Chemical Instrumentation 5. Z. LEWIN, N e w Y o r k University, Washington Square, N e w Y o r k 3, N. Y.
Tecam T h i s series of articles presents a suruey of the basic principles, characte7istics and l i m i t a t i m of those instruments which find important applic a t i a s i n chemical work, running the gamut frmn balances and burets to seruomechanisms and spectrometers. The emphasis is on commacially available equipment; approximate pn'ces are quoted to indicate the order of magnitude of cost of the various types ofdesip.
2.
Temperature-Controlled Baths
A gratifyingly large number of wellengineered thermostats is currently on the market, and the experimentalist should have no trouble finding one suitable for his special circumstances. The following discussion summarizes the characteristics of the more recent examples of the various types of this class of instrumentation. The discussion is limitpd to commercial laboratory instruments designed to operate in the vicinity of room temperature, and with a precision of temperature control of iO.S0C or hetter. Bronwill The Bronwill Constant Temperature Circulator ($210) is 8, unit consisting of sensing element, amplifier and controller, heater, and stirrer, all comprising a compact device and designed to be used simply by inserting the appropriate parts into any desired vessel or container (see Fig. 11). The thermoregulator is of the mercury-in-glass type, similar in appearance to a Beckmann thermometer.
Figure 11. Schematic diagram of the Bronwili Constant Tcmperotvre Circulator.
It is sealed and evacuated, so that the current that passes between the tungsten contaot and the mercury column a t each switohing does not muse corrosion and fouling of the contact. The position of
(Continued)
the metal contact can be adjusted up or down through tho agency of an external magnetic collar (see Fig. 17 B ) , by means of which any desired temperature in the range 0-100°C can be pre-set as the control point, wit,h an accuracy of +lac. (The accuracy with which the control temoernturc can be "re-SP~~hould not be confused with the eon&mcy of temperature rontrol, which in thin case is +O.Ol"C.) The thermoregrdator contacts are connected in the grid circuit of a thyratron (2D21) tube, whieh oantrols a relay that activates the heater circuit. The current through the reguhtor contacts is only 10 microamperes, but this is sufficient to control the tube current, which in turn controls several amperes of heater current. The heater is a resistance winding embpdded in a metal casing shaped in the form of a thin cylindrical shell (somewhat like a nspkin ring) to afford a large exposed surface. I t delivers 700 watts on 115 volts ac. The stirrer, whieh can also he used a8 a ciroulating pump, is operated by an rtc motor that uses 80 watts (0.64 amp a t 115 v). The stirrer blades are fashioned from flexible sheet metal, so that the blades flatten out when the back-pressure increases. This flattening reduces the stirring (and hence, pumping) efficiency and thereby tends to provide an automatic adjustment of the pumping pressure to match the back-pressure in the line. The pumping rate is %djustahle from 0 to 1.5 gallons per minute. Under the best conditions (control temperature of 37OC with room temperature 15' lower, in a n 8-gallon glass container, no largo thermal fluctuations) this unit will maintain the average bath temperature constant to within f0.0l0C. The oontrol tends to be less good under less favorable conditions. The amplitude of the temperature oscillations shove and below the control point, which is independent of the constancy of that point, depends upon the size and shape of the bath.
The Tecam Tempunit ($135), manufactured by Techne, Ltd., Cambridge, England and distributed in the U. 5. by A. H. Thomas Co. and A. 5. LaPine Co., is also a portable constant temperature controller and circulator, consisting, in this case, of a thermoregulator, heater, stirrer combination. It irr a n example of a unit in whieh the temperature sensing element is a bimetal switch. I n order to attain adequate sensitivity t o temperature changes, the bimetallic strip is made as long as is practical, and is wound into a helix. I n some thermostats (as e.g., the Precision-Freas Utility Bath, $2251, the bimetal switch directly opens or closes the ooil circuit of a n electromagnetio relay, passing several milliamperes of current. Such contact points tend to corrode with use, and a rather large temperature effect is needed to assure positive switching ( 0 . 3 T in the PrecisionFreas unit, more in others). To avoid this effect and retain the full sensitivity of the bimetal switch, the Tempunit employs a special pneumatic system to transform the expansion or eontraotion of the bimetal sensor into a switching on or off of the heater (see Fig. 12).
