Anything too light, too fragile or too soft to push a plunger against its spring.

A roller or plunger valve needs a definite force to actuate - enough to overcome seal friction and the return spring. A machine member driven by a cylinder supplies that easily. A carton sliding down a chute, an empty bottle on a conveyor, a light moulded part or a piece of fabric does not.

A whisker deflects at a small fraction of that force. The wire or paddle bends, and that movement operates the valve.

That lets a purely pneumatic circuit sense the presence of product - counting items, confirming a part has arrived, detecting a jam, triggering a reject - which would otherwise need electrical sensing.

It also means the part is not marked or deflected by the detection, which matters on filled containers, finished surfaces and anything unstable that a firmer actuator would knock over.

By choosing the right actuator stiffness, positioning the valve out of disturbance, and shielding it where necessary.

A whisker light enough to detect a carton is also light enough to be moved by other things: machine vibration, the air blast from a nearby nozzle or exhaust, a draught, or a person brushing past.

So actuator stiffness should be matched to the lightest part that must be reliably detected - not made as light as possible. A slightly stiffer whisker that still detects the product but ignores vibration is the better choice.

Positioning matters as much. Mount away from valve exhausts, blow-off nozzles and vibrating panels, and out of the path of operators and material handling.

Where the environment is unavoidably disturbed, a shield or shroud around the whisker allows the product to reach it while blocking air movement.

A circuit that occasionally advances with no part present is usually reporting a whisker being tripped by something other than the product.

Less so, and that is one of the practical advantages.

A plunger valve must be set so the moving part drives it fully through its switching travel but not beyond its mechanical limit - a fairly narrow window, and over-travel breaks it.

A whisker has no such limit in the same sense. The wire or paddle simply bends further as the part continues past, then springs back. So a part that overshoots, varies in position, or arrives at a different height still trips the valve without damaging it.

That tolerance suits conveyors and chutes, where product position varies from item to item and cannot be controlled precisely.

What does still need setting is the height and lateral position, so that every item makes contact and no item passes underneath or beside it - which on a line handling mixed product sizes needs checking against the smallest as well as the largest.

Wire whiskers, spring rods, paddles and levers, each suiting a different product and direction of approach.

A plain wire whisker is the lightest and most sensitive, used for very light product and where contact must be minimal.

A spring rod is stiffer and more robust, tolerating repeated contact from firmer product and resisting disturbance.

Paddle and vane actuators present a larger area, which helps where the product is narrow or where contact position varies - a paddle across a conveyor detects an item wherever it passes.

Lever actuators extend the reach so the valve body can sit clear of the product path.

Direction of approach matters as well: some actuators are designed to be tripped from one side, others from any direction. On a conveyor where product only travels one way, a directional actuator can be used to avoid signals on reverse or on operator interference.

On conveyors and material handling, in packaging, and anywhere a light product must be detected without electricity.

Typical duties are detecting an item arriving at a station, counting product, confirming a carton has been erected or filled, sensing a jam or a backup, triggering a reject mechanism, and confirming a bottle or can is present before a filling or capping operation.

They also appear in textile and paper handling for detecting web breaks and edges, and on chutes and hoppers to sense flow.

The common thread is a light or fragile product and a preference for pneumatic sensing - either because the machine is otherwise pneumatic, because the environment is wet or hazardous, or because the simplicity of a valve and a tube is attractive.

On modern machinery with a PLC, a photoelectric sensor usually does the same job more flexibly and faster, so whisker valves are found where the pneumatic approach is deliberately chosen.

Only by contact - which is the fundamental difference from a photoelectric or proximity sensor, and it constrains where they can be used.

The product must physically touch and deflect the whisker. That means it must pass within reach, on a predictable path, and be substantial enough to bend the actuator even slightly.

So whisker valves cannot detect through a guard or a window, cannot sense an item that passes at a distance, and cannot detect something that is present but stationary and not touching.

They also introduce contact with the product, which on some items - sterile, delicate, freshly coated or high-speed - is unacceptable.

Where non-contact detection is genuinely required and the circuit must remain pneumatic, air-jet sensing is the alternative: a small air stream is interrupted or back-pressured by the presence of the part, giving a pneumatic signal with no contact at all. It uses air continuously, which is its cost.

Very little, but the actuator should be checked for damage and the sensitivity confirmed periodically.

The whisker is a light, exposed component in a product path. It gets bent by a jammed item, knocked during cleaning, or fatigued by repeated flexing over time.

A bent whisker changes both the position and the force at which it triggers - it may detect only some items, or trip continuously. Because the fault is a small change in geometry it is easy to look past.

So periodic inspection is simply a matter of looking at the actuator and confirming it is straight and springy, and testing that the lightest product in the range reliably operates it.

Beyond that the valve itself needs clean, dry air like any other. Keep the actuator clear of product build-up - adhesive, dust or packaging debris accumulating on the whisker changes its mass and stiffness, which drifts the sensitivity without anything looking wrong.