19P External Explained: The Trade-Offs Against Series 18

19P is the external model in Germanjet Series 19. It is mounted outside the machine, does not enter the cylinder and does not take pressure, and transfers the linear motion of the measured mechanism to the magnet ring through a follower structure. It overlaps Series 18 in functional positioning: both are external, neither changes the mechanical body, and both are external non-contact absolute sensors (accuracy rows still follow the series catalogue, not a shared 1–100 μm / ±0.002 mm line). The difference is the product platform: 19P belongs to Series 19 and shares a platform with 19H cylinder-integrated, 19D split and 19F flexible; Series 18 is a dedicated external product line, with more fully developed matching schemes for follower-rod and sliding-carriage follower structures.

The trade-off between 19P and Series 18

The selection logic is similar to the 19H / Series 17 relationship: a single external need with a follower structure that requires fine matching → Series 18; several classes of measuring point on the machine — in-cylinder, external, high-temperature split — and unified spares and procedures wanted → 19P. Structural differences between Series 18 follower-rod and profile-mounted types are in Series 18 external explained.

Shared risks of the external type: misalignment and bracket resonance

Whether 19P or Series 18 is chosen, the two major risks of external mounting are identical, and both sit on the mechanical side rather than in the sensor itself.

Misalignment. When the motion axis of the measured mechanism is not parallel to the sensing-rod axis, the magnet ring becomes eccentric, appearing as a progressive deviation over certain sections of stroke; in severe cases the follower takes a side load, and long running leads to wear or even seizure. The remedy is a floating link at the connection (ball joint, universal joint, flexible connector) that transmits only axial displacement. Practical methods are in External mounting: how to fix the bracket and sliding carriage so they stay stable.

Bracket resonance. When bracket stiffness is insufficient or the natural frequency falls near a machine excitation frequency, readings show regular jitter in operation and recover when stopped. The diagnostic is “jitter frequency tracks machine speed”. The principle is “short, straight and stiff” — keep the bracket as short as possible, fix it directly to the rigid main structure, and avoid thin-plate cantilevers. Mounting points for vibrating environments are in Installing in vibration: locking and vibration resistance.

These two points are the main source of external-type field problems; changing the sensor will not solve them. General installation and commissioning points are in Installation and commissioning in practice.

Typical uses of 19P

Adding displacement sensing to existing plant. When the existing cylinder is a standard bought-in part and cannot be bored, external is the only feasible path. The full decision for retrofit projects is in Selecting for retrofit of existing plant.

Non-hydraulic linear mechanisms. Linear motion driven by a leadscrew, rail, pneumatic cylinder or rack can equally be measured by an external type. The focus then shifts from pressure rating to whether resolution and repeatability match the controller; the criteria are in The three accuracy figures.

Lines that require fast replacement. External replacement does not need depressurising and draining; downtime is far shorter than for a cylinder-integrated type, and is more friendly to continuous lines with a high availability requirement.

Machines mixing several forms. On the same machine the main cylinder uses 19H cylinder-integrated, auxiliary mechanisms use 19P external, and the high-temperature zone uses 19D split. Platform unification simplifies spares and maintenance procedures; the thinking is in 19H cylinder-integrated explained.

The boundary with Series 13

Series 13 is a mobile-hydraulic in-cylinder / head-integrated type, environmentally reinforced for travelling machines: protection up to IP69K, 25 g vibration resistance, 100 g shock resistance. The boundary is clear:

• Equipment fixed, ordinary environment (indoors, no high-pressure wash-down, no continuous strong vibration) → 19P or Series 18 is sufficient;
• Equipment that itself moves, needs high-pressure water wash-down, has shock loads (construction machinery, agricultural machines, marine) → the Series 13 must be used; see Series 13 mobile hydraulics explained.

Do not try to make an ordinary external type do mobile duty by adding a guard — a guard brings heat-dissipation and trapped-water problems, and vibration and shock resistance are structural capabilities that a cover cannot supply. What protection classes actually mean is in What IP65/67/68/69K actually mean in testing.

Where to turn when the stroke is very long

An external type meets physical limits on a very long stroke: the longer a rigid sensing rod, the greater the sag under self-weight, and transport and installation become more difficult. When the stroke is beyond a reasonable range for a rigid rod, turn to the flexible long-stroke option 19F; see 19F flexible long-stroke explained. Overall thinking for long-stroke selection is in Selecting for long stroke.

See the 19P external product page; horizontal selection across all external options is in Comparing the whole external range: how to choose among 13/18/19P/19F.