THROTTLING C&PILLARY
Figure 12. Tempunit.
Schematic diagram of the Tecam
The blades of the stirrer are mounted so that the rotation of this element produces a mild suction in a n adjacent tube immersed in the bath liquid. This suction is communicated to a flexible bellows, pulling i t in and raising a weight that hangs from its end. The lifting of this weight releases a. Microswitch on which i t rests in the normal position, thus producing the switching action that turns the heater on. The bimetal helix is attached to a vane that closes an opening in this pneumatic line. When the temperature rises above the control point, the expansion of the helix moves this vane away from its seat, releasing the suction and causing the flexible bellows to drop its weight on (Continued on page AZW)
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the Microswitch, turning off the heater. The suction can be simultaneously employed to circulate bath fluid through external jacketed apparatus. This pneumatic system is essentially a. mechanical amplifier, equivalent to the electronic amplifier circuits of other thermostats. I t permits positive switching to be accomplished by the bimetal helix in response to temperature changes of as little as *0.05"C. The pneumatic relay also provides a builein "fail-safe" feature, aince any malfunction, such as a tear in the flexible bellows, stoppage of the stirrer, decrease in the level of the bath liquid, etc., will cause the weight to fall onto the Microswitch and turn off the heater. The control point can easily he preset to +l"C a t any temperature in the range 0 to 100°C by a simple mechanical adjustment. The heater is of the tubular immersion type. .4n indoetion-typo 1/20 horsepower motor operates the stirrer. Under average laboratory conditions, this unit can maintain the mean hath
Arthur H. Thomas The A. H. T. Infrared Resoarch Model Confitant Temperature Bath (W20i) utilizes a n external 250-watt heat lamp to provide essentially lag-less heat input.
Figure 13. Arthur H. Thomas Infrared Rerearch Model Condont Temperature Bath. Thermometer and thermoregulator ore risible; heater and stirrer ore obscured by metal baffle plate.
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The thermoregulator is of the sealed mercury-in-glass tvpe, and its rontrol point is set in a fashion similar to that used in adjusting a Beckmann thermometer. This unit does not employ electronic amplification, so the current through the thermoregulator contacts is fairly large, viz., 2 ma. This ourrent energizes a senpitive electromagnetic relay, which in turn operates a heavy duty mercury relay that controls the current to the infrared lamp. The stirrer is a conventional four-bladed propeller driren by a %watt ac motor. One model of this type of thermostat is shown in Figure 13. With the heat lamp, the upper limit of the range of the thermostat is 45°C. By the use of an auxiliary 100-watt immersion heater, this range a n he increased t o 55°C. Under favorable lah-
orstory conditions, this unit controls to *O.OOl°C; under average conditions it is constant to about *0.005'C.
