Pressure Sensors

KNK Sensor

In various industries such as food production, manufacturing of plastic products and construction components, as well as for measuring pressure and liquid levels, sensors with open membranes, including high-temperature sensors, are widely used. One such sensor - the KNK sensor - provides accurate pressure measurements in various systems. The compact PD180 sensor is equipped with a KNK type sensor (silicon on silicon) and a measuring membrane made of stainless steel. 

The KNK sensor, which is semiconductor-based, is used in the KNK sensor to measure pressure. When pressure changes on the sensor's measuring membrane, its shape changes, leading to a change in resistance. This change in resistance is converted into an electrical signal, which is then processed and used to measure pressure.

Pressure Sensors with Open Rigid Membrane

Sensors based on KNK structures ("silicon on silicon") typically have (Fig.1a) a strain gauge with housing 1, a semiconductor sensitive element (SSE) 3, separated from the working medium by a membrane-liquid separator (pos.2 and A). In this case, the separating membrane 2 can be thin-walled, corrugated, or made of elastic (rubber-like) material.

Input part of the pressure sensor with a strain gauge based on the KNK structure with a membrane-liquid separator. Variant with hermetic feedthrough.
a) variant without hermetic feedthrough;
b) variant with hermetic feedthrough.

1 - strain gauge housing; 2 - elastic separating membrane; 3 - semiconductor sensitive element (SSE); 4 - collector housing; 5 - collector board; 6 - sensor housing (fitting, socket, or flange); 7 - leads (micro-wire) from SSE contacts; A - separating liquid; 8 - hermetic feedthrough; 9 - insulator; 10 - tube; 11 - solder for sealing.

The presence of membrane 2 with separating liquid A in the sub-membrane cavity creates several technical problems, as the thermal expansion coefficients of the membranes, housing (pos. 1 and 6), and liquid differ significantly.

When designing sensors, the desire to reduce overall dimensions is one of the key factors. However, this leads to increased membrane stiffness, which in turn causes an increase in basic and temperature errors due to changes in the parameters of the membrane, sub-membrane cavity, and liquid.

To increase the sensor's sensitivity and accuracy, it is necessary to reduce the membrane thickness, but this causes technological problems related to manufacturing and welding thin-walled membranes. Moreover, the thinner the membrane, the more it is susceptible to mechanical and chemical destructive effects.

In membrane liquid separators made in a fitting with M20x1.5 thread, only a very thin membrane (0.025-0.1 mm) can be used, which reduces the sensor's resistance to external influences and may be unacceptable for harsh operating conditions. Additionally, reducing the membrane thickness increases the likelihood of damage and decreases resistance to chemical effects.

Increasing sensor sensitivity by reducing membrane thickness also increases the volume of sub-membrane liquid and the corresponding temperature error. Increasing the membrane diameter allows raising sensor sensitivity at an acceptable metal thickness (e.g., 0.1 mm). However, this increases the volume of sub-membrane liquid and the corresponding temperature error. The frontal load on the sensor also grows (quadratically relative to the diameter).

Sensor reliability is directly related to membrane tightness. The slightest leak in the separating cavity between the membrane and SSE leads to liquid leakage and ultimately sensor failure. For example, in the strain gauge (similar to those produced by VIKA, Kistler, Keller, etc.) shown in Fig. 1b, there are hermetic feedthroughs 8 with insulators 9 and tube 10 and solder for hermetic filling of the sub-membrane cavity with silicone liquid. Leakage through all these elements is possible, reducing the strain gauge's reliability.

Moreover, it should be noted that the membrane is not absolutely hermetic; micro-leaks can occur through microcracks. The thinner the membrane, the greater the leakage, which can eventually lead to creep and sensor failure.

Thus, when designing sensors, it is necessary to find a balance between reducing overall dimensions and maintaining reliability and resistance to various influences. Increasing the membrane diameter can improve sensor sensitivity, but this may lead to an increase in the volume of sub-membrane liquid and the corresponding temperature error. Additionally, technological problems in manufacturing thin-walled membranes and the possibility of leaks through microcracks must be considered.

The use of sensors with membrane-liquid separators in high-temperature environments is limited by the boiling point of the separating liquid. When developing such sensors, several design requirements must be considered, such as membrane elasticity with minimal dependence on displacement, temperature, and time, its tightness relative to the separating liquid and working medium, minimal compressibility of the liquid, minimal volume of liquid in the sub-membrane cavity, and tightness of the chamber with separating liquid. Pressure sensors with a "silicon on sapphire" structure can solve many problems characteristic of KNK structure sensors, but when using a membrane-liquid separator, they do not differ significantly from sensors based on the KNK structure.