Chapter two testing equipment and machinery calibration

Performance evaluation
of agricultural
machinery
Chapter two: Testing equipment
and farm machinery calibration
By Siraj K. Busse (PhD)
Academic year: 2021/2022
Haramaya, 2021

2.1. Introduction
 One category of agricultural machines dispenses materials such as
– seeds,
– fertilizers, and
– chemicals.
 Dispensing machines are required to spread the material at a
specific rate and in a specific pattern.
 Traditional dispensing machines dispensed the material at a fixed
rate.
 These machines are still being used, but dispensing materials at a
fixed rate does not meet the demands of modern agriculture.

 Modern machines are designed to dispense materials at a variable
rate.
 This technology is one aspect of the new agricultural production
method called precision agriculture.
 Dispensing machines used for precision agriculture production must
not only be able to vary the application rate, but some machines
must be able to vary the pattern also.
2.1. Introduction

 A machine that fails to dispense the material at the desired rate
and in the desired pattern will waste resources and/or increase
costs.
 If a seeder does not plant the correct amount of wheat seed, crop
yield will be affected.
 If a sprayer applies more than the desired amount of material,
desirable plants may be damaged.
 The correct pattern is also important.
 The result of a sprayer with incorrect pattern will be streaks or
skips.
2.1. Introduction

 Evaluating the dispensing rate and pattern of dispensing for a
machine is called calibration.
 Historically calibration charts or tables came with dispensing
machines.
 These publications would explain the adjustments or settings
required to produce the desired dispensing rate and pattern.
 These publications are still available for many machines, but
computers are taking over this job for many modern dispensing
machines.
2.1. Introduction

 The purpose of calibrating a dispensing machine is to ensure the
machine is applying the desired amount of material and the material
has the desired pattern of distribution.
 All dispensing machines should be calibrated periodically.
 The following sections will discuss some of the principles of
calibration and provide example problems for calibrating several
common agricultural machines
2.1. Introduction

 Incorrect application rates and patterns have several causes. Five of
these are:
1. The machine may have excessive wear.
2. The machine may have been damaged or modified incorrectly.
3. The machine may have been set incorrectly.
4. Variations in the weight, size, moisture content, and cleanliness
of the material.
5. Variations in the physical condition (lumpiness, flowability) of
the material being applied.
2.1. Introduction

2.2. Principles of Calibration
 All methods used to calibrate agricultural machines are based on
five principles:
(1) Dispensing machines meter the flow (control the rate) of
material at a predetermined rate selected by the operator.
(2) The calibration procedure is completed by collecting material
dispensed by the machine in units of volume, weight, mass, and
number of granules or seeds per unit of area.
(3) Machines can be calibrated using either stationary or mobile
methods.

(4) The calibration procedure must determine the representative
area.
(5) Calibration may include an evaluation of the distribution pattern.
 These five principles will be discussed in more detail in the following
sections.
Material rate
 Two different rate types are considered when calibrating
machinery: flow rate and application rate.
 Flow rate refers to the volume or weight of material being
dispensed over a unit of time.

 For sprayers, flow rate refers to the gallons per minute exiting the
nozzles.
 In calibration of granular dispensing machines, flow rate usually
means a volume or weight per unit area, for example, pounds per
acre. This is known as application rate.
 Some machines, like row crop planters, can maintain an accurate
application rate across a narrow range of speed because the flow
rate of the material increases and decreases as the speed of travel
increases and decreases.
2.2. Principles of Calibration

 For others, like sprayers, the application rate is a function of the
set flow rate and speed of travel.
 During a calibration procedure for most machines, measuring devices
must be capable of measuring fractional units, and care must be
taken to ensure the material is collected and handled properly.
 The amount of time a machine is active during calibration is usually
short.
 This can result in a small quantity of material; therefore care must
be taken to ensure no material is lost and none is accidently added.

Calibration Units
 Calibration methods measure material in units of volume, or weight,
and compare the amount collected to an area.
 The unit of measure depends on the material being dispensed.
 When planting corn the desired rate is a specific number of seeds
per acre or hectare.
 The application rate of sprayers is measured in gallons per acre or
liters per hectare.

 Seeding rate of some grasses is in bushels per acre or liters per
hectare.
 Lime is applied in tons per acre or metric tons per hectare.
 Even though the dispensing unit of area is usually acres or hectares,
the unit of area used during calibration will be a representative
sample because of the problems associated with collecting all of the
material that would be dispensed for an acre or hectare.

