It comprises a 9 to 22.5 1 capacity metallic tank fitted with a manually operated pump; a sieve and a lid on its mouth; and a small delivery pipe ending in either a lance a or a two nozzle boom. The spray tank is loaded on the back of the worker who operates the pump with one hand and directs the lance or the boom with the other as he walks forward and backward. In a day of 8 hrs one person can cover with this sprayer about 0.4 ha area.
The hydraulic knapsack sprayers can develop a spray pressure of up to 12 kg cm-2, but it is better to limit it to 3-4 kg cm-2 to avoid the unwanted drift problems. This can be done by adjusting the nozzle opening and pumping frequency. The hydraulic knapsack sprayers deliver 250-600 1 spray liquid ha-1, depending upon the walking speed of the worker, spray pressure decided, and the type of nozzle used.
To reduce the load on the back of the worker now light weight, plastic tanks are also available in different sizes. However, these are prone to easy damage by rats in stores, and to cracking in hard sun.
The knapsack sprayers are especially suited to spot and directed treatments.
It is a motorised version of the hydraulic knapsack sprayer. It is more convenient to operate than a hydraulic sprayer since it is operated with a small, petrol-run engine. Its spray tank is much smaller in size with a capacity of 6-8 1. The whole equipment is loaded on the back of the worker who holds the delivery hose in one hand, leaving his other hand free. However, here the worker has to move fast to match the high discharge rate of the blower.
This is essentially a low volume equipment, with a delivery rate of about 60 1 per ha. Thus, while using a blower, one requires very little dilution of the herbicide formulation. Here, air acts as the main carrier of the spray particles which can give it a discharge rate as high as 2.5 1pm. But then this heavy discharge is drift-prone too. Therefore, it is best to operate a blower only on calm days, with low, adjusted discharge rates of 0.15-0.25 1pm.
A foot sprayer has manually operated, vertical displacement pump which rests on the ground. As its name indicates, the pump is peddled with the right foot of the worker, while holding the sprayer in position by hand. The intake pipe of the pump is put in the bucket containing the spray material, close to the pump. Another person manages the spray lance connected to the pump through a long flexible pipe.
This provides movement to the spray-man to cover considerable area at a time, where after the pump is moved to another convenient spot.
The foot sprayer develops a spray pressure of 17-21 kg cm-2 and covers about 1 ha area in one day with the help of two persons. There is a provision in the sprayer to use two delivery pipes simultaneously when it will require three workers to operate it. Then it will cover almost 2 ha area in one day. The foot sprayer is a medium volume sprayer with delivery of to 250-600 1 ha per ha.
Equipment # 4. Tractor-Mounted Sprayer:
In this kind of sprayer a large liquid tank (365-400 1) and a long multi-nozzle boom are loaded on the rear of a tractor. The spray pump is operated with power from the tractor engine. The tractor mounted sprayers work under a spray pressure of 22-25 Kg cm-2 and they cover an area of 4 to 5 ha per day, depending upon the boom length.
It is thus a useful spray system for a large holding farmer. The success of a tractor-mounted sprayer lies in proper calibration of the sprayer and its boom adjustment. The height of the spray boom from the ground should be so adjusted as to avoid overlapping, as well as gapy, sprays.
Calibration of Equipment:
Calibration of any sprayer is best done by conducting a test run with plain water. To do so, the tank may be filled with water up to the marked position and then sprayer be operated in an earmarked area. The water consumed can be determined by refilling the tank with known volume of water. Knowing the area covered and the volume of water sprayed, one can easily calculate the application rate in 1 per ha.
If it is not as desired, it can be altered by changing the ground speed, spray pressure, nozzle capacity, and/or number of nozzles on the boom, till satisfaction. In general, as long as the desired quantity of herbicide is distributed evenly on the target area of the field, the volume of spray liquid used in the process may not disturb the results appreciably.
The spray volume is mainly governed by the kind of sprayer used and its calibration. With the kind of sprayers usually used in India for the application of herbicides, spray volume of 250-500 1 pha is considered adequate.
Larger spray volumes mean unnecessary additional effort in transport of water from the source to the action site. In some western countries, now, battery operated, very low volume, hand-held herbicide sprayers, requiring only 20-40 1 spray liquid per ha, are also available. But their use is limited to small gardens and the like areas.
It may be important here to note that while in the case of shoot translocated herbicides it may be enough to wet the top of the weedy vegetation using a low or very low volume sprayer, a shoot contact herbicide must wet the weedy plants thoroughly to obtain their complete kill for which larger spray volumes may be needed.