Our online catalog is being updated! Please contact us for current pricing. Selecting a Valve Since 1958, Thermionics has manufactured over 65,000 qualified valves. Thermionics manufactures the broadest line of vacuum valves. They have been tested and judged the highest quality valves manufactured in the United States. Full Conductance The figure of merit of a vacuum valve is its conductance, i.e., the amount of gas that passes from the plane of the inlet valve port to the plane of the outlet port of the valve in 1 sec per unit of pressure differential. The inside diameter, the length of the tube, and any constrictions or obstructions can adversely affect conductance. Be certain that the gate or poppet of your valve opens fully and does not interfere with gas flow. The inside diameter of the port is almost meaningless as a specification unless full conductance is offered. TLI valves open fully Competitors' valves may not Valves that provide full conductance are qualified valves; those that do not might be referred to as unqualified or counterfeit valves. All Thermionics valves open fully and provide full conductance under all conditions. We have included some computed conductances with some of our valve specifications. Because the short tube approximation to the conductance equation was used, ignoring the larger diameter in the valve body area, computed conductances are slightly understated. Valve Choice Criteria When deciding to specify either manual or electro-pneumatic actuation for the valve, consider frequency of use, available utilities, and operator attention required when the valve is actuated. Select a valve capable of withstanding the bakeout temperature required to reach the base pressure desired in the time required. All Thermionics valves are designed and manufactured to reach and maintain very low vacuum pressures easily. To do this, valves are free of virtual leaks, they have the lowest particulate levels, greatly reduced surface areas, less parts exposed to vacuum, have a smaller number of bearings exposed to vacuum, are smaller in size, weigh less, and are fully welded.      Thermionics valves are welded internally or with full penetration, eliminating the problems inherent with brazed assemblies, and further reducing the internal surface area and profile. The outgassing rate of the materials from which the valve is manufactured must be considered. Materials affect the gas load, and thereby the pump size required. If a valve is to be used in a corrosive environment, the appropriate material must be specified when the order is placed. The smaller the seat or gate conductance leakage, the smaller the pump required. High leak rates make additional pump speed necessary to compensate for leakage through the seal. A valve is often sealed against atmosphere when equipment is shut down for the night or weekends; the leak rate must be small to prevent wasting pumping capacity. Consider the number of cycles for which the valve is guaranteed and if the number is independently verifiable. Is the valve stem seal O-ring or bellows sealed? If bellows sealed, is a welded or formed bellows used? Viton-sealed gate and poppet valves are very forgiving, whereas metal-sealed bakeable valves must be manufactured with greater precision and they are more sensitive to contamination. Determine whether the chamber pressure requirements for your particular needs are met by your proposed system by comparing the gas load at designed base pressure with pumping speed as reduced by any conductance restrictions between pump and chamber. First, the outgassing load Q is simply the product of the outgassing rate(s) of the material or materials used in the system and the respective area or areas. Then, calculate the chamber pumping speed required to maintain the pressure necessary for your vacuum process, S = Q/P where S = Pumping speed in the chamber (l/sec) Q = Chamber outgassing as calculated above (Torr l/sec) P = Pressure in the chamber (Torr) If S appears excessively high, consider: Changing the chamber size or configuration to decrease surface area. Changing to a construction material with a lower outgassing rate. Reducing the outgassing rate by baking the chamber or by special passivation, such as the TLI passivation process. The delivered pumping speed, Sd (pump speed at the chamber port), is limited by component conductance between the pump and the port. Conductance, in turn, is a function of tube size, length and amount the poppet or gate opens. The higher the chamber speed needed, the larger the valve size needed, and the greater the need for full conductance. Usually the conductance of high vacuum valves should be greater than the rated pump speed. Finalize the chamber design and determine how you will connect the valve and other component parts to the chamber system, keeping in mind how each component will affect conductance and associated gas flow pattern. After selecting your chamber, pump, valves, and plumbing, calculate the pumping speed delivered to the chamber as follows: 1/Sd = 1/S + 1/C1 + 1/C2 + 1/C3 + ... + 1/Cn + 1/Cv where 1/Sd = Pumping speed delivered to the chamber 1/S = Pump speed C1 to Cn = Conductance of the components 1 to n between the pump and the chamber Cv = Valve conductance Compare the delivered chamber speed (Sd) to the required chamber speed S as calculated in step 7. If the delivered speed is less than your required chamber speed, then consider: The alternatives in 7A above; Shortening the distance between pump and chamber; Increasing the pipe and valve size between pump and chamber; Increasing the pump speed.