Fisher The Fisher Isotemp Bath ($1200) utilizes a resistance thermometer as the sensing element. This resistance is part of an tto Wheatstone bridge, which is balanced a t the control temperature. For large unbalances, an amplifier circuit turns an 800-watt heater on; far small unbalances, a smaller heater is activated. The magnitude of the power output of the small heater is adjustable by the user between 0 and 500 watts. Thus, this system is a compromise between direct and proportionsl control, and has the advantage of relatively small temperature cycling amplitudes. The range of this instrument is room temperature to 107°C; with special cooling, the range can be The conextended downward to -5'C. stancy of average bath temperature is 10.005'C under favorable conditions. Sargent The resistance themometm type of thermal sensing element is also the basis of the Sargent Thermanitor (Model 8, $275) which is a true proportianal-centrol regulator. I t consists of a thermistor as the eensing element, connected in a bridge circuit. The unbalance current of this bridge is fed into a magnetic amplifier circuit, the output of which provides proportional control of a heater between 0 and 250 watts, depending upon the degree of unbalance of the resistance bridge circuit. The range is 0 to 100°C; constancy of control is &O.OO1° between 0 and 40'; 10.01 st 100°C. Complete controlled bath8 embodying the Thermonitor control system are avaihble; e.g., Model 884810, $440. Other Complete Units The Universal Infrared Bath ($396) of Ralph Kaye and Asaociatettes, Highland Park, Illinois, consists of a mereury-inglass thermoregulator, electronic amplifier controlling a 500-watt infrared lamp, and a centrifugal pump. The operating range is between room temperature and 65'C; by means of auxiliary heaters the range may be extended. Connections are provided for inserting s. cooling coil in the circulating system. The thermoregulator contacts pass 2 microamperes; the precision of control under favorable conditions is 10.OOl°C. The Magni-Whirl Utility Water Bath (Model MW 1110, 800-watts, $169) of Blue M Electric Co. offers a special stirrer arrangement that provides a maximum of free volume in the bath. A perforated plate a t the bottom of the bath is given a '/r-inch movement up and down periodically by the activation of a solenoid mounted below the bottom wall, producing an effective and uniform stirring of the entire bath. The constancy of control is 10.3"C, achieved with an expansiontype metal switch as thermoregulator. A refrieerated modification is also avrtilable ($k9for 800-watt model; the range of this instrume-tis 0 to 100°C.)
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TheResesrch SpecidtiesCo., Richmond, California, offers a thermostat which employs a mechanical shaking mechanism to agitate both the bath fluid and the contents of flasks, beakers, or test tubes attached to the shaker (Model 2156, 25" to 90°C, +0.5"C, 800 watts, $525). The Precision Scientific Company's Temp-Tml Circulating Water Bath ($325) employs a mercury-in-glass thermoregulator, sealed and hydrogen-filled, to operate a sensitive electromagnetic relay that in turn controls a mercury plunger relay ta operate the tubular immersion heater. Stirring is accomplished by a centriiugal circulating pump. Range is from room temporttture to 99%; constancy of mean temperature is 10.3'C. The Wilkens-Anderson Go., Chicago 51, Illinois, has s, wide range precision bath that includes a refrigerator cooler, heater, mercury thermoregulator with electranic amplification, and stirrer. Control to 10.01"C can beaehievedfrom -10 to 6 5 T ; model 882 is 12 in. X 12 in. (cylindrical) and lists a t $390. An example of one of the largest wide range hath8 is their model 884, with inside dimcnsians of 29 X 14 X 10 in. eont,rol to +0.05"C, range -30 to 50DC,$055. A 14-liter eitwlating thermostat bath using a mercury-in-glass contact thermoregulator with electronic relay, giving control to 10.Ol°C is imparted to thc U. S. by Ltlx Scientific Instrument Corp., 835 Broadway, Nem- York 10 ($205). Brinkmann instrument^, Ino., N. Y., has an imported line of "B-H Ultra Thermostats" that are available as portahle units for insertion into existing baths (Model KC for baths up to 10 gallons, $200; Modd E B to 25 gallons, 18420). Other models are complete units, including the vessel, heaters, stirrer and circulating pump. Model F ($290) is a small 0.5-gallon unit designed specifically for t,he circulation of thermostntted fluid through adjacent instruments, such as r~frartomctrrs, etc. Model N (1.75gallon, $430) and Model NB (3-gallon, $175) are litrge enough for the immersion of flasks and other vessels in the bath. All these units achieve control through a mercury-in-glass contact thermoregulator whirh directly operates a relay in Models E C and F (18 milliamperes passing through the regulator contacts), or eontrols an electronic relay circuit in Models EB, N and NB (10 microamperes contsot current). Five companies that specislise in the construction of general a8 well ss special purpose laboratory thermostats are: the Emil Greiner Co., N. Y. 13; Central Seientifir Co., Chioego 13, Ill.; American Instrum m t Co., Silver Spring. Md.; L a t h e Inc., Chiraeo 22. Ill.. and Groenhrier Insts.. Inc., Ronceverte, W. Va. These suppliers have available a. variety of thermostats, hafied upon most of the design types de: scribed above. As a typical example, the American Instrument Co. offers a bath with a particulmly wide range containing x circulating pump, refrigeration and hwting elements, and a bimetal-typc bhermoregulator (Modd 4-8600, 16-gallons, -29 to 71 'C, +0.08 to 0.03'C, $950). (Continued on page A804J
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For some purposes, as, for example, in the use of viscosimeters, good visibility of tho immerscd apparatus is essential. Several companies offer babhs that we specifically designed for such use. T y p i d of these is the Model 111 of Scientific Development Co., State College, Pa., which employs a mercury-in-glass contact thermoregulator with electronic smplificstion, has a. range of room temperature to dose to 10O0C, 8-gallon capacity, is constant to *0.003"C, price $545. Direct and proportional control temporsturt? regulating systems are also available from a number of electronics manufacturers, such as Fenwd, Inc., SimPly-Trol, Minneapolis-Hanqvwell, West Instruments. Fielden In~truments.Sunvic Controls, ~ t d . , and others. since these have been primarily designed for the control of furnaces and ovens, they will be treated latw, in connection with that category of instrumentation.
