Elmar Piel Norwich University Northfield, VT 05663
Simple High Precision Laboratory Valves from Polyethylene Tubing
Folded sections of ruhher hose have long been used as hand held, improvised, and temporary valves. Nevertheless, little other than occasional use has heen made of this tvoe .. of valve. Yet, low density polyethylene especially, and some other varieties of thermoplastic and elastomeric tuhing, can he fashioned readily into folded tuhe valves of great versatility which can allow flow contrul well beyond the possibility of stopcocks and even of simple needle valves. Moreover, the fluid contrnlled never contacts any materials except the wall of the tuhe, and no connecting fittings are required between tuhe and ~~
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One very useful folded tuhe valve configuration originally developed as a microhuret valve and made from 'I8 in. O.D., '/I6 in. I.D. low density polyethylene tubing is shown in A of the figure. It typifies the possibilities of the folded tube valve with thermoplastics such as polyethylene. A fitting to hold the tuhing in the conformation shown is made from a semicircular hand of stiff metal or plastic such as may he cut from metal or plastic tuhing. Holes having the diameter of the tubing are punched into the fitting, one a t each end and two close together in the middle. he axes of the end holes are thus perpendicular to those of the center holes. The tubing is forced into the holes so that a loop is formed between the two center holes and so that the tuhe leaving the fitting is somewhere near narallel to the tuhe enterine. Slots from the side of the hand to the holes can allow for easy pushing of tuhing into the holes fur Droner confieuration. The center of the looo is pinched to form the fold. f i e fold is held firmly in by the tight fit of the tuhing in the holes. In the position shown in A, the valve is closed. To allow liquid to flow under manual control of flow rate one inserts thk tip of the thumh (or index finger) into the loop and pushes against the fold with the index finaer (or thumb). It will he f&d that very exact control over fluid flow rate is obtainable with this system since the pressure used to open the fold to allow flow may he varied easily, quickly, and sensitively and removal of the fingers results in immediate automatic closing of the valve. (Stoococks and needle valves do not have the convenient automatic shut off feature.) Furthermore, if it is desired to set a particular flow rate and hold it for a long period of time one can open the fold to the proper setting hy lengthening (or shortening) one side of the loop to make it asymmetrical. When one side of the loop is about twice the length of the other, the fold will he practically wide open so that very little flow impedance will be present. By proper adjustment of the relative lengths of the arms of the loon anv rate of flow hetween nearlv wide onen and fullv closed
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~nuthe;practical means of making a tul;ingvalvkisshokn in R of the figure. A U or V profile channel of appropriate length fitted with sleeves which can hold the tubing tightly within the channel is satisfactory. This type of configuration is also useful with rubber or elastomeric tuhing in which case it is a normally closed valve which can he manually flowcontrolled. It makes a useful spigot for delivery of liquid from a reservoir. Here it is superior to pinch clamps because control is more sensitive. shut-off is nositive and shiftine ol'the oinch clamp resulting in leaks is eliminated. C illustrates a valve also functioning hy limp n m t n ~whose l only fitting is a polyethylene ring holding the loop together a t its hase. This valve does require two permanent hends made 234 1 Journal of Chemical Education
Usefulconfigurationsof polyethylene tubing for values. by heat forming the tuhing'. The fold tightness can he controlled by the position of the ring on the loop, making the loop larger or smaller. The loop can he made asymmetrical to set any particular flow-rate by slipping one of the atoms of the loop through the ring. Manipulation of the fold is as usual. A variation of this is the use of two rines to hold the loon. The ring farther from the fuld prevents skains in the tubing from affectine the flow-rate settine of the control loon. This variation eliminates the necessity orputting permaneni bends in tubing. D illustrates a simple configuration which is suitable as a spigot a t the end of a tube. The fold may also he closed by a spring for automatic shutoff. E illustrates a spout held in closed position by merely hooking the final bend behind the other &m of theloop Low density~. polvethvlene tuhine is esneciallv useful in the . . lahoratory because of its low cost, inertness, relative transparency, and ability to withstand pressure and vacuum'. The failure of pinch and screw clamps to function with polyeth: ylene tuhing has inhihited its use. The folded tuhe valve now makes the control of fluids in polyethylene tuhing even considerably better than that possihle with normal control in ruhher tuhing. Thiscontrol should help polyethylene tubing realize its full potential in laboratory use. Nylon tuhing is also suitahle for such valves though in heavy wall thicknesses it is impractical. Soft vinyl and urethane tubing of most formulations are also well suited for folded tuhe valve use. Unfortunatelv complete shut-off of folded tuhe valves made of Teflona has nnt been achieved. 'Piel, E...I. CHEM. EDIIC..56, I35 ( 1 9 i 9 ) .
