The functioning of a emitter device depends on its structure and on the method by which it regulates the flow of water. In addition to the design characteristics, the working of the emitters is strongly influenced by the quality and consistency of the materials used in their manufacture. Regulator performance is evaluated by measuring actual flow rates of a representative sample of emitters at nominal pressure and temperature.
Ideally, all emitters in the sample should have exactly the same flow rate and this flow rate should be exactly equal to the nominal flow rate indicated by the manufacturer. In reality, the flow rate of the individual dispensers will vary from one to the other up to a certain degree and the measured average flow rate will vary somewhat from the nominal flow rate due to imperfections that occur during manufacturing processes.
In fact, these processes do not allow perfect homogeneity between the emitters produced due to defects in the calibration of the outlets; the differences are attributable to the quality of the materials, to the construction techniques used and in particular to the variations in pressure and temperature during the production processes and to the cooling method, wear and poor maintenance of the molds [Solomon, 1979].
There are therefore many variables that can produce defects in the manufacture of the emitters, such as the type of resin used, the differences between the plastic compounds that vary in the different formulations, the ambient temperatures, the material of the mold, the age and the wear of the injectors and the mold, the tuning of the mold and so on. These variables determine the quality of the finished product and consequently the real flow rate delivered by the emitters.
The variation between the individual emitters is measured with a value called the coefficient of variation Cv, and with the difference between the actual average flow rate and the nominal flow rate and measured with a value called the deviation from the average flow rate, that is Qd. Both the deviation from the mean flow rate (Qd) and the coefficient of variation (Cv) are data that indicate how accurately the emitters are produced.
An example
In terms of performance, Qd and Cv must be considered separately. Suppose, for example, that a certain sample of dispensers with a nominal flow rate of 4.2 1 / h is subjected to an examination and each of the dispensers has a flow rate of exactly 4.9 1 / h. Since all dispensers have exactly the same flow rate, the Cv for the sample is zero, indicating excellent consistency. The average flow rate, however, is 18.2% higher than the nominal flow rate specified by the manufacturer. On the other hand, the Qaverage for a sample of emitters can be exactly equal to the nominal flow rate of 4.2 L/h (giving a Qd of 0), but if the Cv is 15% between individual emitters there are big variations in the flow rate.
Coefficient of variation
The technological variation coefficient CVT or more simply Cv is a statistical measure of the quality of the emitter and which expresses the variation in the flow rate of the sample outlets as a percentage of the average flow rate. Cv is calculated by dividing the standard deviation of the flow rates of a sample of emitters by the average flow rate, i.e .:
where σ is the standard deviation and μ is the mean flow rate.
In relation to the value assumed by the CV, the drippers can be classified [Solomon, 1979] as:
- excellent if CV< 3%
- acceptable if CV = 4-7%
- mediocre if CV = 8-10%
- poor if CV = 11-14%
- bad if CV> 15%
Deviation from the mean flow rate
The deviation from the avarage flow rate (Qd) is the percentage difference between the nominal flow and the actual average flow, i.e. the average flow rate which has been measured experimentally using a representative sample of emitters. The nominal flow rate Qn is indicated by the manufacturer for a certain temperature and a specific pressure and the average Q is simply the sum of the measured flow rates (Qi) divided by the number of dispensers (n) tested.
Therefore, Qd is obtained from:
The average flow rate for a sample of emitters should be the same as the nominal flow rate Qr specified by the manufacturer at the nominal temperature and pressure. If the average flow rate is considerably higher or lower than the nominal flow rate for which an irrigation system is designed, there will be significant effects on the flow rates and performance of the system.
The average flow rate for a sample of emitters should be the same as the nominal flow rate Qr specified by the manufacturer at the nominal temperature and pressure. If the average flow rate is considerably higher or lower than the nominal flow rate for which an irrigation system is designed, there will be significant effects on the flow rates and performance of the system.
Under operating conditions, knowledge of the q (h) relationship and of the technological variation coefficient may be insufficient to characterize the hydraulic operation of the dripper due to the onset of different phenomena that influence, with the same type of dripper and load on the itself, the entity of the delivered flows. In particular, the phenomena that most frequently occur during the operation of micro-irrigation systems and which can also significantly change the extent of the flows supplied are:
- suspended transport of solid particles larger than that of the minimum section of the dispenser
- transport of organic material, especially present where water from surface supply sources is used
- formation of encrustations caused by the use of excessively mineralized water, rich in iron, calcium bicarbonate and manganese oxide, which reduce the water passage section in the dripper and consequently modify the characteristic delivery curve [Nakayama, 1981]; this problem can be accelerated by variations in water temperature or by variations in pH resulting from fertilizing treatments [Padmakumari, Sivanappan, 1985]
- presence of microorganisms that are formed during periods of plant stasis due to the use of fertilizing substances and which can facilitate the agglomeration and subsequent deposit of suspended solids [Gilbert et Al., 1980].
