Many chemical reactions and experiments may take several days to complete, and, very often, running water is required for cooling during the reaction. In order to have a system which can proceed continuously and unattended, a safety device must be usedto turn off the electrical switch when the water suonlv is low or stonoed. Otherwise, the whole reaction may he ;Aied or even lead t o a hazard, such as fire. device^ for this kind of safety measures are commercially available but very often they are rather expensive and may not be readily obtained locally. A simple and inexpensive device making use of just one microswitch as described in this note (see figure) has oroved useful to stop the Soxhlet extraction of natural products by ether or other volatile and inflammable organic solvents, as soon as the water supply to the condenser is low or discontinued. The heating mantle is controlled by a microswitch which may be opened under the weight of aplastic vial containing some heavy objects. The vial normally floats in the water inside a plastic vessel (e.g., a used plastic bottle for chemicals) so that the microswitch is in the closed position and the extraction can be continued. Water runs into the vessel from the condenser outlet and is allowed to run out both a t the bottom and the top of the vessel. The water flow-rate a t the hottom is adjusted t o he equal to the minimum flow of water required for the condenser to operate properly. Therefore. if the water suonlv .. " to the condenser is s t .o .~ ~ ore d lower than the minimum amount needed, the water level in the vessel will droo and the weieht for the microswitch is lowered. The electrical circuit is now open so that the heating mantle will be switched off to stop the extraction. When the water supply is hack on again, and as soon as there is enough water in thevessel for the weiaht to float, the microswitch d l automatically turn on the Gating mantle to continue the extraction. I t should be noted that the weight does not need to be directly below the microswitch. A Nylon fishing thread and screw eves mav he used to a i d e the weiaht to a desirable DOsition ;here t6e plastic vessel is placed.vhe volume of water in the vessel between the too over-flow outlet, the water legel necessary for the weight tb trip the microswitch, and the water flow rate a t the bottom outlet determine the time constant or sensitivity of the device. These can be tailor-made or tested t o suit the requirement of the individual experiment.
A sealed or sparkproof and heavy duty microswitch is required to provide enough amperage for the equipment needed (e.g., a snap switch of 15A rating). Preferably aswitchof the double-pole single-throw type should he used, so that both sides of the ac line can be interrupted at the same time. The most common snap switches are of the single-pole douhlethrow type. Two of these switches can easily be stacked together to function as a double-pole douhle-throw switch. Spring and lever are already in the switch and it needs only a few ounces of force to operate. As a safety measure, the switch should be enclosed in a metal cover to protect the exposed parts. ~ h & is e no need to connect the mantle or other particular device permanently to the switch. The switch and the electrical socket may be fixed in a wooden hoard as an independent unit for flexibility. If the switch is of a double-throw type, the NO (normally open) side may be connected to an alarm. One can immediately know when the equipment connecting to the other side is off.
The safety device arrangement.
Volume 54, Number 5, May 1977 1 283
