Direct-Reading Instruments

Unlike many chemical hazards, radiological hazards can be easy to detect with highly sensitive, direct reading instruments. Radiological control personnel conduct surveys and post warning signs. 

With particles, the contaminant concentration in the duct is determined by isokinetic sampling with subsequent laboratory analysis use of a calibrated direct reading instrument. If the concentration distribution in the duct is uneven, a complete survey of the concentration distribution with the corresponding duct velocities and cross-sectional area is required. National and ISO standards provide information on isokinetic sampling and velocity measurements. In the case of particles, the airborne emission differs from the total emission, for example in the case of granular particulate. The contaminant settling on surfaces depends on particle distribution, airflow rates, direction in the space, electrical properties of the surfaces and the material, and the amount of moisture or grease in the environment. 

For a direct-reading instrument, sensitivity should be at least of the order of 50 kn/V, and instruments are commercially available with sensitivities of up to and exceeding 1 MQ/V. Direct-reading meters are usually made to show several ranges, which are obtained by the use of suitable resistances placed in series with the indicating instrument (Fig. 10.42). 

Prior to the use of plasma excitation, arc and spark sources were used on multichannel spectrometers, the so-called direct-reading instruments. 

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In the direct-reading instruments the emf of the cell is led through an (operational) amplifier across a standard high resistor yielding a current that is measured by a milliammeter calibrated to be read in pH units or millovolts. So, while the null-point system provides a truly potentiometric (non-faradaic) measurement where the off-balance adjustment remains limited to an interrupted temporary current draw-off, the direct-reading system represents an amperometric measurement where a continuous steady-state current draw-off takes place as long as the meter is switched on. In fact, the latter is a deflection method as a pointer indicates the pH units or millivolts by its deflection on the meter scale. 

In the full scale fire tests some additional gaseous species were studied specifically, i.e. formaldehyde. Not all gas species were studied in every test. Hydrogen cyanide and hydrogen chloride have only been studied in situations where evolution of these species were suspected. HCN and HC1 have only been studied as collective (2, 5 or 10 minutes) samples for each fire test. It is most preferable to follow the concentrations with direct reading instruments. This has been the case for carbon monoxide, carbon dioxide, oxygen and in three out of four cases for nitrous oxide. Drager tubes were used for measurements of nitrous oxides in the DIN 53436 test. 

Results small scale fire tests. Initially we had in mind to make a comparison between NT-FIRE 025 and NT-FIRE 004 "box test" since most lining materials had been classified using that method. It turned out that the gases generated by the method NT-FIRE 004 needed a very high dilution before they could be fed into our direct reading instruments. Instrumentation used for smoke stack sampling. EMP-797, turned out to be suitable for our purpose. 

Direct reading instruments may be used in the studies of fire effluents from different small scale testing methods. Moisture and high concentrations do create problems that has to be considered. Such problems may be overcome by using a dilution system such as the EPM 797-system. The instruments used must be fast responding since in certain situations the whole event is over within 1 minute. This also requires proper attention in the design of the sampling line. 

Direct reading instruments are necessary for recording different stages of a fire in order to compare different methods and materials to each other. 

Direct Reading Instrument A portable device that measures, and displays, in a short period of time, the concentration of a contaminant in the environment. 

The first approach onsite use of direct-reading instruments as initial quahtative identification or screening (note the airborne/volatile contaminant, or the class to which it belongs, is demonstrated to be present but quantitative determination of its exact concentration must await subsequent testing) and... 

Further development of direct reading instruments for analysis may be expected, for example in polarographs, x-ray and emission spectrometers. 

Two general methods for testing air impurities are in use. One involves direct-reading instruments (such as thermal indicators, test paper indicators and other devices described in Ref 16, pp 245-53), the other involves removal of the impurity from a given vol of air and determination of the impurity by a suitable lab method. 

ICP offers good detection limits and a wide linear range for most elements. With a direct reading instrument multi-element analysis is extremely fast. Chemical and ionization interferences frequently found in atomic absorption spectroscopy are suppressed in ICP analysis. Since all samples are converted to simple aqueous or organic matrices prior to analysis, the need for standards matched to the matrix of the original sample is eliminated. 

Direct-reading instrument for automatic coulostatic analysis in the 10-5 to... 

The function of the spectrometer is to accept as much light from the source as possible and to isolate the required spectral lines. This may be impossible where there is a continuous spectrum in the same region as the analytical line for example, the magnesium line of 286.2 nm coincides with a hydroxyl band. In direct reading instruments, electronic devices may be used to supplement the resolution of the spectrometer by modulating the intensity of the analytical signal. In absorption and fluorescence the light source is modulated in emission the spectral line is scanned (816) or the sample flow modulated (M23). 

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