Laboratories and inspection companies verify their tensile and compression testing machines with reference force transducers. These references must themselves be calibrated according to ISO 376 and assigned to a class. MASSLAB calibrates such reference instruments in tension up to 500 kN and in compression up to 4,000 kN.
ISO 7500-1 describes how the force measuring system of a uniaxial tensile or compression testing machine is calibrated and assigned to a class. Reference force measuring instruments are needed for this. ISO 376 specifies how these references are themselves calibrated and which requirements apply to each class.
In an ISO 376 calibration the transducer is loaded in a fixed set-up: several measurement series in different mounting positions, with increasing and decreasing steps. The results yield, among other things, the repeatability without and with rotation, the interpolation error, the zero error, the reversibility error (hysteresis) and, where applicable, the creep. Together with the measurement uncertainty, these determine the class.
ISO 376 defines four classes. The lower the number, the stricter the requirements:
| Class | Typical application |
|---|---|
| 00 | References for calibrating other force measuring instruments |
| 0.5 | Verification of tensile and compression testing machines in class 0.5 |
| 1 | Verification of tensile and compression testing machines in class 1 |
| 2 | Verification of tensile and compression testing machines in classes 2 and 3, and industrial checks |
Which class we can assign depends on the properties of your transducer and on the measurement uncertainty of our set-up. We discuss this with you in advance.
ISO 376 allows a maximum validity of 26 months. Many laboratories opt for annual calibration, so that the reference is always well within its validity when the testing machines are verified. After an overload, a fall or a repair, the transducer must be recalibrated before it is used as a reference again.
The transducer is loaded several times up to the maximum force. This stabilises the measuring element and settles the mounting parts.
We measure in at least eight force steps across the range, in several rotational positions of the transducer relative to the bench, for example 0°, 120° and 240°. This shows how sensitive the transducer is to mounting.
Where required for the classification, we also measure with decreasing force. This yields the reversibility error (hysteresis).
From the measurement series we calculate the repeatability, the reproducibility, the interpolation error, the zero error and the measurement uncertainty. Together these determine the class in which the transducer falls and over which force range.
You receive a certificate with all measurement results, the interpolation equation for converting the reading into force, the class and the range for which it applies.
An ISO 376 calibration includes an equation describing the relationship between reading and force, usually a first, second or third degree polynomial. With this equation you convert any reading within the calibrated range into force, including between the measuring points. When verifying a testing machine according to ISO 7500-1, you use this equation to determine the actual force that you compare with the machine's reading.
Select the direction and the measuring range of your reference transducer. State the required class when you submit your request.
Price on request · state make, type and requirements (display, number of steps, class) in the remarks of your request.
According to ISO 7500-1, the reference must be of at least the same class as the class in which the tensile or compression testing machine is classified. For a machine in class 1, a reference in class 1 or better is therefore required.
We calibrate the reference instruments with which you verify your tensile and compression testing machines, on our benches at our Elspeet facility. For questions about verifying a machine on site, please contact us.
For an ISO 376 calibration we would rather you did not, because the standard assumes the complete measuring chain. If you send only the transducer, we use our own measuring amplifier and the result applies to the transducer with that amplifier.
At low forces, resolution and scatter weigh relatively heavily. As a result, a transducer may achieve a better class in the upper part of its range than in the lower part. The certificate therefore states the force range for which the class applies.
Book your calibration online via the customer portal or request a quote. We collect your force measuring instruments and calibrate them on our tensile and compression benches in Elspeet.