344 On some one-time opening valves, the sealing element is neither broken nor pierced, but melted. The  Angström Space Technology Centre has developed a valve made from two silicon discs or wafers forming a filter and a heater [5]. The gas path is obstr ucted by an indium solder (melting point 156.6°C),  that can withstand 100 bar pressure and is melted within 10 s requiring power of 13 W. The silicon element weighs only 0.4 g, but the housing weight amounts to 140 g. The Jet Propulsion Laboratory has  also built the micro-machined isolation valve, openi ng when a doped silicon membrane melts (less than  30 mJ are needed). The Pyrex housing has a burst pres sure of about 200 bar. A disadvantage of these  valves is that parts from the solder can be re leased into the carrier gas, although both valve types  integrate a filter that captures the larger particles. Another thermally actuated valve using paraffin as seal  was developed, but it can be excluded for the MOMA instrument because of the low actuation temperature.   Basic Design 

Looking at all these valves increased the motivation to  build a new valve. Also, the basics of a sealing  valve were retained: it is always a combination of a sealing element and of an actuator. The sealing  element can be a frangible pillar, a shaft, a meltable element or a metallic membrane. All possibilities of  sealing element are used in single-shot valves. Ho wever not all possible actuators have been tried for  such an application. Particularly interesting are actuators that are activated by a temperature change.  Good examples are the regulation va lve of MOMA, the valve with a wax actuator, or the valve with a  shape memory alloy. Indeed it only requires a heater, which is a very simple and rugged way to transmit energy. Moreover, the heater can be placed outside t he body of the valve, which avoids complications  with cables fed trough the body of the valve. The only problem with thermal actuation is that the activation  temperature has to be above 130°C, temperature of the sterilization process, with a sufficient margin. Ideally, there should exist an activation temperatur e point, and the actuation should not take place  progressively. Which thermal actuator has a well defined actuation temperature? Shape memory alloys  are very interesting, and materials that recover t heir shape at temperature over 150°C were searched.  The Company Dynalloy could provide high temperature SMA made of NiTiPt with a recovery temperature of 300°C. But high temperature SMA is still a field of research, and these are ve ry costly elements, only  available in the form of wires. A rugged design needs an actuator with a longer heritage. A longer  heritage and also high working temperature is given by snap-discs. These are made of two layers of  metals foils with different thermal expansion coe fficients and bend at a determined temperature. This  actuator was favored because it is very easy to fi t inside a housing, very light, and does not produce any  out-gassing, like thermal act uators based on phase change.   Having chosen the actuator, the sealing element remained to be selected. Also here, the criterion was the reliability. Therefore it wa s looked for a component that has a br oad use and that be nefits from wide  knowledge. An attractive device on which the design could be based on is called burst disc. It is basically a thin disc of gas-tight material that ruptures at a certain pressure di fference between the two sections it  separates. These discs are widely used to protect against over-pressures. Similar to the solenoid valve  from Industria, a membrane could be punctured by a pin attached to the actuator. The first test showed that the force and the displacement yielded by the bimetallic snap-disc were sufficient to clearly pierce a membrane similar to a burst disc calibrated at 150 bar.  The next step was to design the body of the valve  with heater, tubes, and adequate pin attached to the snap-disc.   Refined Design 

In order to ensure a high reliabilit y, some important design paramete rs have to be selected carefully. For  example the pin has to be fixed in a manner that it cannot accidentally touch the membrane, the  membrane and the housing have to be so thick that they withstand the high pressure.    First of all, a housing made of stainless steel was chosen, to be weld-compatible with the 1/16'' (1.6 mm)  outer diameter tubes used in gas chromatography. As basic shape, a cylinder was chosen, to just fit the  dimension of a commercial snap-disc that is 12.7 mm. From the beginning, a configuration was found,  NASA/CP-2010-216272