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thickness of the deposit increases, the energy needed to penetrate the deposit is increased. Metals with larger atomic numbers require higher energies for the same thickness. As particles are emitted by the source, the number of active particles remain- ing is reduced. The half-life of an isotope is the time necessary to reduce the activ- ity of the source by one half. As the activity decreases, the instrument requires recalibration for the new activity level. Accurate placement of the part is essential for precise measurement. An aper- ture is provided to control the area exposed to the beta rays. The aperture is mounted in a probe together with the source to control the distance of the source to the sample. The Geiger-M\)ller tube is part of a counting system that records the backscatter and then computes the thickness. A full line of accessories to store data and provide statistical information is available. Many systems pro- vide computer prompting for both measuring and calibration procedures. The beta backscatter method applies to coatings and substrates whose atom- ic numbers differ by at least 5. ASTM Standard Method B 567 details the con- sideration necessary for accurate measurement with this method. COULOMETRIC METHOD The coulometric method is based on Faraday's law. The law states that one gram-equivalent weight of metal will be stripped or deposited for every 96,500 coulombs (ampere-seconds) of electricity passed through the electrolyte. This law is so basic that it has been used to define the international ampere. The international ampere is defined as the unvarying electric current that, when passed through a solution of silver nitrate, will remove 0.000118 gram of sil- ver per second from the anode. This figure (0.000118 gram of silver per second) is called the electrochemical equivalent of silver. The following equation defines the weight of metal deposited according to Faraday's law: M = eit, (2) where M = mass (g), e = electrochemical equivalent (g/ A-sec), i = current (A), and t = time (sec). To apply the coulometric method to thickness testing, four para- Table II. Thickness Conversion Chart Existing Unit of Measure Angstrom Microinch Micrometer Mil Millimeter Inch Desired Unit of Measure Angstrom Microinch Micrometer 1 250 10,000 250,000 10,000,000 0.004 1 40 1,000 40,000 250,000,000 1,000,000 25 1,000 25,000 Mil Millimeter 0.001 0.04 1 40 1,000 0.000025 0.001 0.025 1 25 Inch 0.0001 0.000004 0.0000001 0.000000004 0.025 1 0.000001 0.00004 0.001 0.04 1 How to use: 1. Locate existing unit in vertical column (left). 2. Find multiplier on the horizontal line that is under the new desired unit of measure. 3. Multiply existing quantity times multiplier. Result is the approximate equivalent measurement in the desired unit. Example: 100 microinches = ? micrometers 0.025 is the multiplier 100 0.025 = 2.5 (micrometers). 488