the various equipment types are not available. Nevertheless, some relative measure of performance is desirable, and the ratio of throughput to volume of a theoretical stage has been chosen as a simple characterizing device. Typical values of this criterion have been quoted, under conditions which represent as nearly as possible the best available performance. The figures must be used with considerable caution, since operating conditions were not usually comparable. Furthermore it will be clear from the discussion that this criterion alone is not the only one which should be applied in deciding upon the merits of a particular apparatus. 

Although the capacity of this water-polishing device at ultra-high water cuts was the primary design criterion, several other factors had to be considered. 

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