I/EC
Instrumentation
Low Level Electrical Signals in Industry Careful installation is necessary to minimize electrical pickup a n d stray v o l t a g e problems in plant, instrument systems by P. H. Stirling and Henry Ho, Canadian Industries Ltd.
I HE LAST decade has seen a spectacular progress in electrical measurement. The range of many measurements has been extended by factors of a thousandfold. Development laboratories now speak of nanovolts and nanoseconds (nano S9 10~ 9 ), whereas only a decade ago microvolts and microseconds were their concern. Current and resistance measurements no longer fit any convenient phraseology and need be referred to by indicial symbols. Ranges such as 1015 ohms or 10~16 amperes which used to be in the physicist's realm are now commonly measured in industrial control laboratories using commercial equipment. The products of laboratory development of a decade ago are now reaching the plant instrument engineer as established techniques; and measurements which were only practical in laboratories then, now form the basis of many of his present instruments. Thus he finds himself becoming more and more concerned with measurements at microvolt and micro microampere levels where electrical instrument and stray voltages may present serious problems. The growing demand for improved precision of measurement fostered by computing control developments also requires close attention to possible stray voltages and the development of improved field techniques to reduce these is currently receiving a great deal of consideration.
countered, and this is often traceable to switching transients. Stray signals in the form of circulating currents can also be a cause of shorter component life leading to more frequent maintenance. Dynamic response and sensitivity are also often degraded by pickup and the random variable effects which are characteristic of gross amounts of pickup can make nonsense of control specifications. For these reasons it behooves the engineer to be familiar with the probable sources of stray voltages and to sec that good installation practices are followed. Instrument manufacturers are well aware of the pitfalls and modern instrument designs are greatly improved in their ability to function in the presence of strays. However, poor application engineering and installation practices can often negate the best efforts of the instrument designer. Pickup can be classified roughly into three classes depending on the
frequency range: direct current and low frequency pickup, power frequency and its harmonics, and modulated radio frequency pickup. Power frequency and its harmonics are the commonest type of electrical pickup. This is the most troublesome, because in many forms of electronic instrumentation, modulated power frequencies are used for amplification stages. Power frequency pickup may be in phase or in quadrature with such signals, and either causes insensitivity and sluggishness due to amplifier overloading or random variable offsets. Direct current and low frequency pickup appear as sporadic drifts which can only be distinguished by substituting a stable reference voltage for the suspected transducer. Radio-frequency pickup from induction furnaces is often modulated at power frequencies, and demodulation is often possible in instrument measuring circuits with consequent spurious results. The relative importance of pickup be-
Effects of Stray Signals
Stray voltages or pickup can have serious deleterious effects on electronic instrumentation if present at sufficiently large relative amplitudes. Pickup usually reduces the attainable precision and can in extreme circumstances disable circuit operation if serious saturation occurs. Sporadic momentary loss of control or monitoring action is more frequently en-
Possible stray v o l t a g e sources VOL. 53, NO. 11
·
NOVEMBER 1961
119 A
INSTRUMENTATION comes more pronounced as the i m p e d a n c e a n d sensitivity of measur ing circuits increase. T h e m o d e r n trend toward higher i m p e d a n c e measuring circuits a n d the extension of sensitivity, especially in analytical instrumentation, makes attention to stray signals more i m p o r t a n t t h a n before. How Stray Voltages Arise Pickup is d u e to the coupling of an electrical source to the circuitry of a n instrument. T h e coupling may be resistive, capacitative, or inductive, a n d signals are transferred cither through leakage resistance, leakage capacitances, transformer action, or as electromagnetic radiation. T h e composite d i a g r a m shows possible stray voltage sources. Con sider a transducer A, such as a thermocouple connected to a measur ing instrument B, which feeds a re mote d a t a unit C. Resistive leakage (generator Ec) can occur through poor insulation such as might be found in electrically heated furnaces. At high t e m p e r a t u r e s the resistance of refractories is greatly lowered a n d sizable leakage currents can be encountered. Another form of re sistive leakage can occur in the " g r o u n d l o o p , " wherein the t h e r m o couple a n d well arc grounded and the instrument itself is g r o u n d e d at a separate point (often at two case points a n d through the powerline). Alternating or direct current potentials can exist between the pair of earth points (generator Εχ) a n d sizable circulating currents m a y be met. A similar situation m a y exist between the remote d a t a indication point a n d the measuring unit (gen erator £V) · Electromagnetic transfer can oc cur either as direct transformer action between the pickup source (generator EL) a n d the measuring circuit or by radiative transfer. Sizable induc tively coupled disturbances often arise from direct current e q u i p m e n t which is periodically switched, a n d these must be considered as well as alter nating machinery. Direct c u r r e n t speed controls using thyratrons arc a frequent offender. Capacitative transfer (generator Ec) which is the most commonly en countered form of stray transfer usu ally depends on capacitative leak age to g r o u n d . Transient disturb ances from switchgear are often 120 A
