# Presentation: 406690

## Weight Carts

- Design and Use Issues
- Val Miller
- NIST Office of Weights and Measures
- val.miller@nist.gov

- NCWM Metrology Working Group 2001

## Official NCWM Position On Use of  Powered Weight Carts as Calibrated Standards

- 	NCWM Publication 3(1987), Section 3.2.15 “Field standard test weights (**weight carts**)...  ...are inappropriate as Standard Weights.” (Approved 1980.)

- NCWM Metrology Working Group 2001

## Reasons Given for Weight Cart Use

- Efficiency	Speed of accomplishing scale calibrations	Less manpower required	Budgets requiring “do more with less”
- More closely approximates normal use 	Load application	Weighing system drift

- NCWM Metrology Working Group 2001

## Truck Scale Calibration Using Weight Cart

- NCWM Metrology Working Group 2001

## Remote Controlled Weight Cart

- NCWM Metrology Working Group 2001

## Recap of what has been accomplished

- 1999 Weight Cart Survey263 Weight cart calibrations reported in 23 states	178 Liquid fuel powered 	  78 Electrically powered	    6 Battery powered	    1 using an unknown power source
- 1st draft Handbook 105-x circulated and comments collected during 1999

## Recap of what has been accomplished (continued)

- 2001 State Laboratory Program Workload Survey344 weight carts calibrated by 45 States	39% - Weights &Measures Program	58% - Commercial Scale companies	  3% - Other customers

## Recap of what has been accomplished (continued)

- Additional Weight Cart information survey     Requested Information
- Fuel and Hydraulic Tank shapes and dimensions
- Number and size of wheels;
- Wheels Treaded or smooth
- Empty weight and Maximum loaded weight
- Overall dimensions and wheel base
- Number of Axles

## Recap of what has been accomplished (continued)

- 2001 survey results	25 responses from 7 States

## 1st Draft Recommendations

- Establish Working Group
- Determine size of typical fuel tanks and impact of fuel level errors
- Determine type, capacity and weight of batteries
- Obtain As Found/As Left data
- Draft “Calibration requirements”
- Draft “User requirements” for S&T Committee
- Determine weight impact of dirt/debris on treaded tires

**Notes:**
This will be the basic outline of this presentation.

## Establish Working Group

- Working group established
- Member List Val Miller, NIST (Chair)		Georgia Harris, NISTDavid Ehrnschwender, Fairbanks Scale	LF Eason, NCDA&CSRon Balaze, MI Dept. of Agriculture	Emil Hazarian, LACDAJohn Devald, Tiffin Loader Crane	Tom Schafer, IDDASid Colbrook, ILDA			Bruce Adams, MNDPSJohn Holt, Kanawha Scales		Richard Suiter, NIST
- Meeting September 5 - September 7, 2001 at NIST

## Determine size of typical fuel tanks and impact of fuel level errors

- Survey data requested (25 responses)
- Most reported fuel tanks rectangular at reference level (Colorado retrofits to a 2”x12” cylinder)
- Areas  at reference level range from 28 to 126 square inches (except Colorado carts)
- 0.25 inch error in fuel level: 0.18 to 0.82 lb
- “Full” stated as reference level (no sight glass)

## One Example of Current Fuel Tank

**Notes:**
Can contain approximately 5 gallons of fuel
Deviation of 0.25 inches causes error of approximately 0.4 pounds

Fuel level impact:
Based on the API tables for volume changes with temperature and assuming:
.that the weight cart was adjusted to nominal at 75°F,
. that the fuel used is gasoline fitting the normal wide range of densities, density varying by grade of gasoline and by season, 
.that the gasoline weighs 6 pounds per gallon, 
.that the fuel capacity at the reference is 3 gallons, and 
.that the fuel level can be re-adjusted exactly to the reference level, then
the cart will weigh approximately 0.49 pounds heavy if the fuel level is adjusted when the fuel temperature is 32°F, and approximately 0.59 pounds light if adjusted when the fuel temperature is 125°F as it might be with a dark colored fuel tank in direct sunlight.  

If the level deviates from the reference level, the errors may be greater.

