# Calculating the correct flow rate for open hydroponic systems

There are two basic types of dripper systems used in bag culture; 1 mm spaghetti pipes and 8 mm pressure regulated dripper (also called a PC dripper) which is much more expensive. There are advantages and disadvantages of both these types. The 1 mm spaghetti pipe is very cheap and easy to install. If blockages do occur they are easy to open or if that does work it is very cheap to replace the 50 cm piece with another one. The PC dripper on the other hand is pressure regulated. This is very important. It allows the grower to apply the same amount of water to all his plants even if the area is not level. This is not possible with the 1 mm spaghetti pipe. The other advantage is that when the system switches of, no water will run out of the PC dripper since water is regulated and will only flow if pressure is above 1 bar. With the 1 mm spaghetti pipes, water tends (gravity water) to run out of the pipes long after the pumps have been switched of. This results in some plants receiving more water than other and also results in a lot of wastage. The 1 mm spaghetti dripper cannot be used on systems that are installed on uneven areas due to above mentioned fact. Try not to use the 2 mm spaghetti drippers. Flow rates will decrease significantly over the length of the greenhouse if there is a slight slope.

The optimum flow rate in bag culture is 1 – 2 L.h^{-1} (0.02-0.03 L/min or 17-33 ml/min) The spaghetti drippers have no limits since the flow rate will be determined by the distance of the pipe to the pump and the pressure of the pump.

Determining the length of each cycle of a dripper system of flow rate of irrigation is known.

The abbreviations stand for:

C* _{L}* = Cycle length in minutes

V = Total volume to be applied to the plants in litres

C* _{n}* = Number of cycles required per day

F = Flow rate of the dripper system in litres

The following example illustrates the use of the formula:

We know the following data:

V = 1.5 L total water application to the day

C* _{n}* = 8 : The number of cycles per day

F = 2 L.min^{-1} : The flow rate of the irrigation system

C* _{L}* = Unknown value.

C* _{L}* = 5.625 min

NOTE: A flow rate (*F*) of 1.5 L.min^{-1} is used. You can use L.hr^{-1} if you want to. Just adjust the formula.

Adjust the length of each cycle so that when the water starts to run out of the holes in the bags, the pumps should be switched of. If the water starts running out within 2 minutes, the cycles per day should be increased and cycle length should not be more than 2 minutes. In order to calculate how many cycles are needed if the maximum time for each cycle is known, the flow rate of the irrigation system is known and the total water volume required for the whole day is known, the following equation can be applied:

The abbreviations stand for:

C* _{L}* = Cycle length in minutes

V = Total volume to be applied to the plants in litres

C* _{n}* = Number of cycles required per day

*F* = Flow rate of the dripper system in litres

If we replace the values that we know in the formula, the following result will be obtained:

C* _{n}* = 22.5 cycles (round down to 22 cycles)

The result shows that we must set the pump time to 22-23 cycles per day. Any experienced grower would not agree with above answer, and they are quite correct. The formula does not include the rate of water uptake by the plants over a period of time. So after the pump is switched on for 2 minutes, there might be enough water for at least an hour for the plant to absorb. The next cycle should only be switched on after an hour. If the water lasts for an hour, the maximum number of cycles that are needed is between eight and ten. Since irrigation cycles starts between 07H00 and 08H00 in the morning and ends between 17H00 and 19H00 in the evening, the maximum number of cycles will be 12 for any system.

The number of irrigation applications per day and the length of each cycle in minutes in order to provide at least 1.5L of water to the plant is shown in the table below. The frequency should be divided into the total volume of water required per day to estimate the volume provided in each cycle.

Growth Medium | Frequency per day | Length of cycle |
---|---|---|

Pine shavings | 4 - 6 | 15 - 10 |

Pine chips | 4 - 8 | 15 - 7.5 |

Pine bark medium | 6 - 8 | 10 - 7.5 |

Eucalyptus chips (Multi Medium) | 6 - 8 | 10 - 7.5 |

Choir | 8 - 10 | 7.5 - 6 |

Perlite | 8 - 10 | 7.5 - 6 |

Soil, peat compost mix | 2 - 4 | 10 - 7.5 |

Sand | 6 - 8 | 10 - 7.5 |