transferred this way. Flexing of shielded cables carrying direct cur rent voltages can cause annoying low frequency pickup, d u e to the c h a n g ing capacitance or to electrostatic generation between insulation a n d conductors. Direct c u r r e n t stray voltages (gen erator ET) can also arise from small thermoelectric junctions, corrosion cells, a n d internally generated volt ages (triboelectric, piezoelectric, or contact potentials). T w o modes of pickup transfer are commonly recognized. In differen tial m o d e pickup, the pickup poten tial is applied to terminals B\ a n d #2 in the same way as the transducer signal. Such potentials affect the m e a s u r e m e n t directly. In the other m o d e (common mode), both ter minals rise a n d fall together with re spect to g r o u n d . This form of pickup acts indirectly and must produce a differential signal to affect the meas urement. This can h a p p e n in a n u m b e r of ways a n d in the usual di rect-current c h o p p e r amplifier sys tem, it is usually associated with amplifier distortion a n d stray cir culating currents. Minimizing Stray Signal Pickup Isolation of the measuring circuit by minimizing coupling minimizes pickup. Resistive leakage m a y be cut d o w n by using improved insula tion, a n d the use of metal sheathed thermocouples of the u n g r o u n d e d tip type will often produce a signifi c a n t improvement. T h i s type of thermocouple is now available with p l a t i n u m or platinum-alloy sheath ing (Johnson M a t t h e y & Mallory Co.). High t e m p e r a t u r e wires have been steadily improved, but there is still a d e a r t h of good flexible con ductors. U p to 500° F. there are few p r o b lems in finding wires suitable for long t e r m service. Above this tem p e r a t u r e flexible insulating materials are h a r d to find. Almost all flexible insulating materials involve a n inert material (glass, mica, asbestos) with a binder, a n d the binders almost all evaporate or d e g r a d e after a few hours at 850° to 1000° F. They may still supply sufficient insulation if there is no flexing a n d no ther m a l cycling, but if they are flexed a n d brought back to room tempera ture, they m a y stiffen, crack, a n d de teriorate electrically. Inert braided
INDUSTRIAL AND ENGINEERING CHEMISTRY
sleeving can sometimes be a problem since the sleeving m a y tighten and buckle, breaking wires inside. T h e conductors must also be cho sen to be oxidation resistant. C o m monly, nickel-coated copper wire is used ( S p r a g u e Electric Co.). Anodized a l u m i n u m is also being used in some specialized services. High t e m p e r a t u r e connectors for service u p to 1250° C. are available from Bendix-Scintilla, Amphenol-Borg Corp., a n d A M P I n c . Rockbcstos offers Teflon a n d Teflon-coated glass braid for 600° to 800° F . along with fibrous inorganic insulation using nickel-coated copper for the 1000° to 1600° F . range. Boston In sulated Wire and Cable Co. Superjct wires also use nickel-clad copper a n d fibrous insulation. Lewis Engineer ing, Aero Research, American SuperT e m p Wires, a n d General Electric are all in the midst of extensive de velopment programs in this area a n d can offer a variety of specialized lead wires. W h e r e severe flexing is not a problem, the best solution for process use at elevated t e m p e r a t u r e still a p p e a r s to be the metal sheathed, refractory oxide insulated wires such as those supplied by the T h e r m o Electric C o r p . Capacitative and inductive pickup are best dealt with by avoid ing the source of pickup where ever possible. As inductive transfer de pends on the loop area, this m a y be minimized by using coaxial leads or twisting leads together. Physical isolation of power leads a n d signal leads should be observed if possible, otherwise recourse must be h a d to shielding. It m a y not be generally realized, but simple conduits are quite inefficient as magnetic shields, although they provide fair electro static screening when properly grounded. M a g n e t i c shielding must provide a low reluctance p a t h to bypass magnetic flux a r o u n d a critical circuit. Hitherto m a g n e t i c shields have usually been constructed of very brittle materials which were difficult to fabricate. A new line of magnetic screening materials has been developed by the Perfection Mica Co. u n d e r the names Netic a n d Co-Netic. These materials are available in thin flexible strip or as a bendablc sheet. Some forms use a (Continued on page 122 A)
INSTRUMENTATION
Dow Corning
SILICONE DEFOAMERS
...BEST FOR EVERY JOB Most effective. Dow Corning sili cone defoamers are best for banishing space-filling, time-eating foam in the processing of any foamer . . . from asphalt to beans to peas to zeins. Dow Corning Silicones start quicker, last longer. Eliminate messy and un sanitary foam-overs. Turn mainten ance into production time. Most efficient. Even the most violent foamers are tranquilized with a tiny amount of a Dow Corning silicone defoamer — often one part silicone solids per 10 million parts foamer is effective. For true economy, change to a Dow Corning antifoamer or defoamer. There's one just right for your product and system — oil, aqueous or food.
