ware a a ” ree tt » > aa ‘ Zactory with ene exception: The detent which 2ngaged the groove in the pocket chamber did not guide the chamber in a positive manner whenit was inserted in the tube. Unless an operator has had considerable experience he finds it extremely difficult to insert the chamber and turn it to its proper position. Use of the dosimeters at Operation Green- house indicated a great need for 2- and 5-r instruments, which were not available. (It is extremely difficult to accurately measure doses for a 700-mr mission on a 10-r dosimeter, and some accuracy is lost in reading doses for a 3-r mission.) Many of the missions required exposures which exceeded the range of the 200mr dosimeters but were too low to read with accuracy on a 10-r instrument. 2.9.2 Geiger Mueller Type Field Survey Instruments For TU 3.1.5 two different types of Geiger Mueller (GM) field survey instruments were supplied: 75 Victoreen 263B beta-gamma survey meters and 35 El-Tronics SGM-18A betagamma survey meters. GM instruments that were serviced for other units included the AN/PDR-8B. The Victoreen 263B instruments gave reations noted below. Changes in manufacture as a result of experience gained at Operation Sandstone resulted in easy calibration and good linearity of resultant calibration curves. The parts are easily accessible for repair and maintenance. The following suggestions for improvementare offered: (a) Even though the size of the case has been reduced from that used with the 263A models, it still tends to be top heavy and will fall over easily in a boat or vehicle. The shape of the case could be improved, and the center of gravity should be lowered. (b) The probe has been greatly improved over that of the 263A, but the beta shield is still difficult to remove. This should be given someattention. (c) The life of the 1B85 Geiger tubes was unsatisfactory, and insufficient replacements were provided. (d) The case is not waterproof, and considerable moisture can accumulate inside the instrument, a ah eee position. To remedy this fault it was neces- sary to replace C in several instruments. In some cases the meter absorbed moisture, re- quiring a drying-out process. In the case of nine instruments the phone jack was wired incorrectly which resulted in no pulse in the ear- phones, After 23 monthsof use the following items represented replacements: thirty GM tubes, fifteen 14-volt batteries, thirty-three 300-volt batteries, eight 673-volt batteries, five amplifier units, and two meters. Corrosion was evident on someof the fittings, especially the metal portion of the carrying handles. The-El-Tronics SGM-18A instruments had several desirable design features; but most of the instruments received required changes in order to obtain good calibration, and some trouble was experienced with all tubes. The design of the case is good, and parts are easily accessible for servicing. However, GM tubes required considerable replacementas did 1U5 tubes. The instrument requires selected NC51 neon tubes which are difficult to obtain in quantities from normal stock supplies. The compensating register Rg was not of high enough valueto allow correction for normal 10,000-ohm potentiometers (Rg) were replaced with 50,000-ohm units which permitted a wide range of adjustment and resulted in correct calibration. Apparently some attention should be given to a redesign of circuits, especially the functioning of the NC51 neontube. While TU 3.1.5 was not equipped with AN/PDR-8B GM instruments, considerable servicing was done for other units equipped with them. The halogen filled GM tubes show great promise because of their long life and apparent rugged construction. After the modifications that are detailed below were made, the operation was very satisfactory, and the response was linear and could be directly read for all ranges. However, trouble was ex- — perienced with ruptured tube windows when the instruments were taken by plane to higher aititudes. Four instruments were found to have an open circuit in the “crimp” wire lugs connecting the V201 GM tube cable to the terminal board. This fault resulted in no response to one exception: Moisture affects the 16 pf elecTr trolytic condenser, C, which results in meter readings even though the switch is in the off component variations. For this reason all sonably satisfactory service, with the excep- resulting, in most cases, in excessive meter readings. Such cases respond to drying with N radiation on the 0.5 and 5 mr/hr ranges. Twelve instruments were found to have no response or . ae 3 mr. ee he - a