Every result from a universal testing machine depends on the accuracy of its force measurement. Auditors, customers and accreditation bodies therefore ask to see a valid calibration certificate. In India, force verification of tension and compression testing machines is carried out to IS 1828 (Part 1), which corresponds to ISO 7500-1. This article explains what the verification covers and how to read the certificate you receive.
What the verification checks
Verification compares the force indicated by the machine with the true force measured by a calibrated reference device, usually a proving ring or a load cell-based force proving instrument that is itself traceable to national standards. The check is done at several force levels across each range to be verified, in a series of runs.
From these readings, the calibration laboratory calculates parameters that describe how well the machine measures force, including:
- Relative accuracy error: how far the indicated force is from the true force, on average.
- Relative repeatability error: the spread between repeated runs.
- Relative reversibility error: the difference between increasing and decreasing force, where this is verified.
- Relative zero error: how well the indicator returns to zero.
- Relative resolution: the smallest change the indicator can show, relative to the force.
Machine classes
Based on these errors, each verified range is assigned a class. The standard defines classes 0.5, 1, 2 and 3. Class 1, for example, means the relative errors are within the limits the standard sets for that class, with accuracy error of the order of ±1%. Your test standard tells you which class you need. Metal tensile testing to IS 1608 / ISO 6892-1, for instance, generally calls for a machine of class 1 or better over the range used.
A machine may achieve different classes in different parts of its range. The class applies only between the lowest and highest force levels actually verified. Below the lowest verified force, the machine has no stated class, and results there should not be reported as compliant.
Reading the certificate
| Item on certificate | What to check |
|---|---|
| Standard used | IS 1828 (Part 1) or ISO 7500-1, with edition |
| Machine identification | Make, model and serial number match your machine |
| Mode | Tension, compression or both, as you use them |
| Ranges verified | Cover the forces you actually test at |
| Class achieved | Meets the class required by your test standards |
| Reference device | Identified, with its own valid traceable calibration |
| Uncertainty | Stated where required by your accreditation |
| Date and conditions | Temperature recorded; date within your calibration interval |
Other parts of the machine
Force verification does not cover everything. Extensometers are verified separately to ISO 9513. Depending on your tests, you may also need checks of crosshead displacement, speed, or the alignment of the load train. If you test at specified strain rates, the accuracy of rate control can matter too.
When to recalibrate
- At the interval required by your quality system or accreditation body, often annually
- After the machine is moved or reinstalled
- After repairs or replacement of a load cell, pressure transducer or indicator
- After a software or electronics upgrade that affects force measurement
- Whenever check tests or reference specimens suggest drift
Between calibrations
Good labs do not wait a full year to discover a problem. Simple intermediate checks help, such as testing a reference specimen with known results, comparing with another machine, or using an in-house check device. Recording these checks gives evidence that the machine remained within tolerance between formal verifications.
Calibration, repair and retrofit services
Akuracy provides calibration of material testing machines; see also machine calibration services. Where a machine cannot reach the class you need because of worn components or outdated measurement, upgradation and retrofit can be considered.
Preparing for a calibration visit
A calibration goes more smoothly when the machine is ready. Before the visit, clean the machine, check the hydraulic oil level and leaks, make sure the crosshead moves freely over its full travel and that the indicator or software is working. Let the machine warm up as recommended. List the ranges and modes you actually use, and the lowest force you need to report, so the calibration laboratory can plan the force steps.
Keep a copy of the previous certificate available. Comparing the new results with the last ones shows whether the machine is drifting, even if it still meets its class. A steady trend in accuracy error is an early warning that a transducer, seal or electronic component may need attention.
After calibration, update the calibration label, file the certificate in your quality records and, where the software applies correction factors, confirm that any new values have been entered correctly. Run a check test on a reference specimen to confirm that everything is in order before you restart reportable testing.
Accreditation context
Laboratories accredited to ISO/IEC 17025 usually need calibration from a laboratory that is itself accredited for force calibration, with traceability and stated measurement uncertainty. Check what your accreditation body requires before booking.
Frequently asked questions
What class does my UTM need?
Your test standard sets it. Many metal tensile tests require class 1 or better over the range used. Check the standard and your customer's requirements.
Is a calibration valid below the lowest verified force?
No. The class applies only between the lowest and highest verified force levels. Ask for verification down to the smallest force you need.
Does force calibration cover the extensometer?
No. Extensometers are calibrated separately to ISO 9513, with their own accuracy classes.
Request a quote
Akuracy (Ratnakar Enterprises) can advise on a configuration for your materials, standards and test volume. Share your specimen details and the standard you test to, and our team will reply with a suitable option. Request a quote online or message us on WhatsApp at +91 94038 91579.