Updated: May 20, 2021
Today's post is a quick one covering the basic terms applied to toggles, the first of these is: "Momentary".
A "momentary" toggle stays in one position (usually "on") only as long as you hold it (generally just a moment, hence the term). As soon as you let go, it springs back into the "off" position.
Momentary toggles are frequently used for unit flush (or "purge" - both terms are used frequently and interchangeably). The "momentary" aspect of the toggle is not integral to the function, it simply makes it easier to use for a given application. Generally, a "momentary" toggle is used when it is not desirable to leave the toggle "on" for a prolonged period. One of the most common original applications of the "momentary" toggle, was the cup filler on a cuspidor. By using a momentary toggle, this prevented the patient from running the water constantly.
A toggle that is not momentary is just like a light switch, flip it one way to turn it "on" and the other to turn it "off".
Another term that can be applied to toggles is "exhausting" (also called "relieving"). Often marked "3 way" an exhausting toggle is used to send air to a secondary valve to turn something on. The toggle exhausts the pressure in the line (there will be a short "hiss") when turned to the "off" position so the secondary valve will turn off. While air is what's going through the toggle, the most common use of an exhausting toggle is water activation. That is, it turns water on and off. When the toggle is flipped to the "on" position, the air pressure is applied to another valve turning it "on" so the water flows. When the toggle is flipped to the "off" position, the air flow is interrupted, but just as importantly the pressure is exhausted through the toggle so the water then turns off.
If the toggle fails to "hiss" when turned to the off position, it is failing to exhaust and thus, the water won't turn off. In this case, the toggle valve should be replaced.
Note that these two features of toggles are not mutually exclusive. A toggle can be both momentary and relieving (or neither).