Tubes with Direct Indication

Direct-reading Measurement Techniques 6.9.3.1 Tubes with Direct Indication 

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

It is essential for a quantitative evaluation that the length of the zone displaying a discoloration should be proportional to the concentration of the hazardous chemical in the air. Using a calibration scale printed on the tube, the concentration can be read as function of the volume of air drawn through it. Thus, calibration by the user is not necessary. Sometimes the discolored layer does not develop vertically, i.e. along the axis of the tube (see Fig. 6.52). In principle, the result of the test is evaluated according to the following mles  

If the colored zone develops with a sharp boundary perpendicular to the axis of the tube, the concentration can be read directly (Fig. 6.52 a). 

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To determine an average shift concentration, a measuring time of about 8 h is usually used. Long-term tubes with direct indication are available for inorganic and organic gases, e.g. acetone, ammonia, benzene, chlorine, hydrochloric acid, nitrous gases, sulfuric acid, and vinyl chloride. 

Fig. 6.58 Long-term tubes with direct indication and direct-reading diffusion tubes, (a) Long-term tubes (b) dosimeter tubes (c) tube holder.

A general precondition for a tube system with direct indication is the existence of a specific color reaction of the air contaminant with a substrate reagent The following chemical reactions are used as detecting reactions ... 

The basic principle of reaction papers in the form of badges is very similar to that of the tubes. An agent is applied to a substrate, and the former undergoes a color reaction with the substance to be analyzed. The intensity of the discoloration correlates to the concentration of the air contaminant. The final evaluation can take place either by visual comparison with a calibrated color scale or by photometry. Figure 6.59 shows samples of badges with direct indications. 

A quartz tube with diameter of 100 mm permits direct observation of the process with normal coal. The cyclone furnace produces steady combustion at gas temperature of 700 C but can also be extended to a temperature level of more than lOOO C. Combustion of salt coals is difficult to observe because the alkalines vaporize from the fuel and condense on the colder quartz tube wall to form an opaque layer. This problem is partially overcome by inserting a probe which measures gas and wall temperature and allows the slagging process to be determined as a function of temperature by weighing the deposit. Measurement of deposit hardness supplies additional information on the character of the clinker formed. The cyclone separator behind the reactor allows an analysis of structural changes which occur in the finest grain. The test results obtained indicate the cyclone reactor can be improved by automation (for easier operation) and by standardization to result in a piece of equipment capable of evaluating slagging tendencies. 

Directions Put 5 c.c. of ferric chloride solution into a small test tube, fill the tube with water, shake the solution and then pour a third of it into each of two other test tubes. Dilute and divide into thirds in the same way 5 c.c. of ferrous ammonium sulphate solution. Add a few drops of solutions of potassium ferrocyanide, of potassium ferricyanide, and of ammonium thiocyanate to the three solutions of ferric chloride respectively and likewise to the three solutions of ferrous ammonium sulphate. (1) Tabulate the results obtained in these six tests. (2) Do the results indicate the presence of any ferric salt in the ferrous salt Would you expect the production of any color in the absence of ferric salts (3) State one test for a ferrous salt and two tests for a ferric salt. 

It should be noted that the partition ratio at equlibrium predicts the optimum desorption effiency attainable, and other experiments may be necessary to detect nonequlibrium situations. Desorption efficiency should not be taken as the recovery since other factors may have a significant effect. After a solvent/sorbent system is selected and tested using the phase equilibrium technique, direct injections of the test compound are made into collection tubes with and without air being pulled through. If the desorption efficiencies as determined by direct injection are considerably lower than phase equilibrium values, interaction or reaction on the sorbent surface is indicated. If the total recovery from the simulated air collection is lower than the direct injection efficiency (even through no breakthrough has occurred) hydrolysis, oxidation, or another reaction may indicated. 

The first equation is the famihar law of Laplace for a cylindrical tube with internal radius q. It indicates that the force due to internal pressure, pj, must be balanced by a tensile force (per unit length), T, within the wall. This tension is the integral of the circumferentially directed force intensity (or stress, a ) across the wall ... 

For many solvents and other substances directly indicating detector tubes are available. They are used for rapid assessment of emission. Since they often do not react specifically with a certain compound alone, errors have to be considered due to cross-sensitivity. 

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