An instrument needle valve

An instrument needle valve uses a tapered pin to gradually open a space for fine control of flow. The flow can be controlled and regulated with the use of a spindle. A needle valve has a relatively small orifice with a long, tapered seat, and a needle-shaped plunger on the end of a screw, which exactly fits the seat.
The virtue of the needle valve is from the vernier effect of the ratio between the needle's length and its diameter, or the difference in diameter between needle and seat. A long travel axially makes for a very small and precise change radially .
Needle valves may also be used in vacuum systems, when a precise control of gas flow is required, at low pressure, such as when filling gas-filled vacuum tubes, gas lasers and similar devices.