## Another Example of a Current Fuel Tank

**Notes:**
Can contain approximately 6 gallons of fuel
Deviation of 0.25 inches causes error of approximately 1 pound.

Fuel level impact:
Based on the API tables for volume changes with temperature and assuming:
.that the weight cart was adjusted to nominal at 75°F,
. that the fuel used is gasoline fitting the normal wide range of densities, density varying by grade of gasoline and by season, 
.that the gasoline weighs 6 pounds per gallon, 
.that the fuel capacity at the reference is 3 gallons, and 
.that the fuel level can be re-adjusted exactly to the reference level, then
the cart will weigh approximately 0.49 pounds heavy if the fuel level is adjusted when the fuel temperature is 32°F, and approximately 0.59 pounds light if adjusted when the fuel temperature is 125°F as it might be with a dark colored fuel tank in direct sunlight.  

If the level deviates from the reference level, the errors may be greater.

## Possible Fuel Tank Design

- NCWM Metrology Working Group 2001

- ID = 3.5 inches
- Length = 12 inches
- Sight glass: 0.25 in ID
- Total Volume  0.5 gallons
-  0.07 lb per 0.25 inches fuel level change
- Similar to design currently used in Colorado

**Notes:**
Total cross sectional area including 1/2 inch sight glass less than 10 square inches.

## Determine type, capacity and weight of batteries

- Determine type, capacity and weight of batteries

- Most carts use automotive lead acid batteries
- NC Replacement battery of same manufacturer and model had minimum difference of 3 lb, as much as 15 lb
- Contribution of 0.25 inch error in water level of a standard automotive battery is approximately 0.6 pounds
- Now communicating with battery manufacturers about  possible battery types
- Sealed Lead Acid
- Gel Cell
- appear most likely candidates, some already in use. Charging system specifications are an issue)

**Notes:**
Have communicated via e-mail with the Optima Battery Group of Johnson Controls, and Exide Batteries.  And have obtained representative charging system specs from Kohler, since the majority of carts I’ve seen have Kohler engines.
Charging rate is critical to avoid boiling off the water contained in the sealed battery.  Even though sealed it has a relief valve to protect against over pressurization of the case.  If charging is sufficiently controlled, the hydrogen gasses generated are returned to liquid water state and no mass change is caused.

## Obtain As Received/As Left data

- Data received from   North Carolina Laboratory  Colorado Laboratory  Illinois Laboratory  Los Angeles County Laboratory  Pennsylvania Laboratory  Idaho Laboratory  New York Laboratory

**Notes:**
Data from Idaho was for totally electric or battery powered 2000 and 2500 pound carts.  Variations so insignificant that decided not to include, also smaller carts such as those would not normally be used for 20 or 30 k applications.  Max loaded weight would be 12.5% of 80 000 lb capacity scale

## Gasoline Powered Weight Cart Stability Study

- A field study conducted by the
- Standards Division of the
- NC Department of Agriculture and Consumer Services

- Slide courtesy of LF Eason

- NCWM Metrology Working Group 2001

**Notes:**
This quick presentation demonstrates the type of detail needed for a quality study such as this.

## Weight Carts Used

- NCWM Metrology Working Group 2001

- 3 Tiffen Model C-5500, 5500 lb1 Cargotec Model 40-KVM, 5500 lb
- Two cylinder gasoline engines
- Carried on covered scale trucks
- 3 Axles
- Hydraulic drive
- Tiffen - Single axle drive
- CargoTec - Dual axle drive

## Test Conditions

- NCWM Metrology Working Group 2001

- Time Frame
- September 3, 1998 to June 29, 2000
- 665 Days, 1.8 years
- Special thanks to Ron Murdock for giving us such frequent access to the carts
- Tested on Mettler KG3000 Scale
- NC Standards Lab, large mass lab
- 3000 kg (6600 lb) capacity with 2.5 g division

## NC Standards LaboratoryWeight Cart Calibration

- Slide courtesy of LF Eason

- NCWM Metrology Working Group 2001

## Test Method

- NCWM Metrology Working Group 2001

- Substitution weighing
- Standards placed on scale and zeroed
- Cart centered on scale and weighed
- Cart rotated 90° and weighed
- Cart rotated to original orientation and weighed
- Standards placed back on scale to check drift
- Readings 1 & 3 averaged then averaged with Reading 2

**Notes:**
Method was chosen to minimize the problems of centering the cart on the scale and to monitor the scale drift.

Crane bridge Beams above balance marked to indicate balance center.  Cart centered using that reference point.

