 WHAT IS IT?
This Assembly is made up of two independent SERVOID® Valves which, when driven separately by synchronously, blend two gases to a desired ratio and deliver the blended gases at a desired flow rate and frequency from a common outlet. Maximum flow per Valve is 200 liters per minute @ 10 psid. Each side will shut off and control leakage to less than 50 cc/min. @ 10 psid. The operating pressure range is from 15 to 100 psig. The units are sufficiently accurate and consistent, cycle to cycle, valve to valve, that the drive current level is a very reliable indicator of mass or volumetric flow. The Valve's hysteresis is less than 4% F.S., repeatability less than 0.5% F.S., and response better than 40 Hertz. The curve on the reverse side is typical of either circuit. Resolution, particularly in the low flow range, is quite good. It has a turn-down ratio of 4000:1. |
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 HOW DOES IT WORK?
The SERVOIDS® are current driven devices. A typical design will operate from 0 to 600 mA at 24VDC. By applying a current to one side, anywhere in that range, a pre-set volume of gas will flow into the mixing chamber. Current applied to the other side, determined by the desired mixture ratio, will deliver a proportional amount of the other gas to the chamber. That ratio is then maintained throughout the flow range up to the maximum frequency rate. The ratio can be anywhere from 0 to 100% of each gas. |
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| HOW IS IT USED? |
 While the illustrations depict a dual Valve package, the same scheme can be used to mix and meter and reasonable number of gases. The system designer, faced with having to deliver a pre-mixed blend (where that blend is not fixed), at a volumetric or mass flow and frequency (which are themselves variables), can package a valve assembly to his particular application. The valve shown is used on critical care ventilator applications to regulate the flow and ration of air and oxygen in a patient's breathing circuit. The valve also follows the command signal to determine wave form and frequency of each breath, and is sufficiently responsive to follow a patient's sigh, cough or hiccough if the command signal demands it. The SERVOID® drivers are isolated from the gases. The wetted surfaces are stainless steel, anodized aluminum or fabric reinforced elastomers. There are no dynamic seals. |
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