INSTRUMENTATION
Dow Corning
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C o r n i n g
bonded coating of magnetic powder on a formable metal sheet. Such materials enable critical low level components to be given additional screening in the field. The layered materials can combine electro magnetic and electrostatic screening and are especially useful in shielding scintillation counters, low level chop pers, and computer memories. Ground loops may be minimized in some circumstances by using single-point grounding techniques and running three wire systems. Total elimination of unwanted cir culating currents is extremely dif ficult to do and it is often necessary to float equipment and equipment cases by providing suitable insu lation. In many instances, other remedial measures must be taken, and the use of four terminal am plifiers such as the Kintel 114A may be necessary if the transducer A and the data unit C must inevitably be grounded. Additional electronic filtering in the input circuit is sometimes bene ficial. Capacitors which bypass alternating circulating currents to ground are also useful. If such capacitors are to be connected to Terminals B\ and Bi, care must be taken to sec that they are sized in the ratio of R\ and Ri so as to form a bridge. If telephone lines are used for long distance transmission, then these must also be balanced to ground at both the sending and receiving ends by adding suitable capacitors in pairs at each end. Attention should be paid to the mode of connecting wires so as to prevent thermals, and corrosion of terminal blocks must be guarded against. When special accuracy is desired with thermocouple wires, it is preferable to carry the wires through rather than insert the terminal connector in the circuit. This is easily done by twisting the two wires to be joined together and securing them under a single terminal screw. While pickup problems are often complex because of the many pos sible contributing causes, careful attention during installation lay outs can save later headaches. A logical step by step approach is required to trouble-shoot pickup problems, and early recognition of whether the pickup is internal or external to the measuring instru ment is important.
Circle No. 2 on Readers' Service Card 122 A
INDUSTRIAL AND ENGINEERING CHIEMISTRY
Low Level Measurements
When measuring low-level signals the stray field from the chopper drive may introduce serious dif ficulties, and the better instruments use mechanical drives from a re mote shielded motor. A special lowlevel chopper developed by the National Research Council of Can ada is now commercially available from Guildline Instruments. This chopper ha^ been the basis of several extremely low level amplifiers (0.1 μν. full scale deflection), and has been used in an ingenious transfer circuit to measure differential volt ages directly from grounded thermo couples (Dauphinéc, T. M., Can. J. Physics 31, 577-91). The new styles of chopper such as that put out by Stevens-Arnold all incorporate internal electromagnetic shields and electromagnetic screening. For high impedance inputs the same firm has a vibrating capacitor which enables small voltages in the 100 mv. region to be measured with input resistances in the 100-million megohm area.
Trend toward Electronic Control Systems
The possible use of all-electrical control systems in plants has been receiving a great deal of attention lately. A lively controversy still exists as to the merits of such systems vs. the conventional pneumatic ones. An all-electrical system using electronic controllers, measuring instruments, and electrical actuators is generally feasible today, although it may yet not be the most desirable one. At the present time an allelectrical system is still hampered by disadvantages due to its higher cost, lack of satisfactory actuators, and possible difficulties resulting from power failure. It appears to be generally agreed that replacement of electrical items into an existing pneumatic one brings few, if any, advantages. The situation is however different when new systems are involved. This is particularly true where computer or data logger installations are contemplated for the system. Even in cases where simple controls are involved, the ease with which electrical signals can be manipulated with simple circuits is a