FREE SAMPLE Send today for a generous trial sample and full data. Please specify type of sys tem. Address Dow Corning Corporation, Midland, Michigan, Dept. 5723.
first in silicones
D o w
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
INSTRUMENTATION decided a d v a n t a g e . 1 he extreme reliability of present d a y semi conductor devices, elimination of transfer lag, availability of a wide variety of control functions, a n d the inherent flexibility of electronic sys tems are factors which favor the electrical system. T h e continued development of electrical methods of measurement will undoubtedly accelerate the trend towards in creased usage of electronic con trollers. This growing trend brings to t h e fore the need for more stringent requirements in installation practice. T h e possibility of stray signal pickup in a n electrical system is always possible, a n d electrical cables can not be j u m b l e d together indis criminately as can p n e u m a t i c lines. T r o u b l e can arise in electrical sys tems from the most unexpected areas. T h e b u r d e n of ensuring satisfactory freedom from " s t r a y s " lies with the instrument engineer who must en deavor to develop a comprehensive installation code capable of meeting m o d e r n requirements in this t r a n sition period from the tried a n d true p n e u m a t i c systems to the more elegant: electronic control systems.
SEMI-SOLIDS BECOME FREE Ψ
, INGj
LIQ UIDS
LIST O F S U P P L I E R S Aero Research Instrument Co., Inc., 315 North Aberdeen St., Chicago 7, 111. American Super-Temp Wires, Winooski, Vt. AMP, Inc., Harrisburg, Pa. Amphenol-Borg Electronics Corp., Broad view, 111. Bcndix Scintilla (Pacific Division), North Hollywood, Calif. Boston Insulated Wire and Cable Co., Bay Street, Boston 25, Mass. General Electric, Bridgeport 2, Conn. Guildline Instrument, Smiths Falls, O n tario, Canada Johnson Matthey and Mallory Co., 606 Cathcart St., Montreal, Canada Kintel (Kay Lab), 5725 Kearney Villa Road, San Diego 12, Calif. T h e Lewis Engineering Co., Naugatuck, Conn. Perfection Mica Co., 1829 Civic Opera Bldg., 20 North Wacker Drive, Chicago 6, 111. Spraguc Electric Co., Bennington, Vt. Stevens-Arnold Inc., Elkins St., South Bos ton 27, Mass. Thermo Electric Co., Inc., Saddle Brook, N. J.
m
W I T H THE ADDITION OF
MARASPERSE
LOW COST, MORE EFFECTIVE, DISPERSANTS By adding only .05% to 3.0% of Marasperse (based on the weight of solids ), a viscous pasty mass becomes an easy-to-handle freeflowing liquid. The action of Marasperse prevents, and reverts, the flocculation of insoluble particles in water suspension. Only the state of the mass is changed. The volume is unaffected be cause the Marasperse is taken into solution by the available water. The versatility of Marasperse dispersants is best described by the variety of applications in use, such as oil well drilling muds, gypsum slurries, ceramic slips, pesticides, dyestuffs, industrial cleaners, concrete, pigment, carbon black and clay dispersions, ore flotations, and many others. Marasperses are water-soluble, non-hygroscopic, non-fermenting, free-flowing powders. For additional information use the han dy coupon.
MARATHON A D i v i s i o n of A m e r i c a n C a n C o m p a n y CHEMICAL SALES DEPARTMENT •Registered U. S. trademark MENASHA. WISCONSIN MARATHON · A Division of A m e r i c a n Can Co. C H E M I C A L S A L E S DEPT. · M E N A S H A , W I S .
Please send Information File No. Mil Q Samples D of MARASPERSE for use in
Our authors like to hear from readers. If you have questions or comments, or both, send them via The Editor, l/EC, 1155 16th Street N.W., Washington 6, D.C. Letters will be forwarded and answered promptly.
NAME TITLE COMPANY ADDRESS Please attach
to your
company
letterhead.
Circle No. 55 on Readers' Service Card
VOL. 53, NO. 11
·
NOVEMBER 1961
123 A