Stationary vs. Mobile
 Deciding between a stationary and mobile calibration is influenced by
several factors.
 The two primary ones are the type of material being dispensed and
how the flow of material is regulated.
 Most machines that dispense granular materials use a ground-driven
metering unit.
 To calibrate these machines using the stationary method, the wheel
that drives the metering unit is elevated off the ground, and it is
turned for a calculated number of revolutions.

 This method would be acceptable for a granular fertilizer applicator
but would not be acceptable for a manure spreader.
 For a mobile calibration of a granular dispensing machine, a
container(s) is placed to capture the material flow, and the machine
is driven for a calculated distance.
 This method is effective if just a few containers are required but
becomes problematic if a large number of containers are needed.

 An alternative calibration method for granular dispensing machines
is to weigh the machine, dispense the material for the calculated
distance, and then reweigh the machine.
 This method is acceptable if the machine dispenses the correct or
less than the desired amount, but if an excessive amount of material
is applied, it cannot be removed.

 For machines, such as sprayers, that have an engine-driven metering
unit, a stationary calibration is completed by placing a container
under each nozzle, and the sprayer is operated for a calculated
amount of time.
 For a mobile calibration of a sprayer, containers are placed in the
path of application, and the material is collected as the machine
travels over the containers.
 The container area is used to determine the dispersal rate.

 The advantage of the stationary method is that the machine can be
calibrated in down time such as when the weather does not permit
being outdoors.
 The mobile method may work best when the machine has a large
number of collection points.
 It is important to remember that the larger the area used, the
better the results, but increasing the area collected during
calibration increases the resources required to collect the material.

 Representative Area
 Dispensing machine calibration is accomplished by collecting an
amount of material (volume or mass) and dividing it by the area
covered.
 Two methods can be used to determine the representative area:
(1) calculate the distance to travel or the number of turns of the drive
wheel before starting to ensure the desired area is covered.
(2) Turn the metering unit drive wheel a selected number of turns or
travel a selected distance, and convert the results into the desired
area using the width of the machine.

 Distribution Pattern
 For many dispensing machines, the function of the machine is to
not only dispense the correct amount of material but to also
dispense the material in a desired pattern.
 For example, row crop planters must place the seeds an equal
distance apart.
 The desire for field sprayers is to spray the liquid in a uniform
pattern.
 Methods for evaluating the distribution patterns of machines will
be explained where appropriate in the following sections.

 Calibration consists of comparing the output of the instrument or
sensor under test against the output of an instrument of known
accuracy when the same input (the measured quantity) is applied
to both instruments.
 This procedure is carried out for a range of inputs covering the
whole measurement range of the instrument or sensor.
2.3. Calibration of measuring devices

Calibration of measuring devices…
 Calibration ensures that the measuring accuracy of all
instruments and sensors used in a measurement system is known
over the whole measurement range, provided that the calibrated
instruments and sensors are used in environmental conditions that
are the same as those under which they were calibrated.
 For use of instruments and sensors under different
environmental conditions, appropriate correction has to be made
for the ensuing modifying inputs.

 Instruments used as a standard in calibration procedures are usually
chosen to be of greater inherent accuracy than the process
instruments that they are used to calibrate.
 Because such instruments are only used for calibration purposes,
greater accuracy can often be achieved by specifying a type of
instrument that would be unsuitable for normal process
measurements.
 For instance, ruggedness is not a requirement, and freedom from
this constraint opens up a much wider range of possible instruments.
Calibration of measuring devices …

 In practice, high accuracy, null-type instruments are very
commonly used for calibration duties because the need for a
human operator is not a problem in these circumstances.
 Instrument calibration has to be repeated at prescribed intervals
because the characteristics of any instrument change over a
period.
 Changes in instrument characteristics are brought about by such
factors as mechanical wear, and the effects of dirt, dust, fumes,
chemicals, and temperature changes in the operating environment.

Calibration of measuring devices
 The calibration facilities provided within the instrumentation
department of a company provide the first link in the calibration
chain.
 Instruments used for calibration at this level are known as working
standards.
 As such working standard instruments are kept by the
instrumentation department of a company solely for calibration
duties, and for no other purpose, then it can be assumed that they
will maintain their accuracy over a reasonable period of time because
use-related deterioration in accuracy is largely eliminated.

 However, over the longer term, the characteristics of even such
standard instruments will drift, mainly due to aging effects in
components within them.
 Therefore, over this longer term, a program must be instituted for
calibrating working standard instruments at appropriate intervals of
time against instruments of yet higher accuracy.
 The instrument used for calibrating working standard instruments is
known as a secondary reference standard.

