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Proper Calibration

VIEWS: 35 PAGES: 4

									Vol. 46 No. 3                                                                                          Summer 2009




                       Proper Calibration:
       Are your solid materials spreaders and liquid
         distribution systems calibrated properly?
                                                   by
                         Duane E. “Dewey” Amsler Sr. PE, AFM Engineering Services



t      he Clear Roads
       Continuing
       Pooled Fund
Research Project has
                                                                                             totally compatible.
                                                                                             Most of the observed
                                                                                             inaccuracies during
                                                                                             the project related
                                                                                             to the hydraulic and
recently published
two documents that                                                                           spreader systems
                                                                                             not being able to
may assist you in
                                                                                             totally support the
answering the title
                                                                                             controller. Second, the
question. They are
                                                                                             most sophisticated
entitled: “Calibration                                                                       controller is of little
Accuracy of Manual                                                                           value, unless it is
and Ground Speed                                                                             calibrated to deliver
Controlled Salt                                                                              the desired amount
Spreaders” and                                                                               of product over the
“Calibration Guide                                                                           range of spreading
for Ground-Speed-                                                                            speeds and discharge/
Controlled and                                                                               application rates
Manually Controlled                                                                          being used.
Material Spreaders."
These are available at:                 to be a member of the research team    TYPES OF SPREADER
http://clearroads.org/about.html        that conducted the research and        CONTROLLERS
The former describes the research       prepared the Report and Calibration    There are three types of controllers
efforts that attempted to define        Guide. This article will draw from     in common use:
the accuracy of six manual and          both documents and my personal         e Manual
ground speed controlled solid           experience.                            e Ground Speed - Open Loop
and liquid materials controllers                                               e Ground Speed - Closed Loop
and related distribution systems        FUNDAMENTAL
that are in general use, and the        CONCEPTS                               Manual controllers provide a
variables that impact the accuracy      First, it is vitally important to      constant rate of product delivery
of the calibration process and what     understand that having a spreader      (weight or volume per unit of time)
actually ends up on the road. The       controller (manual or automatic)       for each setting as long as the
“Calibration Guide” provides step-by-   does not guarantee that accurate       capacity of the hydraulic system
step instructions for calibrating the   discharge/application rates will be    is not exceeded. By knowing the
common manual and ground-speed-         achieved on the road. The controller
controlled systems. I was fortunate     is part of a system that must be                             continued on page 2




SALT & Highway Deicing / Summer 2009                                                                                  1
 Proper Calibration             (continued from page 1)


relationship                                                                                    controller setting
between the                                                                                     and discharge/
rate of delivery                                                                                application rates
for each setting                                                                                are determined by
(from calibration)                                                                              mathematically
and truck speed,                                                                                relating the
some control                                                                                    discharge rates to
of discharge/                                                                                   likely operational
application rate is                                                                             truck ground
possible by driving                                                                             speeds.
at prescribed
speeds. This is                                                                                  Ground speed
probably the least                                                                               controlled systems
accurate control                                                                                 are calibrated by
method, as the                                                                                   determining the
truck operator                                                                                   weight or volume
usually cannot                                                                                   of the discharged
maintain constant                                                                                material over a
speed.                                                                                           known distance.
                                                                                                 There are three
                                       During operations the only variables
Open Loop ground speed controllers                                            methods in common use for
                                       that can practically be changed are
are the next most accurate method                                             determining distance (speed x time):
                                       conveyor, auger or pump speed, and
of controlling discharge/application                                          •	 Connecting	an	electronic	speed	
                                       truck speed. However, most systems
rate. The controller senses the                                                  simulation device to the controller
                                       have a spinner speed control that
ground speed of the spreading                                                    that provides the same data as the
                                       may be able to change the number
vehicle from the speedometer or                                                  truck speedometer and collecting
                                       of lanes being treated, if there is
other sensing point, and changes                                                 material discharged at desired
                                       no downstream deflector control.
product delivery rate in proportion                                              speeds and time
                                       Ground speed controllers are
to vehicle speed.                                                             •	 Jacking	the	truck	drive	wheels	
                                       designed to take truck speed out of
                                                                                 (with proper safety methods and
                                       the equation, leaving the operator
Closed loop ground speed controllers                                             devices) and collecting discharged
                                       to concentrate on the multitude of
are the most accurate method of                                                  material with the wheels turning
                                       other required tasks.
controlling discharge/application                                                at desired speedometer readings
rate. The controller senses ground                                               and time
speed from the speedometer or
                                       CALIBRATION                            •	 Operating	the	truck/spreader	
other point, and the speed of the
                                       METHODS FOR                               over a known distance, at desired
auger/conveyor or the liquid flow
                                       MANUAL AND                                speeds, and capturing the material
rate on the discharge end. The
                                       GROUND SPEED                              discharged by the spreader
controller uses both data sources to
                                       CONTROLLERS
control and balance the application    The methodology for calibrating        These are arrayed in order of
rate.                                  all spreading/distribution systems     decreasing accuracy.
                                       is fundamentally the same. The
OPERATIONAL                            weight or volume of the discharged
                                                                              There are two “constants” developed
APPLICATION RATE                       product is determined for a known
                                                                              for most ground speed controllers
VARIABLES                              or simulated distance of travel or
                                                                              during the calibration process.
                                       unit of time. Control elements are
The primary application rate                                                  The first is sometimes called the
                                       changed until the desired discharge/
variables are:                                                                “speed constant” and is developed
                                       application rates are achieved.
•	 Gate	or	orifice	opening	that	                                              by traversing a known distance with
   discharges material on to the                                              the truck, and entering the data in to
                                       Manual control systems are             the controller. This makes sure that
   spinner
                                       usually calibrated with a stationary   the controller is using the speed/
•	 Conveyor,	auger	or	pump	speed
                                       truck having the engine RPMS           distance relationships correctly.
•	 Truck	speed
                                       at a level normally used during        The second or “rate constant” is
•	 Number	of	lanes	being	treated
                                       operations. The discharge per          developed by determining the weight
                                       minute is determined for each
                                                                                                   continued on page 3