Modification of a NIST HB 145 Modified Substitution weighing procedure

## Analysis

- NCWM Metrology Working Group 2001

- Carts were tested as used in the field
- Oil and gas level adjusted before test
- Wet carts were allowed to air dry overnight
- If adjustment was required, the amount of adjustment was recorded so that the unadjusted weight of the cart could be tracked (“Adjustment compensated” lines on graphs)

## 11/17/99  Water added to low battery11/18/99  New battery installed  6/21/00  Adjusted, Oil drain extended past frame

- 11/17/99  Water added to low battery11/18/99  New battery installed  6/21/00  Adjusted, Oil drain extended past frame

- Slide courtesy of LF Eason

- NCWM Metrology Working Group 2001

**Notes:**
Bob Albright’s cart.

## 11/17/99  Cart very dirty

-   3/10/00  Cart very dirty

- Slide courtesy of LF Eason

- NCWM Metrology Working Group 2001

**Notes:**
Frank Wood’s cart.
Noted that cart was very dirty on Nov 11, 1999 and March 10, 2000 tests.

## 6/29/99  Adjusted, Calculations re-referenced

- Slide courtesy of LF Eason

- NCWM Metrology Working Group 2001

## Slide courtesy of LF Eason

- 6/28/99  Battery replaced with larger battery, Battery cover removed and discarded
- 6/27/00  Oil drain extended past frame. Adjusted and re-referenced calculations

- NCWM Metrology Working Group 2001

## Colorado Data

- Adjusted to zero error at each calibration1996 Weight increased from 3100 pounds
- Inquiry about repeated changes gained information that user likes to tinker and modify resulting in apparent instability.

- NCWM Metrology Working Group 2001

**Notes:**
Picked one cart of many for which data was submitted.  
Even electric carts displayed similar shifts, though not as consistently indicating better stability, but still under operator influence.

## Illinois Data

- No known cause for instability.

- NCWM Metrology Working Group 2001

**Notes:**
Found policy statement requiring that the battery of the cart be replaced with a new heavy duty battery and that all servicing and maintenance , including painting, be performed prior to each submission for calibration.  This could be the reason for the apparent instability.

## Los Angeles County Data

- Hydraulic Tank welded and battery changed during study

- NCWM Metrology Working Group 2001

**Notes:**
Cart was adjusted to 4.4 pounds heavy and left there

Exact statement:
The Laboratory adjusted the Fuel to its reference level and tested the cart without any ballast adjustment.  Kern Co. removed ballast, after the the cart returned from testing to bring the cart to 3000 lb.  After taking the battery change and under filled gasoline into account the actual weight change over the study period was +3.7 lb.

## Los Angeles County Data

- Oil changed, battery water added, cart lubricated.

- NCWM Metrology Working Group 2001

**Notes:**
Lab adjusted the fuel to the reference level, installed the new battery, tested the cart and adjusted the ballast to bring the cart to 5000 lb.  Cart weight change over the study period was 0.1 lb.

## Pennsylvania Data

- No special instructions to operator as to how to prepare cart for calibration.  Values taken “As Delivered”

- NCWM Metrology Working Group 2001

## Why were adjustments made?

- Changed battery
- Welded hydraulic tank
- Added motor oil drain to clear chassis
- User modified cart
- Lost battery box cover
- No guidance provided for submission
- Painted cart...

**Notes:**
Actual data should be recorded for As received or As maintained condition.

## As Received/As Left data Recommendations

- Very detailed information is required as to causes of weight deviations, e.g. fluid levels, contamination, repairs, modifications, etc., for meaningful analysis of the weight cart stability over time.
- As Received data is required for reverse traceability and for determining the impact of errors on completed scale calibrations.  Also, is important information when setting the re-calibration interval of the weight cart.

## ISO/IEC FDIS 17025: 1999(E)

- 5.10.4 	Calibration Certificates
- 5.10.4.3	“When an instrument has been adjusted or repaired, the calibration results before and after adjustment or repair, if available, shall be reported.”

**Notes:**
One note that I found was the weight cart maintenance and annual calibration instructions.

One instruction was to install a new heavy duty battery prior to calibration.  Yet all maintenance to the cart which might change the weight require re-calibration.  While not inconsistent, this destroys the validity of all as received data, and the possibility of reverse traceability.