 When the working standard instrument has been calibrated by an
authorized standards laboratory, a calibration certificate will be
issued.
 This will contain at least the following information:
– the identification of the equipment calibrated
– the calibration results obtained
– the measurement uncertainty
– any use limitations on the equipment calibrated
– the date of calibration
– the authority under which the certificate is issued

 The establishment of a company standards laboratory to provide a
calibration facility of the required quality is economically viable only
in the case of very large companies where large numbers of
instruments need to be calibrated across several factories.
 In the case of small- to medium-sized companies, the cost of buying
and maintaining such equipment is not justified.
 Instead, they would normally use the calibration service provided by
various companies that specialize in offering a standards laboratory.

 National standards organizations usually monitor both instrument
calibration and mechanical testing laboratories.
 Although each different country has its own structure for the
maintenance of standards, each of these different frameworks
tends to be equivalent in its effect in ensuring that the
requirements of ISO/IEC 17025 are met.
 This provides confidence that the goods and services that cross
national boundaries from one country to another have been
measured by properly calibrated instruments.

 The national standards organizations lay down strict conditions that
a standards laboratory has to meet before it is approved.
 These conditions control laboratory management, environment,
equipment, and documentation.
 The person appointed as head of the laboratory must be suitably
qualified, and independence of operation of the laboratory must be
guaranteed.
 The management structure must be such that any pressure to rush
or skip calibration procedures for production reasons can be
resisted.

 As far as the laboratory environment is concerned, proper
temperature and humidity control must be provided, and high
standards of cleanliness and housekeeping must be maintained.
 All equipment used for calibration purposes must be maintained to
reference standards, and supported by calibration certificates that
establish this traceability.
 Finally, full documentation must be maintained.

 Primary reference standards, describe the highest level of
accuracy that is achievable in the measurement of any particular
physical quantity.
 All items of equipment used in standards laboratories as secondary
reference standards have to be calibrated themselves against
primary reference standards at appropriate intervals of time.
 This procedure is acknowledged by the issue of a calibration
certificate in the standard way.
 National standards organizations maintain suitable facilities for
this calibration.

 Calibration has a chain-like structure in which every instrument in
the chain is calibrated against a more accurate instrument
immediately above it in the chain

 All of the elements in the calibration chain must be known so that
the calibration of process instruments at the bottom of the chain
is traceable to the fundamental measurement standards.
 This knowledge of the full chain of instruments involved in the
calibration procedure is known as traceability, and is specified as a
mandatory requirement in satisfying the ISO 9000 standard.
 Documentation must exist that shows that process instruments
are calibrated by standard instruments that are linked by a chain
of increasing accuracy back to national reference standards. There
must be clear evidence to show that there is no break in this chain.

Calibration of farm machinery: Calibrating Fertilizer
Applicators
 Fertilizers are applied in liquid, gaseous, or granular form.
 The application of liquid fertilizers follows the same principles as
spraying chemicals.
 Sprayers are discussed in a separate section. Gaseous fertilizers
require specialized equipment and are very hazardous to use.
 The equipment to dispense gaseous fertilizers is not included in this
text.
 The following section will discuss the calibration of granular
fertilizer applicators.

 Most granular fertilizer applicators will be one of two types,
broadcast or gravity flow.
 A common agricultural broadcast spreader has a hopper to hold the
material, a metering mechanism, and a spreading mechanism (Fig.
2.1).
Applicators

 Hoppers have tapered sides to feed the material to the conveyor
chain that moves from front to back along the bottom of the hopper.
Hopper capacities are usually measured in tons of fertilizer.
 Metering is usually accomplished by a variable speed conveyor chain in
the bottom of the hopper and an adjustable opening at the back
where the fertilizer drops onto the spreader(s).
 The chain auger is operated by a drive train powered by one of the
wheels of the spreader.
 Because a ground wheel drives the chain auger, the amount of
material dispensed changes as the speed of the spreader changes.
Applicators

 The spreaders are usually powered by the tractor PTO and must
turn at a constant speed for the spreader to have a consistent
dispersal pattern.
 In this design the application rate of the material is set by
changing the speed ratio of the drive train that powers the
conveyor chain and/or changing the size of the opening at the
metering gate.
 The process of using a variable opening for metering is called bulk
metering.
Applicators

 The material is spread as it drops onto one or more rotating disks,
impellers, which propel it in a wide arcing pattern.
 Large spreaders have the impellers mounted on the back of the
machine; smaller spreaders may have the impellers mounted in front
of the hopper.
 A common horticultural design powers the impellers from the ground
wheels, eliminating the need for a PTO or additional power source.
 Gravity flow applicators also have a hopper, but the hopper extends
across the width of the machine.
Applicators

