1942
ANALYTICAL CHEMISTRY
Constant-Pressure Device for Gas Flow System. Alban Charnley and Geoffrey L. Isles', Chemistry Department. Manchester University, Manchester, England.
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gas is flowing through a system to a region of lower pressure, as in a condensation process, the higher pressure must often be maintained a t a constant value. Constant-pressure devices such as rubber membranes ( 1 j or buoyancy manometers ( 9 ) depend not only on the backing pressure but also on the rate of gas flow. A solenoid-controlled plunger has been used by Clow and Shand ( I ) , but the actual cutoff was made of glass which would clearly not stand up to fast vibrations for long periods. HEN
thermostating is essential if reproducibility is required over long periods. The plunger is 10 cm. long with a diameter of 1 cm. and a taper of 45'. The solenoids are controlled by a telephone relay and sparking is reduced by 2 pf. condensers. Since the controlling solenoids are identical, solenoid S B helps to pull the plunger back with an acceleration of a t least 2g and so reduces the time lag as well as giving a better cutoff. Both solenoids have 10,OOO turns, a resistance of 300 ohms, and internal diameters of 2.5 cm. JVhen Pp equals 100 mm. of mercury and air is flowing through at the rate of 4 liters per minute a t pressure PI,the amplitude of the plunger vibration is about 1 mm. This gives 100 to 150 vibrations per minute, according to the fineness of the adjustment. The pressure variation measured in a mercury E-tube is a barely perceptible movement of the meniscus. The plunger always seats itself properly, as the clearance in the tube is only about 1 mni. The maximum gas flow used is 5 liters per minute.
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LITERATURE CITED /
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(1) Clow, A,, and Shand, G., J . SCL. I n s t r . , 16, 354 (1939). (2) Huntress, E. H., and Hershberg, E. B., ISD. ENG.C H E M h, i i r . ED.,5 , 144 (1933).
Simple Multiunit Magnetic Stirrer. I,. A. Wollermann and R. G. Tischcr, Iona State College, .hies, Ion-a.
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an immediate need for an additional magnetic stirrer. HA"' the authors assembled the apparatus from items common1:h~
available in a chemical laborator!-.
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A small stirring motor, preferably one 7vith a variable-speed drive, serves as the source of power and n small Imr magnet, wired across a pulley activates the stirring bar. The shaft for the pulley is a short iron rod on which the pulley may turn freely. This rod is inserted in a rubber stopper and the stopper is mounted in a buret clamp with a sleeve or a number of washers between the stopper and the pulley to allow free rotation of the pulley. The motor shaft may be grooved or, if not, a pulley may be mounted on it. A belt of rubber bands or heavy cord will be sufficient to turn the assembly, and the Ion- speed at tvhich magnetic stirring is commonly done olwiates the need for bearings.
Figure 1
Instead of this the authors used a cone-tipped iron plunger and n seating made from a soft pencil eraser having a hole made by
a standard (No. 2) cork borer. h t the beginning of an experiment the relay is in position Y (Figure I), and the bottom solenoid is then actuated. ,4s soon as tap T 2is opened, gas is allowed to flow into the 250-cc. bulb, E, until the desired worliing pressure is reached. ?'p is then closed. \\-hen any further pressure increase occurs in the system, the rising mercury in the sensitive manometer, 31, makes contact and completes thr circuit, causing the solenoid, Sa,to lift the plunger. \Vhen the pressure falls sufficiently, contact is broken and the I)ottom solenoid, SB, pulls the plunger back. An ordinary barometer control ( 1 ) is subject to violent oscillations even under steady flow conditions. In the controlling manometer the oscillations are damped by partially closing tap 1', and the control is further improved by having the capillary (2 mm. in internal diameter) a t a small angle to the horizontal. The sensitivity of such a device is clearly dependent upon the flow rate, backing pressure, and size of the hole in the cutoff. The effective size of the hole can be altered by varying the height of the plunger oscillation with a rubber-tipped brass bar, n hich can be externally adjusted using flexible copper bellow s. The rubber seating and the copper bellows unit are both held i n position by vacuum wax. A Miter ballast bulb, D, is used with the higher flow rates and the manometer unit is well lagged; 1 Present address, Gas Research Department, Fuel Department, Leeds University, Leeds, England.
EAR M A G N E T PULLEY SLEEVE
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R I N G STAND-
As only the pulley assemblv need be mounted under the material to be stirred, a considerable saving of space is affected; moreover, the material is not subjected to the heat that Jvould arise from the motor directly under the container. -2 number of such assemblies may be operated from one motor, thus enabling one source of power to serve in the handling of several samples simultaneously. JOURYAL Paper No. 5-2283, I o n a .igrIcultilral Experiment Station, htnes, Iowa. Project KO.1123.