2                                                                         SALT & Highway Deicing / Summer 2009
 Proper Calibration                  (continued from page 2)


or volume of product discharged per
                                           Method Variables
revolution of a drive shaft that is
                                           There are many acceptable                Product Variables
associated with the delivery of the
                                           methods and procedural variations        Solid products should be
product.
                                           that can be used during the              representative of what is actually
                                           calibration process. The controller      used in the field. Variations in
APPLICATION RATE                           manufacturer’s recommendations           gradation and moisture content
AND DISCHARGE                              are usually the best starting point.     can affect the accuracy of the
RATE                                       There are generic procedures found       calibration. A prepared sample
I have used the term discharge/            in the “Calibration Guide” that          of about 1/2 truckload of product
application rate above. The reason         is the subject of this article. The      that has been screened to exclude
is that they are often separate and        “Calibration Guide” also contains the    material larger than the gate or
distinct entities. Discharge rate is the   Salt Institute procedure and form        orifice opening is a good idea.
amount of product being dispensed          for calibrating manual and ground
by the spreader per unit of distance       speed controlled material spreaders.
(gal or lbs/mi.) or (l or kg/km).          This is also available at: http://       Truck, Controller and Delivery
Application rate is the discharge          www.saltinstitute.org/content/           System Variables
rate divided by the number of              download/605/3484                        There are a number of procedures
lanes being treated (lbs or gal/lm                                                  that can be used to control truck
or l or kg/km). If the discharge rate      The minimum weight of solid              and delivery system variables:
of a spreader is 200 lbs/mi, and a         material discharged during a “catch      •	 Warm	the	truck	and	hydraulic	
centerline, or 2 - lane spread is being    and weigh” test is often debated. In        system sufficiently, prior to
performed, the application rate is:        my judgment, 100lbs is a reasonable         calibration.
200 lbs/mi divided by 2 = 100 lbs/mi.      minimum. It is also a good idea          •	 Tailgate	type	spreaders	should	be	
If that 200 lbs/mi is being distributed    to observe the delivery stream              calibrated with the bed level that
on one lane, the application rate          of material being discharged. If            is used during operations.
is 200 lbs/lm. CALIBRATION ONLY            discontinuities are observed, the test   •	 “Gate”	openings	should	be	straight	
DETERMINES DISCHARGE RATE.                 must be aborted.                            (top of opening parallel to the
                                                                                       bed).
HOW TO ASSURE                              Equipment Variables                      •	 Make	sure	any	controller	
ACCURACY DURING                            There are a number of items                 “programming” that opens control
THE CALIBRATION                            used for measurement during the             valves to “full open” for a period of
PROCESS                                    calibration process. These may              time, before going to the desired
There are a number of nuggets of           include: scales, liquid measurement         opening, has been disabled.
wisdom that come in to play during         containers, distance measuring           	•	When	doing	repetitive	collections	
calibration. These include:                equipment and procedures,                   of discharged material, use the
“A product is only as good as its          stopwatches and shaft rotation              “power” switch on the controller
individual parts,” “garbage in -           measurement aids. These devices             to start and stop material flow
garbage out” (the GIGO rule) and           should all be calibrated.                   (assuming the control valve
“think critically about every task                                                     opening does not change with loss
you perform, every time                                                                         of power).
you do it, and commit the
accumulated knowledge                                                                          Verification Testing or
to policy and procedure”                                                                       Checking
(the doctrine of                                                                               There is a fairly simple
continuous improvement).                                                                       procedure in the
In a practical sense,                                                                          “Calibration Guide” for
this all boils down to                                                                         ground speed controlled
systematically paying                                                                          systems that provides a
attention to, and                                                                              basis for setting an initial
controlling variables                                                                          “gate” opening, allows
during the calibration                                                                         the assessment of the
process.                                                                                       need for calibration, and
                                                                                               provides data necessary to

                                                                                                          continued on page 4



SALT & Highway Deicing / Summer 2009                                                                                       3
 Proper Calibration                             (continued from page 3)

“fine tune” calibrations over the full
                                                      Discharge Rate Set     Test Speed       Test Time    Target Discharge Acceptable Range
operating range of discharge rates                       in Controller     (miles per hour)   (seconds)        (pounds)         (pounds)
being used. It involves performing                    (pounds per mile)                                   * due to rounding
“catch and weigh” tests for specified                                                                     to the nearest full
                                                                                                                second
time intervals at various discharge
rates. The time intervals are chosen                         200                 25              73           101.4*          97.3 - 105.5
to provide a discharge of about 100
                                                             300                 25              49           102.1*          98.0 - 106.3
lbs. of solid material. Variations
outside of the “acceptable” range                            500                 20              36            100.0          96.0 - 104.0
can be corrected by fine - tuning
                                                             600                 20              30            100.0          96.0 - 104.0
the “rate” constant in the controller,
and re-testing until an acceptable
result is obtained. There is a similar
procedure for liquid pre-wetting                        FINAL THOUGHT
systems.                                                Calibration is a procedure and a process. As with most things in life,
                                                        maximum accuracy is achieved by “paying attention to business”.
A composite table (not in the
“Report” or “Guide”) for solid
material that is based on data from
Tables 5-1 and 5-2 in the Calibration
Guide appears to the right:




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4                                                                                             SALT & Highway Deicing / Summer 2009

								
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