The optional surcharge velocity sensor (SVS) allows Flo-Dar to continue measuring flow during surcharge conditions. The sensor uses an electromagnetic type sensor to measure velocity. The sensing electrodes of this sensor are located about 8" below the reference point for sensor offset (top of the tubular mounting frame). The elevation of the mounting frame is extremely important for proper operation of the SVS sensor. The sensing electrodes should be located at a particular elevation below the crown of the pipe to measure surcharge velocity correctly. This would be 2 inches below the crown of the pipe for pipe IDs up to 24" and 3" below the crown for larger pipe sizes. Because the electrodes are about 8" below the top of the mounting frame this means the sensor offset should be 6" greater than the pipe ID for small pipes (8" - 6" = 2") and 5" greater than the pipe ID for large pipes (8" - 5" = 3").
It is also important for the Flo-Dar sensor be mounted as close to the where the pipe exits into the manhole as possible. This is to catch the flow jetting out of the pipe before it starts to disburse in the manhole. Even though the SVS sensor is situated in the manhole the core flow exiting the pipe stays together for a while in the manhole even if the manhole is flooded. Think of a garden hose in a swimming pool. You can still feel the jet of the water quite a distance from the end of the hose but the closer the better.
There are two ways the system determines when to switch into the surcharge condition. The primary switching mechanism uses distance and conductivity. When distance is less than 8" the SVS sensor is turned on. If the sensor frame is mounted correctly there will 2" of air gap in small pipes and 3" in large pipes. The sensing electrodes at this point will have just become wetted. When distance becomes 7" or less the system will switch into the surcharge mode. The electrodes will be submerged at least 1" at this time making surcharge velocity measurements valid.
Because Flo-Dar typically cycles on once every 15 or 5 minutes it is very possible a surcharge can occur when the sensor is off. The sensor could wake up for the next sample when it is already under water. Because distance measurements are not reliable when the ultrasonic depth sensor is submerged there needs to be a secondary switchover method. Surcharge level and conductivity are used for the secondary switchover method. When surcharge level is greater than the surcharge level trigger (1.0") the SVS sensor is turned on. If conductivity is greater than the conductivity trigger (50) the system will switch into the surcharge mode regardless of the reported distance.
When the system switches into the surcharge mode level or depth is set to the pipe ID and surcharge velocity is substituted for velocity. Flow is calculated using the full area of the pipe and surcharge velocity. There is a surcharge calibration adjustment (default = 1.0) in the SVS setup screen. This can be used to match or calibrate surcharge velocity to radar velocity during transitions. It is rarely used because surcharge velocity usually closely matches radar velocity when going into and out of the surcharge condition. When going into or out of the surcharge condition the median filters are reset to avoid carrying over any erroneous data from one state to the other.
Flo-Dar with SVS Theory of Operation