ow ever,
Thermoregulators The assembly of a "do-it-yourself" thermostat in the laboratory from the component parts has been greatly facilitated by recent developments in the design and eommereial availability of thew items. Thia is part,iculrtrly true of thc thermoregulator, which once x-as exclusively of a single type that required great skill in glassblowing, care in filling and adjusting, and cont,inuous attention bo maintaining clean contact surfaces, in order to attain a sensitivity of the order of hundredths of a dpgree or better. Kaw, a variety of thermoregulstors can be purchased as rugged, yet extremely sensitive devices that are very easily adjustable to any desired control temperature. A. Metal Ezpansion Type. There are two principal types of metal expaneion regulators currently available. One type consists of the himetal strip, which is generally wound in a helix to provide minimum hulk, combined with maximum linear extension, as s h o r n in simplified form in Figure 14A. Well-engineered examples of this type of component are: Aminco Model 4-235A, range -73 to 177"C, sensitivity +0.02T, price 523; Aminco Model 4'235F, range -95 to 18!L0, sensitivity *0.005"C, price $23; Cenro No. 99005, range -45 to %OD, sensitivity *O.Z°C, price 822.50. The second type of metal expansion regulator, illustrated in Figure 14B,consists of a cylindrical brass or stainless steel tube as the temperature-sensitive element, to the ends of which are attached a pair of non-expanding struts carrying the electrical contacts. Expansion of the metal tube stretches the struts apart, thus breaking the electrical contact. The amount of shell movement necessary to cause the contacts to open or close is set by a n adjusting errew. A given linear expansion of the shell separates the contacts by approximately twenty times that distance. This type of detector is manufactured hy Fenwal, Inc., Ashland, Mass., under the trademark "Thermoswitch." I t is available through several chcmieal supply houses; a typical example is tho Ccnco l i a . 99180, range (Conlinued on page -4806)
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-74 to 300', sensitivity +0.05'C, price $48. B. Mercury-in-Class Types. The classical form of the mercury-in-glass thermoregulator consists of a bulb terminating in a capillary tube; one contact ia sealed
Figure 14. Differential metal expansion thermoregulators. A. Bimetal strip type. 8. Cylindrical shell (Fenwal Thermoswikh) type.
into the bulb, the other contact being mounted a t the mercury meniscus in the capillary. The control temperature of this type of regulator can he adjusted either hy moving the contact in the capillary up or down, or by removing or adding mercury. This adjustment can be aeromplished readily enough if the thermorogulator is not a. sealed unit; a particularly convenient form of thin type of unsealed thermoregulator is the "RspidSet" device of Precision Scientific Co., Chicago 47, Illinois, shown schematically in F i w m 15. However, a mercury "c"'
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m n?.S r".,,
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sw .
Figure 15. Unsealed contoct mercury fhermoregulator [Precirion ScientiRs Co!r Rapid-Set).
surface exposed to labocttor,v air will not remain clean for long, and the electrical (Continued on page Ad08)
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ront,art hrcomes fouled. This fact,o
