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Filter Media Expansion Measurement System - SID-10500

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Achieving proper media filter bed expansion during backwash in gravity filters is a crucial step in maintaining filter performance and longevity.

The US EPA recommends 20% to 25% filter bed expansion to adequately clean the filter media (anthracite, sand). Failure to expand the bed sufficiently will result in "mud balls" that degrade the filter's ability to trap dangerous microorganisms such as Cryptosporidium oocysts and Gyardia cysts.

Until now, the only options for measuring filter bed expansion were home-made type devices or expensive fixed mount microprocessor based echo-time measuring equipment. With the portable Media Expansion Measurement System, Raven offers an affordable solution for small plants on a tight budget as well as large plants establishing a filter surveillance program.

This simple two part system includes Raven's proven infrared Interface Detector coupled with the Expansion Measurement Arm. It's lightweight, rugged and easily transported throughout the plant.

Round railing clamp and square railing clamp are interchangeable. Choose either or both.

Carrying Case*:

Measurement Arm*:

Railing Clamp*:


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This instrument brings precise and repeatable measurements to the process. The measurement arm is mounted to the railing with a non-marring quick release clamp. A pulley at the tip of the arm allows easy deployment and retrieval of the sensor probe. The sensor drop point can extend out to 3 feet over the filter from the safety rail and can easily be moved to other locations within the same filter to check for dead spots or uneven expansion. A pivot point connection on the measurement arm allows the operator to range the probe over 9 linear feet of expanding media in a semi-circle to verify uniformity of expansion at multiple points.

FILTER MEDIA BED EXPANSION MEASUREMENT - see Measurement Sequence Illustration
Prior to backwash, the flat bottom sensor probe is lowered to rest on top of the filter media bed. With the cable taught, it is cinched to the sliding marker block on the measurement arm. The sliding ruler is positioned so that the marker block is on zero. This is the reference point for measuring filter bed expansion as shown below:

Sensor cable is cinched to marker block with sliding
ruler set to zero.

At the start of the backwash, the sensor will react to the turbidity
and debris being evacuated from the bed with illumination of all 20 red LED's as shown below:

LED's fully illuminated as sensor probe is fully
engaged in backwash debris and turbidity.

The measurement arm reaches out 36" (91cm)
from the sidewall to avoid any interference the wall
may have on the movement of the filter media.

Upon clarification of of the backwash water into the waste troughs, the operator slides the marker block, with cable cinched, away from the filter to raise the probe with the expanding media bed. While raising the sensor probe the red LED's will begin to go out. This is the top of the expanding media bed. The operator can lock the sliding marker block on the measurement arm at this point to allow the expanding media bed to catch up to the sensor probe. This procedure is repeated until the media has achieved full expansion. The operator will be able to lock in on the top of the expanded media with 1/8 inch. At this point, the operator notes the point on the scale where the marker block pointer is positioned over the scale and records the precise amount of expansion. In the image below, the media bed achieved 10 inches (25cm) of expansion as shown by the horizontal pointer on the marker block.

The marker block has been pulled back 10" (25.4cm)
representing the same amount of media expansion.

When the filter bed has settled after the backwash is terminated, it is expected that it will be at a lower level than before the backwash. The probe is lowered back down to the top of the bed by moving the marker block toward the filter chamber. The operator can easily verify that the bed has settled to a lower point as the marker block pointer will be in negative territory on the scale.

Analyzing the dynamics of the expansion from within the media bed, the infrared sensor probe reports to the operator via the interface detector mounted on the Expansion Measurement Arm. There is no need to calibrate the sensor. It automatically references changes in density. A sensitivity knob fine tunes the probe. Measure the "ragged edge" on top of the expanding filter bed by adjusting sensitivity to "Max" to determine if the top of the bed has a gradual or well defined interface. How do you interpret a gradual uneven interface over several vertical inches between the expanded bed and the wash water above?

Infrared sensors are uniquely suited to give more than just up or down information about the dynamics of the media bed during backwash. It's the only technology that lets the operator interact with the even in so many ways.