Calibration
of Hand-held and Backpack Pump-up Sprayers
Extension Agronomists
Various types of hand-held and
backpack sprayers are commonly used for spot treating or spraying small
turfgrass areas infested with weeds.
These types of sprayers are extremely useful for this purpose but like
larger types of spray equipment they must be calibrated in order to properly
apply the recommended rate of a herbicide. Each year numerous instances of excessive
turfgrass injury occur due to improperly calibrated pump-up sprayers.
The overwhelming majority of
herbicides used to control weeds in turfgrasses are selective in that they
control susceptible weeds without adversely injuring the turfgrass. However, selectivity is dependent upon the
amount of herbicide applied per unit land area.
The amount of herbicide applied to a unit of land area is more commonly
referred to as the recommended rate of the product and is usually expressed in
amount of the product to be applied to one acre or 1,000 square feet. For example, the highest recommended rate of
Trimec Classic for bermudagrass and tall fescue is 4.0 pts./acre
or 1.5 fl. ozs./1,000 square feet. When
applied at this rate and according to label directions this herbicide will
cause only minimal injury to labeled turfgrasses. But, what would happen if we applied 3 times
this amount of Trimec Classic. First,
certain turfgrasses would be injured (noticeable yellowing and browning of the
foliage) for several weeds after application.
Secondly, the client/customer would extremely disappointed
due to the high level of turfgrass injury.
And, finally, the applicator could be found liable for not following
label directions and subject to fines and/or other penalties.
Unless a sprayer is calibrated it is
not possible to apply herbicides at recommended rates. It doesn’t matter if it is a large commercial
sprayer with a spray boom or a type of hand-held, pump-up sprayer. There are various methods that can be used to
calibrate sprayers. One method that is
simple, easy to do and easy to remember is referred to as the 1/128th acre
method. In this method
an area is sprayed that equals 1/128th of an acre. There are 43,560 square feet in one
acre. If one multiples 43,560 square
feet by 1/128 the answer is 340.3 square feet.
So an area of 340.3 square feet is equal to 1/128th of one acre. Different sizes can be used, but a commonly
used dimension to calibrate pump-up sprayers is 18.5 by 18.5 feet. The following steps are used to calibrate
pump-up sprayers.
1.
Mark off a calibration plot that measures
18.5 ft. by 18.5 ft.
2.
Fill the sprayer to normal capacity with
water.
3.
Pump the sprayer to the pressure normally
used for herbicide applications. (Hopefully, your sprayer has a pressure
gauge!)
4.
Spray water over the plot area while
maintaining normal (and constant) operating pressure.
5.
Record the time in seconds it takes to
spray the plot area: (example: time = 30 sec).
6.
At the same constant pressure,
by pressure gauge or constant pressure on the handle, spray into a
bucket for the same time (number of seconds) it took to spray the plot area.
7.
Measure the volume of water in fluid
ounces: (example: volume collected = 25 fl. oz.).
The number of fluid ounces collected is equal to number of gallons that would be applied to one acre, if the sprayer is operated at the same pressure and walking speed used in the plot area. In this case the sprayer is applying 25 gallons of water per acre. This quick conversion from fluid ounces per 1/128th acre to gallons per acre (gpa) works since there are 128 fluid ounces in one gallon of water. It doesn’t get much easier than this. Now all there is to do is to determine how much herbicide to add to the pump-up sprayer.
Example:
Using
the 128th acre method as described above we have determined the pump-up sprayer
is applying 25 fl. ozs. to a 1/128th acre plot area
(18.5 by 18.5 ft.), or 25 gpa. The
recommended rate for Trimec Classic is 4.0 pts./acre,
which is equal to 0.5 gallon of Trimec Classic per acre (remember 8.0 pts. =
1.0 gal.). Now divide 0.5 gallon/acre of
Trimec Classic by 25 gpa.
0.5
gallon =
0.02 or 2.0%
25
gpa
Now
multiply the operating capacity of the sprayer (let’s use a 2.0 gallon sprayer)
by 2.0%.
2.0
gallon sprayer
x 2.0%
0.04
gallon
Since
it is impractical to measure 0.04 gallon, multiply this amount by 128 fl. ozs./gal. = 5.1 fl. ozs.
So
we would add 5.1 fl. ozs. of Trimec Classic to the
sprayer and fill to the 2.0 gallon capacity with water.
By
using the 1/128th acre method and some basic math we have: a) calibrated the
pump-up sprayer, and b) determined the amount to herbicide to add to the spray
tank. But remember, you must walk the
same speed you used in the calibration plot area, you must maintain the
pressure you used in the plot area, and you must hold the spray nozzle tip at
the same height you used in the plot area.
If you do these things, then you will apply the recommended rate of the
herbicide, you will control the target weed, and you will drastically decrease
the chance of adversely injuring the turfgrass.