Increasing Reliability by Regular Measuring Equipment Calibration

Almost every company is certified according to the relevant quality standards (probably the most well-known is EN ISO 9001) – a prerequisite that must be fulfilled to be eligible as a supplier to a wide range of sectors in production and service. The standard also stipulates that measuring equipment must be traceable.

What Is Traceability?

Traceability means that a continuous chain of calibrations capable of being traced to the national standard must be proven and the measurement uncertainty of each calibration is known and documented.

In practice, the national metrological institute (in Germany, the Physikalisch-Technische Bundesanstalt, PTB) cannot calibrate all measuring equipment used in industry due to capacity reasons. Because of this, accredited calibration laboratories are connected to the PTB via the national accreditation body of the Federal Republic of Germany (DAkkS). This implies that high-precision comparison measurements between the DAkkS laboratories and the PTB's calibration machines ensure that a Newton is consistently a Newton or a Newton meter always a Newton meter. 

HBM's calibration laboratory has been one of the world's most renowned calibration laboratories since 1977 and has been accredited by DAkkS.  It is common practice for customers to send their force transducers or torque transducers to HBM for calibration in the interest of low measurement uncertainty. The requirement for traceability is, in this way, fulfilled: The HBM calibration laboratory is connected to PTB (accuracy of the measured values and measurement uncertainty are known and documented) and the sensor has been calibrated with known measurement uncertainty (documented in the calibration certificate) at HBM.

For measuring equipment that cannot be dismantled and shipped due to the installation situation or the cost and time it would entail, another solution is applied that meets the quality standards required for audits:  Using precise reference transducers (transfer standards), calibrations can also be performed on site, installed in the same conditions as the transducers in operation. HBM offers force and torque transducers for this calibration task, which not only offer extremely small measurement uncertainty but are also robust and can be combined with amplifier modules specially designed for this purpose to form economical reference measuring chains. 

The U15 has proved its worth in the force range, covering measuring ranges up to 2.5 MN. The sensor is suitable for tensile and compressive forces. It is made entirely of stainless steel, hermetically sealed, and meets the requirements of the IP67 degree of protection. According to ISO 376, the class accuracy is 0.5*. The U15 measuring range guaranteed by HBM is exceptional: Class 0.5 is maintained from 10% of the maximum measuring force up to the full-scale value.

The C15 is available for applications that require compressive forces only. It complies with the requirements of Class 00 per ISO 376. Both C15 and U15 provide an unusually high output signal. With more than 4 mV/V, the input levels of the amplifier modules can be fully utilized, resulting in excellent resolution. 

For the torque range, the TB2 is recommended as a reference transducer. Owing to an optimized design, the measuring body made of one piece in the form of a closed, hollow body, the measuring system is protected against environmental influences such as moisture. Because of this, and the IP67 degree of protection, the sensors are ideally suited for calibrations in industrial environments. Excellent performance can be achieved in combination with a suitable precision measuring instrument. For the range from 10 to 100 % of the rated torque, the highest class, 0.05, as per DIN 51309 is achieved.  

These robust sensors offer practical accuracy and can also be utilized as master sensors in calibration systems. The U15 and C15 are also available in double-bridge versions. This allows using one measuring bridge as a reference source and the other output for machine control. 

HBM offers amplifier modules that are ideally matched to the TB2, C15, and U15 reference sensors in terms of their input range, carrier frequencies, and supply voltages. The QuantumX MX238B and MX430B modules provide low measurement uncertainties and are therefore an optimal match for the TB2 and C15 or U15. The amplifiers are compact and lightweight and thus perfectly suitable for mobile use. With additional modules from the QuantumX series, they can simultaneously acquire further physical variables such as temperature or strain at any time. 

Provided that a DAkkS calibration is performed, modern factory standards ensure traceability to the national standard. Assessing measurement uncertainty enables you to place the last piece of the puzzle. Along with the measurement uncertainty calculation, the reference sensor, and the calibration, the required traceability to the sensor on site is accomplished. HBM also helps you assess measurement uncertainty by offering seminars on ways to calculate the measurement uncertainty of force and torque measuring chains. 

*Class accuracy 0.5 according to ISO 376 does not signify that the measurement error is 0.5% of the measured value. In fact, ISO 376 distinguishes 4 different quality classes regarding the transducers’ ability in terms of repeatability, hysteresis, creep, and other parameters. The transducers with the smallest uncertainty belong to Class 00, followed by 0.5, 1, and finally 2.