Pioneer Scale has a long reputation for understanding the critical components required to calibrate and certify the scales utilized in concrete batching systems. Our trained technicians understand the basics of yield calculation, material inventory and specifications of mix design to obtain the highest quality in batched concrete. Each scale calibrated from cement/fly-ash to aggregate used for concrete includes a repeating load test to insure that each batch maintains a consistent weighment. An increasing load test is performed to insure that the scale performs accurately in the full range of the scale. All scale tests are performed using the requirement as set forth by the National Ready Mix Concrete Association -NRMCA.
There are some differences in what the scale companies are accustomed to doing in accordance with NIST Handbook 44 and what is required by the NRMCA plant certification and ASTM C94. Some of these differences are described in the Plant Inspector’s Guide. The following should be communicated with the scale company before they do scale checks:
Minimum quantity of test weights should be at 10% of scale capacity. Aggregate scale capacities will generally govern the minimum amount of test weights required – about 4000 lbs is typically needed. (There may be situations where the plant configuration does not permit the minimum test weights to be used and that has to be verified by the plant inspector).
Pioneer Scale's test weights are be certified to be accurate to 0.01% of their indicated load within the last twelve months and registered with the State Weights and Measures Department in the State where the concrete plant is located.
Scale checks and test are conducted through the range of use of the scales. Pioneer Scale verifies the scale through 50% of the scale capacity.
Up through 50% of the scale capacity, scale checks should be done using a build-up test load using a combination of product and test weights in a process called a substitution loading. Scale increments should not be skipped. In substitution loading, product in the scale should only be to the load previously verified – as close as possible.
Over 50% of the scale capacity, strain test loading is permitted. An unknown quantity of product is charged and the incremental weight indication with the test weights is verified. At least two points should be tested in this portion of the scale – through typical range of use.
Scale accuracy requirements (ASTM C94) is the greater of
● ±0.15% of scale capacity (governs at the lower end)
● ±0.4% of applied test load
● If it’s not accurate the scale has to be adjusted.
Maintenance tolerances in accordance with Handbook 44 are stated on the basis of scale divisions (min grad) but are generally more restrictive than those in C94 that state tolerances based on applied load or scale capacity.
A copy of the scale verification data sheets are available and kept with each scale test vehicle and technician. Calibration, test and certification are provided electronically to indicate the details of the test loads used, test load increments, load indications and load error.
Definitions of load testing, discussions and numerical examples of the scale accuracy verification are available in the NRMCA Plant Inspector Guide.
Scale accuracy should be verified anytime the plant is moved (portable plants), maintenance activities on the plant impact the weighing systems, or when there is a concern on scale accuracy determined from the batchman operating the batching process or the quality of concrete.
Water meter accuracy verification
Pioneer Scale provides testing of most water meter and scale water discharge systems to NRMCA standards and applicable testing requirements. Accuracy of water meters should be verified at least once every six months. Pioneer Scale maintains electronic documentation of these verifications available through our website for review by the inspector.
Accuracy of water meters is the same as that required for batching accuracy – ±1.5%.
Measure accuracy using a large enough volume of water – at least 50 gallons.
Discharge water into a 55-gallon drum and weigh the quantity – can be suspended from the cement scale.
Perform this at least 3 times to obtain an average.
Some discharge 200+ gallons into a truck mixer if truck scales are available. After weighing the truck, water is discharged and the truck is weighed empty to determine water in the drum.
Discharge water into a calibrated container of at least 50 gallon capacity. Calibrations on the container should be accurate enough to measure to within the required accuracy tolerance.
When water is measured by weight, accuracy requirements for scales apply.
More details and numerical examples of water meter accuracy verification are in the NRMCA Plant Inspector Guide.
Cardinal Scale’s THTC series load cell kits are designed for use with suspended hoppers and tanks. Sixteen models are available in 3 or 4 cell arrays all featuring environmentally-sealed stainless steel load cells