Indicator tubes

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. 

CH3-COOC2H5 + Cr — Cr + various oxidation products This reaction is not substance-specific. Other alcohols, at varying cross-sensitivity as well as methyl ethyl ketone, are reactive as well. 

2 Quantitative soivent determination in exhaust gas of plants by means of gas-chromatography 

A defined amount of exhaust gas is fed into an adsorption medium (activated carbon, thenax, etc.) and its concentration is enriched. There is a subsequent extraction by means of a solvent. The mixture is then injected into a gas chromatograph. The individual components are determined according to Guideline VDI 34 82, page 1.  

Example Emission measurement in an industrial varnishing plant The measurement was done in a furniture company, which applies different varnish systems on several varnishing lines.  

Detector tubes contain sorption-active agents. A gas flow of a defined rate is fed through and the substance in the gas flow produces a color reaction with the agent in the tube. The color intensity is checked against the scale on the test tube and the ppm value can be registered. 

---

Check for leaks with soap solution, aqueous ammonia or colour indicator tubes. 

Blockages or faulty pumps. Pumps should be checked periodically as instructed by the manufacturer. They can be calibrated using rotameters or bubble flowmeters. Unless pumps possess a limiting orifice they should be calibrated with the air indicator tube in position. 

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. 

Since no device is completely specific for the substances of interest, care must be taken that interferences do not invalidate the sampling results. Many common gases and vapors react with the same chemicals, or have similar physical properties, so that the instrument may give falsely high or low readings for the substance being sampled. The manufacturer s data for colorimetric indicating tubes lists those substances which may interfere with the desired determination. 

Hongmanee P., Stender H., Rasmussen O.F. Evaluation of a fluorescence in situ hybridization assay for differentiation between tuberculous and nontuberculous Mycobacterium species in smears of Low-enstein-Jensen and mycobacteria growth indicator tube cultures using peptide nucleic acid probes. J. Clin. Microbiol. 2001 39 1032-1035. 

Do not rely on the sense of smell to detect hydrogen sulphide leaks. Strips of v/et lead acetate paper turn black on exposure to hydrogen sulphide and offer a simple indicator, as do colour indicator tubes. For plant-scale operations, instrumental multi-point detectors and alarms are likely to be more appropriate. 

Flammable atmospheres can be assessed using portable gas chromatographs or, for selected compounds, by colour indicator tubes. More commonly, use is made of explos-imeters fitted with Pellistors (e.g. platinum wire encased in beads of refractory material). The beads are arranged in a Wheatstone bridge circuit. The flammable gas is oxidized on the heated catalytic element, causing the electrical resistance to alter relative to the reference. Instruments are calibrated for specific compounds in terms of 0—100% of their lower flammable limit. Recalibration or application of correction factors is required for different gases. Points to consider are listed in Table 9.10. 

Access Indications Tube Placement Options Advantages Disadvantages... 

EN can be administered through four routes, which have different indications, tube placement options, advantages, and disadvantages (Table 58-1). The choice depends on the anticipated duration of use and the feeding site (i.e., stomach versus small bowel). 

Colorimetric indicator tubes required 5. Flammable vapor detection apparatus ... 

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

H2S exposure may be monitored by diffusion type colorimetric dosimeters (such as Vapor Gard) color changes from white to brown-black dosage exposure measured from the length of stain in the indicator tube. 

Analysis may be performed by passive indicator tubes, various passive samplers, and electrochemical instruments. 

Browne s steriliser tubes contain a liquid that undergoes a slow chemical reaction at elevated temperatures, changing from red to amber then green. Only when they are green has the correct temperature time combination elapsed. Several of these indicator tubes should be distributed in each batch of material to be sterilised. It is important when autoclaving liquids, to place a Browne s tube in a bottle of liquid as the conditions within such a bottle differ from those outside. Different types of Browne s tubes are available for sterilisation using steam or dry heat. Type I (black spot) tubes are... 

Figure 7.36 (Plate 1) Top section of a condenser. White indicates tubes plugged with extensive internal corrosion blue indicates tubes identified with internal corrosion to be monitored and orange indicates tubes previously replaced. (Reproduced from COM 1999 and 2005 with permission from the Metallurgy Society of CIM)...

Navy. 1984a. Indicator tubes for the detection of explosives. Report no. NWC TP 6569. China Lake, CA Naval Weapons Center. Document no. AD A152385. 

---