Testing standard
Standard Practices for Force Calibration and Verification of Testing Machines
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ASTM E4 covers the force calibration and verification of tension and compression testing machines using standard calibration devices. It works to a single baseline of ±1 % of the applied force over the verified range, rather than to a ladder of accuracy classes.
Standard calibration devices, traceable to the SI through a National Metrology Institute, are installed in the machine's own load train and aligned as a specimen would be. Force is applied slowly through the range while the applied and indicated forces are recorded, and the series is repeated as the practice requires. The relative error is calculated at each point and checked against a single baseline of ±1 % of the applied force. The outcome is a verified range — the interval of force over which that baseline was actually demonstrated — rather than a class.
Whether the force the machine reports is the force it is applying, and over what span that holds. The practice is used by machine users, manufacturers and calibration providers alike, and it covers machines that have no force-indicating system of their own as readily as those that do. Its most useful feature is an unusually candid one: the standard states that, combined with measurement uncertainty, a verified machine may deviate from the true force by more than ±1,0 %. Verification demonstrates a result under defined conditions; it does not abolish uncertainty, and a certificate is not a guarantee.
ASTM asks whether the machine is good enough. ISO asks how good it is. That is the structural difference.
Verification is bounded by a range. Outside the verified range the practice says nothing about the machine, and a certificate that does not state the range cannot be acted on.
(indicated force − applied force) / applied force × 100
A percentage of the applied force, so the absolute tolerance shrinks as the force falls — which is why a large frame verified to ±1 % is not a good instrument for a very small load.
The interval of force over which the ±1 % was demonstrated
Reported with the result. It is the half of the certificate people forget to read.
Combined with measurement uncertainty, the true deviation may exceed ±1,0 %
Stated in the practice itself. It is a statement about what verification can and cannot promise, not a loophole.
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.
SpecificationsAccepting a calibration device in the alignment a specimen would occupy, and resolving force finely enough that the ±1 % is not limited by the display. Because the tolerance is a percentage of the applied force rather than of full scale, it shrinks in absolute terms as force falls — one per cent of a hundred kilonewtons is a kilonewton, one per cent of twenty newtons is two hundred millinewtons. Holding the baseline at the bottom of a large frame's range is a different problem from holding it at the top, which is the practical argument for sizing a load cell to the force rather than to the frame.
Reading the tolerance and not the range, which is the commonest failure and the one that quietly invalidates low-force work. Treating verification as a guarantee, when the practice itself says the combined deviation can be wider. Citing E4 where the test method named ISO 7500-1, or the reverse — they are not interchangeable on a report even though both deliver traceable force. And letting the certificate expire in a folder while the machine keeps running, since nothing about a drifting load cell announces itself in the data.
| ASTM E4 | ISO 7500-1 | |
|---|---|---|
| Result | Verified, or not | A class: 0,5, 1, 2 or 3 |
| Tolerance | ±1 % baseline | The class number, in per cent |
| Reference devices | Traceable via an NMI | Force-proving instruments to ISO 376 |
| Also covers | Force calibration and verification | A general inspection of the machine and its accessories |
They answer different questions about the same machine, and neither substitutes for the other on a report. A laboratory serving both ASTM and ISO methods normally holds both certificates.
It is the ASTM practice for the force calibration and verification of testing machines, using standard calibration devices. It applies to tension or compression machines, or both, static or quasi-static, and explicitly includes machines that have no force-indicating system of their own. The current edition is ASTM E4-24, approved in September 2024.
Structurally. ASTM E4 works to a single baseline — ±1 % of the applied force over the verified range — and returns a yes or no. ISO 7500-1 hands out a class from 0,5 to 3, where the class number is the tolerance in per cent, so it tells you how good the machine is rather than only whether it is good enough. Test methods name one or the other, and a laboratory serving both families usually holds both certificates.
No, and the practice says so directly: combined with measurement uncertainty, a machine verified under it may deviate from the true force by more than ±1,0 %. That is an unusually candid statement for a standard, and it is worth quoting to anyone who treats a calibration certificate as a guarantee. Verification demonstrates a result under defined conditions; it does not eliminate uncertainty.
It is the interval of force over which the ±1 % was actually demonstrated, and it is the half of the certificate people forget to read. Outside that range the practice says nothing at all about the machine. Running a small specimen on a large frame very easily puts the test below the verified range, and nothing in the data will show it — which is why the range is worth marking on the machine itself rather than leaving it in a folder.
Because the tolerance is proportional to the force being applied, so it shrinks in absolute terms as the force falls. One per cent of 100 kN is a kilonewton; one per cent of 20 N is 0,2 N. Holding that at the bottom of a large frame's range is a different engineering problem from holding it at the top, which is why load cells are chosen for the force rather than for the frame.
Yes, at the 2024 edition. E4-20 and its predecessors were titled *Standard Practices for Force Verification of Testing Machines*; E4-24 is *Force Calibration and Verification*. ISO 7500-1 made the same move at its 2015 edition, promoting calibration alongside verification — the two families changed in step, and older references will still carry the old titles.
Yes. The practice explicitly covers machines that may or may not have force-indicating systems. In that case the verification establishes the relationship between the machine's applied force and the reference device rather than checking a displayed number, which is how dead-weight and lever-operated machines are handled.
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 | Every range the machine will be used over | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | Traceable to the SI through a National Metrology Institute | ISO 7500-1 Class 0.5, verified to ASTM E4, DIN 51221 and BS 1610 |
| Gripping | Standard calibration devices placed in 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.
