In a concrete batching plant typically all of the ingredients that go into the concrete are measured by weight with certified digital scales. As you can imagine, those scales that are batching and mixing of concrete, need to be calibrated on a regular basis in order to maintain accurate weighing.
At Central Carolina Scale, we have checked and tested ready mixed concrete hopper scales for many years. Over all these years, concrete batching plant calibration procedures have evolved and we have also made changes to adapt to new guidelines and requirements. Whether it’s asphalt plant calibration or concrete batching, we have many years of experience repairing and calibrating these digital scale systems throughout central and eastern North Carolina.
While we don’t sell Command Alkon or Mettler Toledo indicators, we have worked on many systems that include those units. Popular instruments that we offer for sale include the Cardinal 205.
For concrete producers looking to achieve National Ready Mixed Concrete Association (NRMCA) Plant Certification, they need to adhere to the Guidance to Concrete Producers maintaining plant certification. This includes a list of Primary Requirements, Plant Inspector Guide and most importantly to us, the Scale Accuracy Verification guide.
Scale Accuracy Verification
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.
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).
Test weights should be certified to be accurate to 0.01% of their indicated load within the last two years. This is typically not a problem with commercial scale companies.
Scale checks should be done through range of use of the scales. Scale companies may only verify it 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 should be obtained to indicate details of the test loads used, test load increments, load indications and load error. A certificate just indicating a scale is OK is not acceptable.
Definitions of load testing, concrete batching plant calibration format, discussions and numerical examples of the scale accuracy verification are available in the NRMCA Plant Inspector Guide.
Batching Plant Calibration Frequency
Accuracy checks of measuring devices (scales, water meters, admixture dispensers and moisture probes) should be performed at least once every 6 months.
State DOTs may have a requirement for these to be performed more frequently. The requirement with the greater frequency governs. Documentation of these verifications should be maintained and made available to the inspector during the plant inspection.
Also, 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 batch man operating the batching process or the quality of concrete.
For customers throughout the central part of North Carolina, adhering to the NRMCA requirements can be accomplished with the help of the experienced service technicians of Central Carolina Scale located in Sanford, NC.
We have the trucks to handle these requirements and we have the large certified test weights (and small weights too) needed to accurately test your digital scales.
And we also stock a huge amount of replacement load cells, digital weight indicators, load cell cable, and other scale accessories to keep your batching plant up and going year round.
Contact our service department today (919) 776-7737 or fill out the RFQ button on our website for additional information.