The pneumatically operated recorder is still used today, particularly in remote locations. It does not use electricity so it does not need any special intrinsic safety measures when it is used in hazardous areas. Examples of the pneumatic recorders are shown below manufactured by Foxboro. They are the series 120 Consotrol recorder and the series 40 circular chart recorder; both made by Foxboro.
The Foxboro Series 120 Consotrol Recorder
Foxboro produces a range of pneumatic instruments called “Consotrol”. The Foxboro series 120 Consotrol recorders is one of the most common pneumatic recorders. It is still manufactured and you can get spares for it. You may see other types of pneumatic recorders on older sites but they will work in much the same way as the Foxboro.
Operation of the pneumatic recorder:
- The incoming pneumatic signal is applied to a bellows unit which works by compression against the range spring.
- As the input signal increases the pen is rotated across the scale. The scale indicates from 0 to 100% for an input signal of 0.2 to 1 bar (3 to 15 psi).
- The rotating mechanism has span and zero adjustment screws (as shown)
- The driving rod from the bellows to the rotating mechanism has a linearity adjustment. This can adjust the linearity of the pen travel from 0-100%.
- The recorder can drive a maximum of 4 pens.
- The ink of the pens is supplied from bottles which are refillable. The colours available are red, green, blue and violet.
The chart is driven round by a motor (either electric or pneumatic) at a standard rate of 19 mm per hour. There is enough paper on a roll or flip chart to record around 30 days of continuous operation.
Circular Chart Recorder
Foxboro also produces a circular chart recorder for field use. The pen drive system is the same as in the 120 Consotrol recorder. The chart is normally driven by clockwork. The instrument technician must wind it up from time to time. The circular chart completes one revolution every 24 hrs. The chart can be linear or square root. The square root chart can be calibrated to give a direct reading of flow rate.
The measurement unit can be selected from various measuring elements, e.g. gauge pressure, absolute pressure, temperature, etc. The recorder can have a maximum of 4 elements to drive 4 pens. The newer recorders use disposable fibre tip pens but the older ones use a capillary tube with a refillable ink bottle.
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