A gate valve is a valve that opens by lifting a round or rectangular gate out of the path of the fluid.Instrumentation Ball Valves

A needle valve is a type of valve usually used in flow metering applications. It is similar in operation to a gate valve but tends to use a smaller, more precisely controllable orifice. When the handle is turned, a plunger slowly descends to block flow.

Manual means that a person has to phisically open/close the valve by turning a handle or wheel.  Actuated means the valve is opened/closed by a mechanical, or electrical, source.Needle & Gauge Valves

The key difference is called "linearity".  For a needle valve, the flow vs. valve opening is nearly linear (e.g., if the valve is 30% open then it will pass about 30% of it's maximum flow).  

The linearity of a gate valve is nearly non-existant.  A gate valve will pass 100% of maximum flow at about 25% open.  At 10% open it will pass about 5% of maximum.  A plot of flow vs. percent open (for a given set of pressures and temperatures) is sharply non-linear.  If you ever try to throttle with a gate valve you'll see that the quiver in the valve position will give you a wildly varying mass flow rate.

I've never seen a case where a gate valve had a legitimate place in any measurement application.Instrument Manifolds

Gate valves are designed to pose the least possible restriction to the flow when they are open.  They then have an unobstructed flow path.  The only valve with less drop would be a full-port ball valve or possibly a split-body butterfly valve with a thin single-piece vane (Keystone 99, for example) It'd probably possible to find a small gate valve but they are of questionable merit below 1" NPS and highly rare smaller than 1/2"

Needle valves are a special case of globe valves where the ports are drilled. 1/2 NPS would be on the large end for a needle valve, at least for the port size, but there are scads of them with 1/4" ports and smaller, even if they have 1" connection sizes.   The flow path is tortuous and discontinuous, both in direction and in cross-sectional area.Regulators  Needle valves are used where restriction to the flow is either not a problem or is actually wanted.  Needle valves are good for regulation of the flow because the valve plug can be manufactured with a long <needle>  taper, so there is very high resolution of the flow vs valve-stem position.  Blunt valve plugs are also available, and these are NOT useful for precise regulation but usually have a little more capacity.Globe Valves

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