AWS F1.5M pdf free download
AWS F1.5M pdf free download.Methods for Sampling and Analyzing Gases from Welding and Allied Processes.
4. Summary of Analytical Techniques 4.1 General. The following are three major analytical methods for gases formed during welding: (1) Instruments (2) Colorimetric or length of stain (commonly referred to as detector tubes or passive samplers) (3) Chemical Each of these methods has advantages and disadvan- tages which shall be considered by the user. Since weld- ing by-products are a complex mixture of gases and fume, the analytical method must be chosen to suit the specific welding situation.
Instrumental methods are considered the method of first choice. They are easily and accurately calibrated, give immediate results, and can be used to show the variation of a contaminant concentration over time. An additional advantage is the ability to electronically collect data which can be downloaded to a computer for evaluation. A distinct disadvantage may be the need for long sampling lines to obtain a breathing zone sample and the relatively high cost of the equipment.
Detector tubes are used as a convenient, inexpensive way to measure concentrations of gases (see Figure 1). Normally, such tests provide only a snap-shot view of a contaminant concentration during the working time. However, detector tubes can be used with personal sampling pumps or as passive samplers, for sampling up to eight hours to provide data for calculating a time- weighted average concentration.
Chemical methods have a particular advantage when solid sorbent tubes are used, because sorbent tubes can be used to obtain time-weighted average concentrations. Chemical methods are the most cumbersome, however, since they require special sampling techniques and a higher degree of analytical expertise to assure accurate results. They are the only recommended methods for sampling and analysis of NO and gaseous fluorides.
4.2 Safety Considerations. Safety is an important consideration in all analytical techniques. General safety requirements should be reviewed and implemented as prescribed in laboratory manuals that provide specific information on particular analytical techniques, and the appropriate annexes found in this document should be consulted. Guidance on protection of persons from injury and illness during welding operations can he found in ANSI Z49. 1, Safety in Welding, Cutting, a,id Allied Processes, as well as AWS Safety and Health documents that specifically address the actual sampling and analysis of welding fumes and gases.
4.3 Instrumental Methods. There are both active and passive types of instruments available. In active instrumental methods, an air sample is drawn to the sensor or collector by a pump. Passive sampling devices do not aspirate an air sample, but rely on passive molecular diffusion to bring the contaminant to the sensor. Using either method, it is possible to collect the sample in an evacuated cylinder or bag for testing. Since contaminants can react with the collection media, an interference check shall he made to ensure accuracy of the sample. Some of these devices are too large to he worn. Small personal monitors are available for CO; these instruments can incorporate alarms which can he set at predetermined levels.
4.4 Detector Tubes and Passive Samplers (see Annex A). Detector tubes consist of sealed glass tubes containing a sorbent (such as silica gel which has been impregnated with a reagent). When a specific contaminant is present in the air being drawn through the tube, a chemical reaction takes place that results in a color change. The concentration of a contaminant present in a fixed volume of air yields a given length of stain. The accuracy and reproducibility of the method is highly variable. Factors such as cross sensitivity (interferences), packing of the reagent, air flow patterns through the reagent, actual air volume, and sampling rate can affect the final reading. Precision, accuracy. and reproducibility vary with the age. the conditions of storage, and the lot-to-lot manufacturing variability of these tubes. Despite the disadvantages, detector tubes do provide a convenient method for preliminary screening. Where high precision is not critical, they can be used to measure concentrations over short periods. Using special sampling and statistical techniques. short-term tubes can be used to estimate the average concentration over extended periods.AWS F1.5M pdf download.
