11 January 2014 – Using Variable Resistance To Control Irrigation Flow
My friend Ryan Fabris asked my to look at the design of his irrigation system that supplies bore water to his organic saffron crop he is trialling in the A.C.T. The irrigation system is quite simple in concept and uses:
- Direct current electrical pump with around 30 litres per minute capacity to pump water through a series of flexible pvc pipes
- The pipes are sunk into trenches dug into the soil at a depth of around 15 centimeters and these pipes run approximately 50 centimeters from the saffron plants
- The electrical pump needs to be carefully controlled to ensure that the flow of water through the system meets the requirements for watering the crop but at the same time ensuring that water is preserved wherever possible. The A.C.T. has fairly strict water usage legislation and it is imperative that the available ground water is used appropriately.
Controlling The Water Pump By Using A Wire Wound Potentiometer
My early foray into electronic control circuitry came into use in this project. I established that the power rating of the pump was sufficient to achieve maximum water flow to the irrigation system but by carefully controlling the current fed to the pump i could also control the amount of water flow – thereby maximising usage of the available water but ensuring adequate water was fed to the crop. I ordered a wire wound potentiometer from Sydney based Resistor Specialists. The wire wound potentiometer has a 3 watt rating and can be mounted into the irrigation control panel easily via a simple threaded bush assembly on the potentiometer. A resistance value of 330 ohms (ohms is a measure of electrical ‘resistance’, which is the measure of how much current is or is not passed in the electrical circuit). By being able to control the resistance value via the potentiometer, feeding to the water pump from the 30 watt Meanwell power supply, i was able to create an elegant way of controlling the water flow into the irrigation system. The wattage of the potentiometer seems to be important. Many potentiometers are made from a carbon wiper assembly, which controls the available electrical resistance. Most carbon potentiometers have only a small power rating – typically 1/4 watt up to 1 watt (wattage is a measure of electrical power and is calculated by the mulitple of the square of the current flowing times the the resistance, or in mathematical terms Power = I squared times R, where I= electrical current and R = resistance).
Correct Water Flow Achieved
It is satisfying to be able to assist in a simple engineering problem and find an elegant, cost effective (the potentiometer, wiring and mounting system cost less than $100 to install) solution. The water flowing to the crop can now be easily controlled ensuring maximum crop yield and minimum water wastage. Q.E.D.