Testing standard
Metallic materials — Calibration and verification of static uniaxial testing machines — Part 1: Tension/compression testing machines — Calibration and verification of the force-measuring system
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ISO 7500-1 covers the calibration and verification of the force-measuring system on tension and compression testing machines. It assigns a class — 0,5, 1, 2 or 3 — where the class number is the maximum permissible relative accuracy error in per cent. The class belongs to a force range, not to the machine as a whole.
Force-proving instruments traceable through ISO 376 are installed in the machine's own load train, aligned as a specimen would be, and force is applied slowly through each range while the indicated force is compared against the true force. The series is repeated to establish repeatability, and zero readings and the resolution of the indicating system are recorded. Alongside this calibration sits a general inspection of the machine, including the accessories used to apply force. Each force range is then assigned a class — 0,5, 1, 2 or 3.
Whether the number on the screen is the force in the specimen. Almost every test method on this site names ISO 7500-1 Class 1 as its accuracy requirement, and this is the standard behind that phrase: the class number is the maximum permissible relative accuracy error in per cent, so Class 1 means ±1 % of the indicated force. Accuracy alone does not earn the class — repeatability, zero error and resolution each carry their own limit, and a machine can meet the accuracy figure and fail on repeatability. The error is expressed as a percentage of reading, not of full scale, which is why holding a class at a tenth of capacity is much harder than at full capacity.
The class number is the permitted error in per cent. That is the whole scheme, and it is why a Class 1 machine is a machine good to one per cent.
There is no such thing as a Class 1 machine in the abstract. There is a Class 1 range on a machine, with a lower limit, and testing below that limit is testing outside the certificate.
q = (Fi − F) / F × 100
Expressed as a percentage of the true force, not of full scale. That distinction is why a percentage-of-reading specification gets tighter in absolute terms as the force falls, and why low-force work is hard.
The class number is the maximum permissible relative accuracy error, in per cent
Class 1 permits ±1 %, Class 0,5 permits ±0,5 %. Repeatability, zero error and resolution each carry their own limit for the same class.
The range it applies over
Always read the certificate's lower limit alongside the class. The two are meaningless apart.
The force-measuring system is what this standard classifies. A frame carries several interchangeable cells, and each one is calibrated and classified over its own range rather than inheriting the frame's certificate.
SpecificationsBeing measured honestly. In practical terms that means a load train that accepts a proving instrument in the same alignment a specimen would occupy, an indicating system with enough resolution that the class is not resolution-limited, and grips, platens and couplings in good enough condition to pass the general inspection — they are explicitly in scope, and a worn coupling puts bending into the load train that no amount of load cell accuracy corrects. Where a frame carries several interchangeable cells, each is calibrated over its own range rather than inheriting the frame's certificate.
Quoting a class without its range, which is the single commonest error and makes the certificate unreadable. Testing at forces below the classified lower limit — easy to do when a large frame is used for a small specimen, and invisible unless someone checks. Treating a static certificate as covering dynamic work: the standard states that its values are not necessarily valid for high-speed or dynamic testing, so a fatigue frame certified under it was certified standing still. And assuming a good force certificate implies good results, when a modulus also depends on an extensometer certified under a different standard entirely.
| ISO 7500-1 | ASTM E4 | |
|---|---|---|
| Structure | Classes 0,5 / 1 / 2 / 3 | A single ±1 % baseline, no class ladder |
| Reference instruments | To ISO 376 | Traceable to SI through a national metrology institute |
| Scope of verification | Machine inspection plus force calibration | Force calibration and verification |
| Result | A class, per range | Verified, or not |
Both give traceable force. ISO tells you how good the machine is; ASTM tells you whether it is good enough. Test methods name one or the other, and a laboratory serving both usually holds both certificates.
It is the ISO standard for calibrating and verifying the force-measuring system of tension and compression testing machines. It covers a general inspection of the machine, including the accessories used to apply force, together with a calibration against force-proving instruments, and it assigns an accuracy class. The current edition is ISO 7500-1:2018, the fifth.
That the maximum permissible relative accuracy error is ±1 % of the indicated force. The class number is the tolerance in per cent, which makes the scheme unusually easy to read: Class 0,5 is half a per cent, Class 2 is two. But accuracy alone does not earn the class — repeatability, zero error and resolution must each also fall within that class's own limits, so a machine can hit the accuracy figure and still fail.
Strictly, no machine is. The class is assigned to a **force range**, not to a machine, and this is the most commonly misread thing in the standard. A 100 kN frame might be Class 1 from 2 kN to 100 kN and carry no classification below 2 kN. Testing a 500 N specimen on it is then testing outside the certificate, however impressive the certificate looks. Always read the class together with the lower limit of the range it covers.
Because that is what the standard specifies, and it has a consequence worth understanding. A percentage-of-reading tolerance stays proportionally constant but shrinks in absolute terms as the force falls, so holding ±1 % at a tenth of capacity is far harder than at full capacity. It is precisely why low-force work — catheters, closures, fine films — needs a load cell sized for the force rather than for the frame.
Not while it is running. The standard says its calibration values are not necessarily valid for high-speed or dynamic testing. A frame certified under it has been certified standing still, so a dynamic test may deliver a force meaningfully different from the one commanded. That is why dynamic force verification at the test frequency is a separate exercise, and why fatigue standards ask for it.
Yes, in the verification part. The standard requires a general inspection of the testing machine including its accessories for force application, so the grips, platens and couplings are in scope rather than the load cell being treated in isolation. A worn grip or a sloppy coupling can put bending into the load train that no amount of load cell accuracy will fix.
Different standard. ISO 9513 covers the calibration of extensometer systems used in uniaxial testing. Force and extension are certified separately, which matters because a modulus depends on both — a machine with an immaculate force certificate and an uncalibrated extensometer will still produce a wrong modulus.
Dak verifies against whichever standard the method names, and where a class applies our frames sit a class tighter than it asks.
| The method asks for | Dak supplies | |
|---|---|---|
| Capacity | Whatever the machine's ranges are — the point of the exercise is to classify each of them | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | Force-proving instruments to ISO 376 | ISO 7500-1 Class 0.5, verified to ASTM E4, DIN 51221 and BS 1610 |
| Gripping | Force-proving instruments to ISO 376, applied through the machine's own load train | Wedge, vice-action, pneumatic and hydraulic grips, built to the specimen |
| Environment | 23 ± 2 °C standard laboratory atmosphere | 3009 series chambers, −150 °C to +400 °C — temperature only |
This page describes the method as practised. The governing text is the current edition from the issuing body. Tell us what you are testing and we will answer with the machine, the fixture and a quotation.
