Direct-reading tubes

By far the most well-known sampling systems are the direct-reading tubes commonly called Draeger Tubes [6-99]. 

Fig. 6.52 Different discoloration zones of direct-reading tubes, (a) Sharp zone of discoloration (b) angular zone of discoloration (c) diffuse zone of discoloration.

The main application area for direct-reading tubes is the measurement of shortterm exposure concentrations or momentary concentration peaks. In order to use them for determination of the median shift concentration, a large number of single measurements have to be performed using statistical modeling (see Section 6.9.1.4). 

As well as direct-reading tubes for short-term measurements, tubes are also available for long-term monitoring. Here, the air sample is sucked through the tube by use of a personal-air-sampling pump (PAS-pump). These all look more or less the same and have a similar kind of indication to that of short-term tubes.Their area of application corresponds to that of PAS procedures (see Section 6.9.1.4.1). In contrast to these, the concentration can be read directly at the end of monitoring. Personal exposure is calculated based on the air volume pulled through. 

NIOSH approved tubes are direct reading and are graduated in parts per million from 0-60. The pump draws a measured volume of air through the tube, and contaminant reaction causes a color change within the direct reading tube. 

Unlike water and solid waste, no comprehensive study has been published on air pollution from textile operations. Textile mills produce atmospheric emissions from all manner of processes, and these have been identified as the second greatest problem for the textile industry [8], There has been much speculation about air pollutants from textiles but, in general, air emissions data for textile manufacturing operations are not readily available [9-11]. Most published data are mass balance not direct measurements [12, 13], Direct reading tubes and gas chromatog-raphy/mass spectrometry (GC/MS) have been used more recently to get more reliable data [14, 15]. Hopefully, in the future air emissions data will continue to be collected from textile operations, and better definitions of industry norms can be expected. Considerable effort is now underway in that regard [14, 16]. 

Direct-reading polychromators (Figure 3b) have a number of exit slits and photomultiplier tube detectors, which allows one to view emission from many lines simultaneously. More than 40 elements can be determined in less than one minute. The choice of emission lines in the polychromator must be made before the instrument is purchased. The polychromator can be used to monitor transient signals (if the appropriate electronics and software are available) because unlike slew-scan systems it can be set stably to the peak emission wavelength. Background emission cannot be measured simultaneously at a wavelength close to the line for each element of interest. For maximum speed and flexibility both a direct-reading polychromator and a slew-scan monochromator can be used to view emission from the plasma simultaneously. 

Direct reading samplers include simple devices such as colorimetric indicating tubes in which a color change indicates the presence of the contaminant in air passed through the tube, or instruments which are more or less specific for a particular substance. In the latter category are carbon monoxide indicators, combustible gas indicators (explosimeters) and mercury vapor meters, as well as a number of other instruments. 

If a pitot tube is inserted in a circular cross-section pipe in which a fluid is in streamline flow, calculate at what point in the cross-section it should be situated so us to give a direct reading representative of the mean... 

Derive a relationship between the pressure difference recorded between the two orifices of a pitot tube and the velocity of flow of an incompressible fluid. A pitot tube is to be situated in a large circular duct in which fluid is in turbulent flow so that it gives a direct reading of the mean velocity in the duct. At what radius in the duct should it be located, if the radius of the duct is r l... 

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. 

Chemical measurements can be performed during developing stages of fire using direct reading measurements and sampling techniques like sorbent tubes. 

Lungs, brain, blood Gases (carbon monoxide, cyanide) Direct-reading colorimetric indicator tubes Head-space Microdiffusion GC... 

Direct reading. a instalments Continuous monitors Pumps—sorbent sampler Detector tubes Passive badges... 

One should note that Fig. 6.1 displays how any pipe experiment can be set up to find this unknown / factor. The hL value is experimentally determined by taking readings of v and P The P value is directly read as shown, by reading the liquid column heights. The velocity, however, must be read by adding a venturi tube or orifice flange to Fig. 6.1 for the velocity reading. 

Not available with metal scale. Specify percent scale or direct reading scale on tube. 

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