Following is a plan for a water-level sensor for the pondless waterfall tank. It is just a plan. It has not been built.
The system consists of two units: a submerged sensor and a control box.
The submerged sensor has three IC's on a PC board, sealed in a PVC pipe case. Four stainless-steel screws serve as the level-sensor probes. A four-wire cable connects the submerged sensor to the control box.
In the contol box is a power supply, relays, and a water solenoid. The control box has a pendant cord to connect to the power timer, a socket to plug the pump cord into, and two hose connections with which to atach the tank fill lines. The box is water-tight and the fill section is isolated from the electrical section.
In most pondless waterfalls there is a considerable amount of water in the stream bed. This complicates the auto-fill system because the tank cannot be filled completely while there is water in the stream. If it was filled completely, the tank would overflow when the pump turned off.
By using four water-level sense points, this complication can be overcome. The four sense points are:
FULL - The level in the tank when the pump is off and the stream water has flowed back into the tank ( the water is as high as possible).
MEDIUM - The level in the tank when the amount of water in the flowing stream is subtracted from the FULL tank.
MINIMUM - The level where re-filling the tank is desired to ensure it will reach the MEDIUM level before the EMPTY level is reached.
EMPTY - The level where the pump will shut off. This level is still above the pump inlet and protects the pump.
If there is 75 gallons in thestream when it is operating, and the tank is a 100 gallon tank, then the FULL level is 100 gallons, and the MEDIUM level is 25 gallons.
If 5 gallons of water in the tank is enough to cover the pump inlet, and not have it suck some gulps of air, the 5 gallons is the EMPTY level.
The MINIMUM level is between EMPTY and MEDIUM. If it is too close to MEDIUM, the fill solenoid may oscillate on and off. If it is too low, splaching may cause the EMPTY point to trigger.
The logic of the level sensor is to turn the fill solenoid on at the MINIMUM level and off at the MEDIUM level. If the level gets down to the EMPTY level, the fill solenoid turns on and stays on until the level reaches FULL. The MINIMUM / MEDIUM circuit is ignored in this case. During the filling to the FULL level, the water in the stream has time to drain back into the tank and thus the overflow of the tank due to the tank being full when the stream draining is prevented.
My four sense points are:
FULL - 20 inches.
When the stream water flows back into tank the level will be 20 inches.
This is 3/4-inch above the concrete blocks and about 4-inches below the tank lip.
The extra depth is to allow for rain and allow a safety margin so I don't have to re-make the sensor.
MEDIUM - 11 inches.
The water in the tank will rise 8-3/4 inches if the pump is turned off.
MINIMUM - 8 inches.
This gives a margin of 3 inches above the EMPTY level.
EMPTY - 5 inches.
The top edge of the pump inlet is 3-1/4 inches.
