Sludge bed level probe has 'X-ray' vision
January 2004
Sensors & Transducers
A vertical probe, submerged in the tank, contains 64 high-intensity infrared light emitting diodes stacked vertically, which shine horizontally across a probe gap to 64 phototransistor detectors. This gap contains either liquid or sludge. See the cross section of the 'probe active element' illustrated here, which shows one of the LED/phototransistor pairs. The 64 pairs are sequentially scanned by a microcontroller to locate the sludge bed. Beam power intensity is automatically adjusted to accommodate different sludge and overlying liquid concentrations. At its highest power level the beam will pass right through a hand (you could count the bones) but there is no hazard since the beam is only on for a few microseconds. This high intensity beam easily penetrates fouling on the probe's windows, and as long as the solids in the probe gap are high relative to the solids in the scum layers on the windows, the meter continues to read correctly. Eventually, the probe has to be removed and washed.
A desludge pump can be operated automatically from the low set-point relay along with a pump run interval timer, and the high set-point used as an alarm for excessive sludge depth. Alternatively, the desludge pump can be turned on at the high set-point and off at the low set-point, or the 4-20 mA output can be used for continuous variable speed drawoff. Either the 4-20 mA or the RS232 and optional RS485 can link the meter to the user's computer for centralised process control.
For more information contact Collin Smith, Gemini Instruments, 016 422 5460, [email protected]
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