## ExampleWeight Cart Log Sheet

**STATE OF GEORGIA**
**WEIGHTS AND MEASURES LABORATORY**
**WEIGHT CART INSPECTION**
**Date of Inspection: ____________________ Inspector: ________________________ **
**Company Name: _________________________________________________________**
**City: _____________________________________ State: ________________________**
**Cart Manufacturer: ________________________Date of Manufacture: ___________                                                              **
**Model # ___________________________ID #_________________________________ **
**Nominal mass weight of cart: _____________LBS.     Number of Axles: ___________**
**Powered by: Gasoline___________ Diesel _____________Electric________________**
**Fluid level; Fuel: full_________ Hydraulic: full_______ Battery: full_____________**
**General Condition: Cleaned and Painted_____________________________________**
**Drain holes in cargo cage_______________ Adjusting cavity accessible____________ **
**Adequate size:(approx 100 lbs. capacity)________Cavity sealable with wire and seals______ **
**List and report any repair and maintenance performed, parts replaced etc., i.e., new battery, carburetor, exhaust system, wheels changed, welding performed etc.**
**Include any comments, changes since last test.**
- ____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

- NCWM Metrology Working Group 2001

## Draft “Calibration Requirements”

- Standard Operating Procedure Required
- Need well documented history
- Must establish criteria for conditions at time of test: thermal soak time, cleanliness, before or after maintenance, fluid levels as delivered or after adjustment, fluids hot or cooled to lab conditions, etc.
- Minimum weighing equipment requirements: resolution, weighing system type, uncertainty, weighing platform size, etc.

**Notes:**
The same requirements exist for these weight carts as has been put forward for railroad test cars.

## Pennsylvania Weight Cart Calibration

- NCWM Metrology Working Group 2001

**Notes:**
Note that the scale deck must be extended using the planks.

## North Carolina Weight Cart Calibration

- NCWM Metrology Working Group 2001

**Notes:**
Cargotec cart approx 9 inches wider requiring use of 4x4s in balance rails for calibration.  

Cart purchasers must coordinate with their supporting laboratory prior to purchase to ensure that the cart purchased can be supported.  

Scale companies are traveling tremendous distances to obtain cart calibrations.
Possibly NY to Maryland or NC and Western TN to NC, South Ga to NC simply because the state that normally supports them does not have the capability.  Not fault of Lab.

## Draft ‘User requirements’ for S&T Committee

- Probably as suggestions for Test Notes
- Must outline maintenance requirements
- Detailed maintenance documentation is required
- Provide temperature guidelines for adjusting fluid levels
- ANY maintenance requires re-calibration of cart
- Investigate use of fuel density correction table or limit volume of fuel tank

**Notes:**
Any Input?

## Determine weight impact of dirt/debris contamination

- No treaded tires!
- No data obtained on impact of tire tread debris contamination at this time
- Contamination on cart causes non-repeatable errors and loss of traceability as the errors cannot be repeated in the laboratory
- Rain soaked 3000 pound cart error of more than 5 pounds
- More than 2 pounds of condensation collected on cold cart (40°F) brought into lab (75°F)

**Notes:**
Mud weighs approximately 120 pounds per cubic foot.  

## Additional concerns

- Hydraulic reservoirs: size, shape & ability to seal
- Footprint relative to typical truck footprint
- Limit load concentrations on scale decks to avoid damage
- EPO for Weight Carts use
- Minimum use requirements

**Notes:**
Based on current data, pressure under wheels of carts (contact patch area) less than 300 psi. 

## Weight Cart Tolerances and Calibration Uncertainties

- NCWM Metrology Working Group 2001

## “Weight Cart”

- NCWM Metrology Working Group 2001

**Notes:**
Used as approx. 11000 lb substitution load.

Has hydraulically actuated ram to load in 4x8 foot print.
Towed empty behind truck then loaded as required.
Note Treaded tires!

## Weight Cart Tolerances

- HB 44 Appendix A, 3.2 Tolerances for Standards - “The error in a standard used by a weights and measures official should be known and corrected for when the standard is used; or if the standard is to be used without correction, its error should be not greater than one-third of the smallest tolerance to be applied when the standard is used.”

