Even More Accurate Measurements

Measuring pressure is very simple if you use a pressure sensor. But how do you turn a good sensor into a perfect pressure transducer? Keller's Technical Director Bernhard Wetterli gives a brief insight into what is needed to create accurate and reliable measurement systems.

Information is the foundation of any decision and any action. That is why measurement, verification, comparison, and testing are the starting points for all kinds of optimization. Without data, there can be no feedback loops and improvement processes. In the world of measurement systems, this plays a crucial role. This is how we have been able to improve our products over decades.

The better we understand our products, the more opportunities we have to develop and optimize them, as well as to increase the reproducibility of final results. The quality of products depends on design and manufacturing processes. Close collaboration between the design team, electronics and software development departments, and the production department is a key success factor for KELLER.

The quality mark for KELLER sensors – original since 1974

Measurement and testing in product development

Pressure sensors convert pressure, which is a physical quantity, into an electrical signal. To determine how good a measuring instrument is, it is necessary to accurately measure and compare physical and electrical factors. Ultimately, KELLER is responsible for the new product and must be able to demonstrate that it meets or even exceeds expectations and standards.

Electronics

Electromagnetic compatibility (EMC) requires that pressure sensors withstand electromagnetic radiation up to 6 GHz, electrostatic discharges (ESD), as well as voltage pulses and surges (lightning strikes). At KELLER, sensors are tested for additional factors that may cause problems in real conditions. For example, we optimize the electronics of new sensors so they can operate in close proximity to current inverters, during interference or power failures. Additionally, each sensor is equipped with overvoltage and reverse polarity protection.

EMC testing laboratory

Preparation for vibration and shock testing

Mechanical design

Housing components, weld seams, diaphragms, and pressure connections are designed using simulation tools. This allows identifying and eliminating potential design errors already at the development stage.

Pressure testing

We subject our high-pressure gauges to burst tests and long-term pressure tests up to 4000 bar not only at room temperature but also in climate chambers at temperatures up to 200 °C.

Burst pressure tests and long-term pressure tests in the mechanical testing laboratory

Long-term testing

Our pressure cycling systems operate around the clock and allow creating pressure fluctuations up to 1800 bar. During the design phase, the test specimens are subjected to 10 million pressure cycles ranging from 0 to 100% of the specified pressure range, which we guarantee in our technical specifications. The Wöhler curve shows what resistance can be expected for any number of load cycles.

Our cyclic loading systems are in constant use

Measurement and testing processes guarantee the quality of our products. But no less important are our highly qualified employees who think independently, work carefully, and are well versed in methods and processes. Comprehensive testing procedures in production and long-term measurements are ultimately only as good as the people who conduct them. That is why we pay great attention to the accuracy, responsibility, and contribution of each individual employee.

Intermediate testing in production

All these efforts and measurements, as well as deviations from specified parameters, are characterized and corrected so that our customers can always rely on accurate measurement results. We supply them with pressure transducers with a standardized output signal and a total error of ± 0.05% FSO over the entire temperature range.

During the sensor production process at KELLER, the following measurements and tests are carried out.

Sensor chips

At the core of our sensors lies the pressure sensor itself. It consists of a silicon chip located behind a metal diaphragm that converts physical pressure into an electrical signal. Before new batches of chips can be put into production, they undergo a sensor verification procedure controlled by the development department. For this, sensor chips are embedded in measuring cells and subjected to thorough testing. This is not just incoming goods inspection; it includes comprehensive characterization measurements using pressure and temperature, as well as long-term measurement over 3 months at high temperature.

Silicon wafer with approximately 1200 individual sensor chips

Long-term measurements of new chip batches

Mechanical components

During the assembly of mechanical components, all connections undergo helium leak testing. This test is directly integrated into the system for embossing thin metal diaphragms in the pressure sensor to exclude further processing of defective components. Of course, each finished pressure device is also subjected to pressure and temperature exposure to verify its pressure resistance and structural integrity.

Helium leak testing of mechanical components

Characteristics of each individual sensor

Once the pressure sensor chips are installed in a housing with easily adjustable connections for pressure and signal supply, control measurements and initial evaluation begin. These can last from several hours to several days. The piezoresistive sensor technology with characteristic temperature behavior has allowed us over the years to establish more than 60 measuring systems with a total of 12,000 measuring stations. Although materials, production equipment, and process reproducibility are constantly improving, each pressure sensor differs slightly from others and has its own characteristics. For this reason, KELLER carefully tests and evaluates the behavior of each individual sensor under pressure and temperature.

Installing sensors in adapter racks for pressure and temperature testing in measuring systems

Measuring system for performance measurement via pressure and temperature

Evaluation of measurement data

Compensation

Such an individual characteristic of sensors allows mathematically describing the deviation from ideal behavior and understanding how this deviation can be compensated. Then this information can be provided to the customer in the form of a polynomial coefficient. In the case of pressure transducers, the coefficients are fed directly into the compensation electrical circuit. This is the key to creating the "ideal pressure transducer" mentioned at the beginning.

Modeling the physical characteristics of the sensor element

Intermediate and final tests

After completing important work and before packaging and shipping the pressure gauges, they must undergo comprehensive functional tests and accuracy verification on the so-called test bench. The test bench was developed by KELLER using special software and hardware. It is continuously improved and can perform automated testing.

Final test using the internal "Unitester"

Subsequent measurements and tests

Extensive measurements and tests conducted by KELLER for sensors provide direct benefits to customers through individual and comprehensive sensor characteristics.

Calibration certificate with pressure and temperature measurement data

The large volume of measurement data also provides KELLER with the necessary statistical data for a better understanding of the properties of various sensor designs and thus serves as a basis for the continuous improvement of sensors.

Our equipment manufacturing team develops automation solutions that meet the requirements of our production department and adhere to high standards of quality and accuracy. This allows us to continuously improve our products, process stability, and efficiency over the past 50 years.

For more information about the instruments, you can contact IZMERKON by phone at +7 (812) 309 56 05 or via the feedback form.