Deadweight Force Machine Stability: Long-Term Reliability in Force Calibration
Morehouse has a new paper on Deadweight Force Machine Stability titled "Deadweight Primary Standards: Best Practices and Their Associated Risks for Stability Determination in Compliance with ISO/IEC 17025"
Overview
Deadweight force machines are considered the pinnacle of force calibration standards, delivering the lowest possible measurement uncertainties and unmatched long-term reliability. This comprehensive technical paper by Henry Zumbrun, President of Morehouse Instrument Company, presents detailed insights into the construction, performance, and enduring stability of deadweight systems. Through extensive data from institutions like NIST and NPL, the paper establishes the superiority of deadweight machines in ensuring metrological traceability and low uncertainty for decades. It also outlines best practices for minimizing risk, maintaining compliance with ISO/IEC 17025 and ILAC G24, and extending recalibration intervals.
Why Deadweight Force Machines Matter
In the hierarchy of force calibration equipment, deadweight force machines represent the most accurate systems available. They apply force directly through known masses without intermediate mechanisms like levers or hydraulics. According to ASTM E74, these systems are defined as devices where “force is applied directly without intervening mechanisms such as levers or hydraulic multipliers, whose mass has been determined by comparison with reference standards traceable to the International System of Units (SI).”
Due to this direct application of force and precise correction for air buoyancy, local gravity, and material density, deadweight machines regularly achieve expanded uncertainties below 0.002 %—often as low as 0.0008 % of the applied force. This positions them as the only suitable calibration method for high-accuracy classifications such as ISO 376 Class 00 and ASTM E74 Class AA.
The Case for Long-Term Stability
A major theme of the paper is the stability of deadweight force machines over extended periods. Unlike other calibration systems that require frequent recalibration due to wear and drift, deadweight machines—when properly designed and maintained—can operate with remarkable stability for 20–30 years or more.
Multiple studies from the National Institute of Standards and Technology (NIST) and the National Physical Laboratory (NPL) confirm this claim. For example, NIST found that weights from their 498 kN machine showed an average mass change of less than 1 part per million (ppm) over a 24-year span, well within acceptable uncertainty limits. Similarly, NPL observed that stainless steel weights used in their machines exhibited negligible systematic change—less than 0.2 ppm over ten years.
These findings emphasize that deadweight force machine stability is not a theoretical benefit—it’s a measured, empirical fact supported by decades of high-precision data.
Risks of Unnecessary Recalibration
Despite their long-term reliability, deadweight machines are sometimes recalibrated too frequently. The paper argues that dismantling these systems can introduce more risk than reward. Handling and transporting large calibrated weights may lead to mechanical damage, contamination, or surface changes that increase uncertainty rather than reduce it.
A case study included in the paper highlights NIST’s teardown and rebuild of their 4.45 MN deadweight force machine—the world’s largest. Even after decades of use, the weights had not meaningfully changed in mass. However, the restoration effort required significant labor, heavy lifting equipment, and costly downtime. This reinforces the idea that deadweight force machine stability should not be disrupted unless necessary and backed by data.
Best Practices to Preserve Stability
To preserve the long-term performance of deadweight systems, the paper offers a number of best practices:
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Use of corrosion-resistant materials: Stainless steel or plated weights are preferred, as they resist environmental degradation.
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Avoiding porous cast iron: Porous materials are more likely to absorb contaminants, increasing mass drift over time.
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Environmental controls: Maintaining stable temperature, humidity, and air density helps minimize buoyancy correction errors.
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Automated weight application: Eliminating hydraulic or manual lift mechanisms reduces the risk of shock loads and improves repeatability.
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Statistical Process Control (SPC): Regular monitoring using control charts ensures early detection of drift or anomalies.
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Intra-laboratory comparisons: Using the same load cell across multiple machines provides confidence in performance without disassembly.
These measures align with ISO/IEC 17025 and ILAC G24 guidelines, both of which stress the importance of metrological traceability, intermediate checks, and data-driven justification for calibration intervals.
Compliance with ISO/IEC 17025 and ILAC G24
To maintain international accreditation, labs must establish a structured process to justify calibration intervals. ILAC G24 explicitly requires that recalibration intervals be based on observed stability and usage, not arbitrary timelines. The paper explains how deadweight force machine stability supports extended calibration intervals and outlines the documentation methods—such as control charts, interlaboratory comparisons, and in-situ checks—that demonstrate continued compliance.
Clause 6.4.10 of ISO/IEC 17025:2017 mandates intermediate checks, while Clause 7.7.1 requires use of statistical tools to validate ongoing measurement accuracy. This reinforces the argument that internal verification practices—not routine disassembly—are the most effective means to ensure traceable, low-uncertainty measurements over time.
Monitoring Stability Without Teardown
To avoid disturbing stable machines, the paper promotes non-invasive monitoring tools. For example:
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In-situ checks: Repeating force application using the same weight set verifies system consistency under actual conditions.
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Control charts: Tracking a load cell’s output across multiple machines reveals variations at the ppm level.
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Cross-checking: Comparing results between different machines allows labs to identify discrepancies without disassembling equipment.
These techniques enable labs to track system performance over time, making it possible to extend calibration intervals without compromising accuracy or compliance.
Conclusion: A Gold Standard in Force Calibration
The paper concludes that deadweight force machine stability is not only well documented but critical to modern calibration practice. When maintained correctly, these machines outperform all other force standards in both uncertainty and longevity. They eliminate the need for frequent recalibration and reduce lifecycle costs, while satisfying international standards for traceability and reliability.
Ultimately, the deadweight force machine remains the gold standard in force measurement—not just for its unmatched precision, but because it offers a level of long-term stability that no other system can rival.
As one experienced United Kingdom Accreditation Service (UKAS) assessor aptly said, “The worst thing you can do to a working deadweight machine is take it apart.” With proper design, maintenance, and monitoring, these machines remain the most accurate and dependable force calibration standards.
The new paper can be downloaded Deadweight Force Machine Stability and Best Practices
More Information about Morehouse
We believe in changing how people think about Force and Torque calibration in everything we do, including discussions on deadweight force machine stability.
This includes setting expectations and challenging the "just calibrate it" mentality by educating our customers on what matters and what may cause significant errors.
We focus on reducing these errors and making our products simple and user-friendly.
This means your instruments will pass calibration more often and produce more precise measurements, giving you the confidence to focus on your business.
Companies around the globe rely on Morehouse for accuracy and speed.
Our measurement uncertainties are 10-50 times lower than the competition, providing you with more accuracy and precision in force measurement.
We turn around your equipment in 7-10 business days so you can return to work quickly and save money.
When you choose Morehouse, you're not just paying for a calibration service or a load cell.
You're investing in peace of mind, knowing your equipment is calibrated accurately and on time.
Through Great People, Great Leaders, and Great Equipment, we empower organizations to make Better Measurements that enhance quality, reduce risk, and drive innovation.
With over a century of experience, we're committed to raising industry standards, fostering collaboration, and delivering exceptional calibration solutions that build a safer, more accurate future.
Contact Morehouse at info@mhforce.com to learn more about our calibration services and load cell products.
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