Optimal Results Under Pressure: A Digital Sensor at the Core of a New High-Tech Compression Tester

The Hohenstein Laboratories are the only test centre authorised by the Gütezeichengemeinschaft Medizinische Kompressionsstrümpfe e.V. (quality mark association for medical compression hosiery). This organisation gives companies the right to use the quality mark for medical compression hosiery. The RAL Quality Certification Mark is based on tests carried out in accordance with RAL-GZ 387/1 and /2 and is one way of verifying the medical benefit of the hosiery. In addition to medical products, compression textiles for other purposes can also be tested. On clothing, the sleeves, legs, torso and trouser sections can be tested. This allows the compressive effect, for example of sports-wear or shapewear, to be determined.

From his tower room at the Hohenstein castle, Florian Girmond enjoys the view of the entire HOHENSTEIN Group facilities, which, since its establishment in 1946, has had its headquarters on the premises of the Renaissance castle in Bönnigheim, Germany. More than 600 of the company's worldwide staff of over 1,000 work at the German headquarters. The independent textile testing institute is run by the third generation of the Mecheels family and it holds important accreditations and certifications as a test facility. The laboratories’ range of testing and service offerings includes the testing of fit and workmanship of products, the development of functional wear, and the certification of personal protective equipment (PPE). Compression tests are primarily performed on medical textiles and sportswear, which involve the measurement of the pressure and pressure gradients that a textile exerts on a body part.

In the beginning of the 1980s, the HOHENSTEIN experts developed the HOhenstein SYstem (HOSY) for the testing of the impact of medical compression wear. Meanwhile, HOSY has established the standard in compression wear testing and has become the mandatory testing system for compliance with different standards such as RAL and DIN. HOHENSTEIN utilizes the system for its in-house textile testing services and also sells it to customers.

“HBK technology has always been integrated into the HOSY,” explains Gunther Kretzschmar, who has worked for HBK Sales and has also been the contact for Hohenstein for over 20 years.

HOHENSTEIN has now enhanced its testing system to meet an increased number of individual customer requirements and to make the HOSY compatible with IIoT (Industrial Internet of Things) applications. In doing so, the company has outlined specific requirements for the digital sensors that measure force. “The sensors must be very precise to enable us to obtain accurate measurement results,” explains Florian Girmond. Additionally, the individual sensors need to be narrow to ensure that a maximum width of 5 centimeters, including the mount for the textile jig, is not exceeded. It is also essential that the sensors have a high overload stability to avoid damage to the sensors in the event of unexpected load peaks. The sensors are compatible with the CAN bus technology applied in the testing system due to their CAN bus interface. Hence, they are a perfect fit for the new HOSYcan and can transfer the measured data quickly and efficiently to the software, which, in turn, calculates and visualizes the compression. “FIT5A is the digital sensor that matches our high demands,” says Florian Girmond.

There are 20 tension testing units that have been arranged parallelly at the core of the new HOSYcan that uses the CAN bus technology. Textile tubular knitwear such as medical compression stockings or sportswear tops is clamped onto these units and extended in an absolutely non-destructive way. When the desired perimeter, between eleven and 150 centimeters, has been reached, the HOSYcan measures the required forces and, based on the measured values, calculates the compression that the textile exerts on the body. The results are listed in a table and, additionally, represented graphically in a compression profile. “We determine the tension force along the entire length of the test sample in sections of five centimeters each. This allows the calculation and visualization of pressure and pressure gradients at any position,” explains Florian Girmond. The precise digital sensor. with integrated electronics, is suitable for both static and dynamic measurements.

The FIT5A digital sensors have been integrated into the tension testing units, where they acquire tension force.

The testing unit applies CAN bus technology to simultaneously transmit the measured values provided by the 20 tension testing units to the analysis software. “We knew right from the start that we would use a digital sensor with the CAN bus technology,” says Florian Girmond. “It is less sensitive to the electromagnetic fields emitted; for instance, those from nearby electric lines or radio waves.” The new CAN bus system enables the measured data to be processed directly in the sensor’s electronics and transmitted to individual channels. Additionally, the FIT5A swiftly measures and boasts of an overload protection of 1,000 percent of its capacity. This is important since the measurement is determined by the strain that the fabric is subject to and not by the force that is applied to achieve this. For this reason, the sensor’s overload range is achieved with certain textiles such as inelastic knee bandages; however, it is essential that the system continues to provide precisely measured data after exiting the overload range again.