**Notes:**
Standard in this case is tolerance of weight cart plus tolerance of each weight loaded into the cart.

## Proposed Weight Cart Tolerance

**Notes:**
Based on calculations of d= 5, 10 and 20 lb at 500d and 1500d.  The places of greatest sensitivity to the standard.  Max d before next tolerance change.

Calculated using acceptance tolerance for IIIL scale. 

Would a cart be used for a Class III scale?
May have to be a requirement that carts only be used for IIIL scales.  Otherwise tolerance for cart will be smaller.

Calibration uncertainty required to be less than 1/3 of cart tolerance.  Will require adequate balance resolution (0.05 lb per div. or less)

## Tolerance ComparisonClass IIIL Scale Acceptance Tolerance and Standards Tolerance

- NCWM Metrology Working Group 2001

**Notes:**
Note that the tolerance of the weight cart and weights is marginally acceptable for the test.

## Tolerance Comparison Class IIIL Scale Acceptance Tolerance and Standards Tolerance

- NCWM Metrology Working Group 2001

**Notes:**
Note that the tolerance of the weight cart and weights is marginally acceptable for the test.

## Tolerance Comparison Class IIIL Scale Acceptance Tolerance and Standards Tolerance

- NCWM Metrology Working Group 2001

**Notes:**
Note that the tolerance of the weight cart and weights is marginally acceptable for the test.

## Tolerance Comparison Class IIIL Scale Acceptance Tolerance and Standards Tolerance

- NCWM Metrology Working Group 2001

## Tolerance Comparison Class IIIL Scale Acceptance Tolerance and Standards Tolerance

- NCWM Metrology Working Group 2001

## Tolerance Comparison Class IIIL Scale Acceptance Tolerance and Standards Tolerance

- NCWM Metrology Working Group 2001

**Notes:**
Carts smaller than 4000 not shown the number of 1000 lb weights required to reach 30 000 pounds begins to exceed the capabilities of the cart.

## Weight Cart Working Group Goals

- Review a draft NIST Handbook 105-x for weight cart design
- Review an EPO for the testing of large capacity scales utilizing a weight cart as part of the calibrated load
- Develop guidelines for the use and maintenance of weight carts as calibrated standards
- Develop testing criteria and process for the calibration of weight carts

## Weight Cart Working Group Goals (continued)

- Draft a policy statement for submittal to NCWM S&T committee for review and possible adoption
- Identify and deal with other areas of concern involving the use of weight carts in large capacity scale testing

## Things to Think About

- Most stability issues are user dependant
- Changing battery, even with one of the same size & model, can make up to 15 lbs difference
- Changing filters, routine maintenance, any repairs, or other changes require recalibration
- Fluid levels (fuel, oil, battery water, fluid filled wheel bearing, etc) must be carefully monitored
- Must be kept contamination free
- Weights on a cart create even more surface areas to trap and hold water

**Notes:**
Two battery change stories:
Operator changed battery without consideration of weight.  Got a different case size.  Steel battery cover would not fit over new battery, so he left it off.  Even without the steel cover, the cart was almost 3 lbs heavy
Replaced a battery with same brand, size, and model number.  Took a package checking scale and weighed all on shelf.  Closest battery weighed 2.5 lbs more than battery being replaced.  Some were almost 15 lbs heavier.
As the variability due to the fuel tank temperature and motor oil levels can be so significant, any changes in hard mass of the cart must be monitored and compensated.  

## Things to Think About

- NCWM Metrology Working Group 2001

- Design Considerations
- Fluid level sight gauges should be clearly marked and readable,
- Adjustment box must be adequately sized
- Fluid drains should extend beyond frame
- Floor deck should not be multi-layered

## Things to Think About

- NCWM Metrology Working Group 2001

- Design Considerations (continued)
- Multiple drive axles help improve smooth tread traction
- Sealed bearings should be used instead of fluid filled bearings
- Hard shelled paints should not be used in weight box
- All surfaces must have good drainage

## Recommendations

- The weight of liquid fueled weight carts can be stable enough for high capacity scale tests with proper care and operator training
- Design considerations and maintenance procedures must be followed to limit potential errors
- Weight carts should not be used in adverse weather

## Recomendations (continued)

- Very detailed maintenance records are required
- Existing weight carts will require modifications, over a period of time, to comply with the requirements of Handbook 105-x (once the requirements have been set)

**Notes:**
Stress “AFTER THE REQUIREMENTS HAVE BEEN SET”