 The material is dispensed through adjustable openings along the
bottom of the hopper.
 The rate of material flow is set by the size of the opening.
 This design uses an agitator along the bottom to prevent the
material from bridging across the hopper and to break up lumps,
thus improving the uniformity of flow.
 The material flow drops through the openings and onto the soil
surface.
Applicators

 Figure 2.2 shows a common horticultural gravity flow fertilizer
applicator.
Applicators

Gravity Flow Stationary Calibration
 Stationary calibration is the preferred method for calibrating
gravity flow fertilizer spreaders because the material flows out of
the entire width of the machine.
 This makes it difficult to collect material from the spreader in a
mobile calibration.
 During a stationary calibration, the material can be collected for a
portion of the width, but the preferred method is to capture
material from the entire width.
 For a stationary calibration, the spreader must be secured so the
drive wheel(s) are suspended and can be turned by hand.

 For the conversion method, the application rate, R, is determined by
collecting the fertilizer for a selected number of turns of the
metering drive wheel.
 The application rate is the pounds of material collected for the
selected number of turns of the drive wheel divided by the area
covered.
 The area covered is determined by multiplying the width of the
spreader times the distance traveled.
 The distance traveled is determined by calculating the perimeter of
the wheel (2πr) and multiplying it by the number of turns.
Gravity Flow Stationary Calibration …

 The distance traveled is:
 Where D = distance covered, in;
 2πr = circumference of circle, in;
 n = number of turns.
 The area is:
 Where A = area, in2;
 w = width of machine, in;
 D = distance traveled, in.
 Note: because the standard unit of area is acres and the standard
conversion from area to acres is feet squared.
n
r
D 
 
2
w
D
A 


 Note: because the standard unit of area is acres and the standard
conversion from area to acres is feet squared. The area should be
converted to square feet:
 Knowing the area in square feet and the weight of material collected,
the application rate can be determined.
 In an equation this relationship is:
 Where R = application rate (lb/ft2);
 W = weight of material (lb);
 A = area (ft2).
  2
2
2
2
144
1
in
ft
in
ft
A 

A
W
R 

 The pound per square foot is converted to the desired units of
pounds per acre:
 These multiple steps can be combined into one equation using a unit
conversion.
 Entering this equation into a spreadsheet would reduce the
computation time and the chance for errors when completing
multiple calibrations.
ac
ft
ft
lb
ac
lb 2
2
560
,
43



 The equation is:
 Where R = application rate, lb/ac;
 W = material collected, lb;
 0.0001442 = unit conversion;
 r = tire radius, ft;
 w = machine width, ft;
 n = number of turns.
Problem Determine the application rate for a 36.0 inch gravity flow
fertilizer spreader. It has a 14.0 inch drive wheel, and 20 turns of the
drive wheel produced 14.6 ounces of fertilizer.
n
w
r
W
R




0001442
.
0

Broadcast Spreader Mobile Calibration
 Mobile calibration may be difficult for large spreaders because of
the width of the broadcast pattern and the volume of material that
will be collected.
 On some machines the impellers can be disengaged and the
fertilizer collected as it flows off the conveyor chain.
 The difficulty is insuring all of the fertilizer is collected.
 Another method is to collect a sample area of the broadcast
pattern.
 Both of these methods will be illustrated with a problem.

Calibrating Grain Drills
 The traditional end wheel grain drill includes a hopper plus a
metering unit, seed tube, and furrow opener for each row.
 They use a ground wheel and drive train to power the metering unit
The rows are usually spaced 6–10 inches (15–25 cm) apart.
 Combining the row spacing and number of metering units is the
traditional method for indicating the width of the drill.
 A drill identified as 13–6 would have 13 metering units spaced 6
inches apart or a width of (13 x 6)/12 = 6.5 feet.

 The calibration of grain drills is more critical than the calibration of
fertilizer spreaders because a small error in seeding rate can have a
greater impact on the yield than an error in fertilizer application.
 In addition, it is more important that the seeds are planted
uniformly.
 Grain drills can be calibrated stationary or mobile.
 The unit cancellation method can be used in either situation.
Calibrating Grain Drills

Grain Drill Stationary Calibration
 End wheel grain drills drive half of the metering units from each
wheel.
 For a stationary calibration, it is recommended to elevate one wheel
at a time.
 A collection container is attached to each of the metering units
driven by the elevated wheel.
 The drive wheel is turned a desired number of revolutions, if the
area or conversion method is used.
 The process is repeated for the other drive wheel.

