Patent ID: 7525306

Claim:
Apparatus comprising: a first element defining a contaminated environment containing a fluid under pressure; a second element movable within the contaminated environment; and a magnetic encoder for measuring movement of the second element within the contaminated environment, the magnetic encode comprising: a wheel rotatable about an axis in the contaminated environment, the wheel having an outer periphery arranged to roll on the second element to rotate the wheel as the second element moves in the contaminated environment; at least one permanent magnet mounted on the wheel such that the magnet moves past a sensing location in the contaminated environment each time the wheel rotates to provide a count representative of the rotation of the wheel; a sensor responsive to presence of a field generated by the presence of the magnet; a dividing wall separating the wheel in the contaminated environment from the sensor in a separate environment, the dividing wall being formed of a material which is not ferro-magnetic; the dividing wall being arranged to maintain the sensor in the separate environment separated from the fluid in the contaminated environment and separated from the pressure in the fluid in the contaminated environment; and a ferro-magnetic pin extending through the dividing wall from a first end of the pin at the sensing location in the contaminated environment at the magnet on the to a second end at the sensor in the separate environment, the ferro-magnetic pin being arranged such that the presence of the magnet at the sensing location causes actuation of the sensor.