Mobile Grain Drill Calibration
 During a mobile calibration, collection containers are attached to
each metering unit, and the drill is driven a measured distance.
 Mobile calibration is more problematic than stationary because of
the problem of attaching containers to the metering units that
stay in place.
 The calibration process is the same as the one used for fertilizer
spreaders—collecting the seeds and calculating the seeding rate.

Calibrating Row Crop Planters
 Row crop planters or precision planters are used to plant crops in
rows while maintaining spacing between seeds within the row unlike
those planted by grain drills.
 They use a different type of metering mechanism. Row crop planters
are commonly used to plant large seeds, such as corn, soybeans, and
sunflowers, but they are also used to plant small-seeded vegetable
crops such as radishes.
 The common metering units for row crop planters are called plate,
disk, and drum.

 A common feature of these types of metering units is that they
adjust the planting rate for changes in velocity, within limits, using a
mechanical drive wheel or computer controlled electric drive motor
(Fig. 2.5).
Calibrating Row Crop Planters …

Row Crop Planter Stationary Calibration
 Precision row crop planters can be calibrated using the stationary
method.
 The seeding rate is calculated by dividing the seeds planted per
revolution of the drive wheel by the representative acres covered
per revolution of the drive wheel, or:
 This equation can also be written as:
 where R = seeding rate (seeds/ac);
 sd = number of seeds; nr = number of revolutions of drive wheel;
 A = area, ft2.
r
r
n
A
n
Sd
R 
A
n
n
Sd
R r
r



Row Crop Planter Mobile Calibration …
 Mobile calibration of precision row crop planters can be completed
the same as fertilizer spreaders and grain drills, but because they
place individual seeds in the ground a different method can also be
used.
 For each row spacing and planting rate, there is a unique spacing
for the seeds in the row.
 The planting rate of row crop planters can be determined by
operating the planter in the field and measuring the seed spacing
in the row.

 To complete a mobile calibration of a row crop planter, seeds are
added to the hopper, and the planter is lowered into the ground and
driven for a short distance to ensure the system has stabilized.
 Next carefully uncover several seeds in a row and measure the
distance between the seeds.
 On some models the press wheel can be disabled during this step to
make seed spacing measurement easier.
 The average distance is used to determine the seeding rate.
Row Crop Planter Mobile Calibration …

Calibrating Sprayers
 Accurate calibration of spray equipment is very important because
small variations in the application rate can cause chemical damage to
the crop or the environment, be wasteful of materials or be
ineffective.
 There are two important differences between the design of
sprayers and that of grain drills, and row crop planters.
 (1) Sprayers do not use ground-driven metering units.
 The application rate (gal/ac) is a function of the flow rate of the
nozzles (gal/min) and the velocity of the sprayer (mi/hr).

 The flow rate from the nozzles does not change when the ground
speed changes; therefore changing the sprayer velocity changes the
application rate.
 (2) Most agricultural sprayers use PTO or engine-driven pumps.
 To maintain a constant application rate, the PTO or engine speed
must be constant.
 Ground driven pumps do exist, but they are not as common as PTO
and engine driven pumps in dedicated spray equipment.
 The focus of this text will be on sprayers with PTO or engine
equipped pumps.
Calibrating Sprayers …

 The problems associated with variability in speed and flow can be
managed by using a sprayer controller.
 Sprayer controllers are designed with a variety of capabilities.
 Simple controllers may just monitor system pressure and provide
switches to operate the sprayer systems.
 The most expensive controllers will monitor and regulate all the
sprayer systems.
 They have the ability to the vary application rate to compensate for
changes in sprayer speed or changes in flow from the pump to insure
the application rate is constant.
Calibrating Sprayers …

Sprayer Stationary Calibration
 When the stationary method is used, collectors are placed under
each nozzle, and the sprayer is operated for a specific time period.
 The specified time period can be preselected, or a recorded amount
as long as it provides an adequate sample without exceeding the
capacity of the collection vessel.
 The volume per time is converted to gallons per acre.
 The application rate can be calculated with the unit method or by an
equation.

Mobile Sprayer Calibration
 To conduct a mobile sprayer calibration, the sprayer is set up ready to
go to the field with collection containers under each nozzle.
 If the area method is used, the required distance is calculated before
collecting any liquid from the sprayer.
 If the conversion method is used, a measured distance is marked out.
 The sprayer output is collected travelling the distance and the
application rate is then calculated.

Chapter two testing equipment and machinery calibration

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