Pressure sensors are a key component in the monitoring and validation of steam temperature and other conditions in autoclaves commonly used to sterilize medical and dental equipment. Nominal conditions are around 2.1bar, 134°C (maximum pressure is about 3.8 bar/55 psi) steam from pure distilled water. Some of the requirements for pressure sensors are typical of most of today’s equipment calling for smaller form factor and cost-effective solutions. Autoclaves have additional application-specific needs for pressure sensors in that they must deliver high accuracy, be able to support elevated operating temperatures, feature high pressure capabilities, and offer harsh media compatibility.
Autoclaves are typically built with an internal chamber that is filled with pressurized steam media and held at a specific temperature necessary to sterilize medical and dental instruments. The autoclave’s pressure and steam media need to be directly monitored by a pressure sensor. Pressure sensors that work here must be small, capable of 2.1 bar/30.46 psi, and importantly continue to provide accurate readings after multiple exposures to extreme steam temperatures and soakings.
Most standard PCB-level pressure sensors are specified for operating temperature ranges lower than those of the typical autoclaving temperature of 134°C, therefore, they cannot be used in the very hot and wet environment of an autoclave. Specifying pressure sensors for an autoclave, designers look for harsh media features that ensure they can operate reliably throughout the following sterilization cycle phases.
Phase 1: Chamber pre-heating at startup — sends plant steam to the jacket while the condensations are purged.
Phase 2: Vacuum performance — takes the air out from the chamber and replaces it by pure steam injections. That operation could be done thanks to dilutions in order to reach a high percentage of steam vs air.
Phase 3: Sterilization — according to the type of the materials to sterilize and based on the bacteriological level on the load, the right temperature and time needs to be determined.
Phase 4: Vacuum drying — evacuation of the steam by a vacuum pump and replacing it by sterile air. Dilution operations could be performed as well.
Phase 5: Return to atmospheric pressure — must be performed before the door can be opened.
Selecting the Sensor
Pressure sensors work by converting the pressure of the air, gas, or liquid they are exposed to into an electrical signal. When evaluating pressure sensors, there are a couple of important attributes to be judged. The pressure and temperature range of the sensor and its media compatibility compared to the application’s pressure measurement and feedback needs. Pressure sensor accuracy is another important performance feature to review. Package size and power consumption are also critical in many space-constrained applications.
The construction matters. Many of the latest MEMS-based harsh media pressure sensors are built using a stainless-steel intermediate diaphragm and an oil filled cavity to prevent the steam from degrading the die bond on regular pressure sensors. A key consideration when evaluating board-level pressure sensors is to look for designs based on an adhesive-free die that is mounted using a eutectic die bond on ceramic. This type of construction results in a robust structure capable of handling high pressure even at a temperature as high as 150°C.
It is desirable to combine the adhesive-free design with backside pressure measurement, which enables construction with a small number of media-resistive wetted materials. Backside sensing is a type of pressure sensor design whereby the measured media only touches the backside of the measurement element. A distinct sensor accuracy benefit is that all electronic components and other sensitive surfaces are automatically isolated from the media.
Small form factor PCB-mount packaging is another desirable feature of this new generation of pressure sensors that meet autoclave specifications. New construction techniques make it possible to integrate the measurement functionality of stainless steel and media-isolated pressure sensors at the PCB level. This new sensor brings measurement functions and additional value into designs that were either impossible or very difficult to supply in the past. By adding more functions to the PCB or into one housing, today’s pressure sensors support miniaturization and contribute to reduced wiring complexity. This also lowers the risk of signal distortion because of environmental noise and helps reduce the number of sealed electrical connections. These are all important features and benefits to consider when selecting the right pressure sensor for a harsh media application.
Autoclaves are a good example of an application where severe conditions along with chemicals and temperature can threaten its performance, reliability, and longevity. Certain advanced pressure sensors have been designed with extended harsh media compatibility as defined from a very short list of “wetted” surfaces, which are all materials that come in contact with the measured media. Therefore, the wetted materials are most critical in terms of media resistivity of the sensor.
For autoclave applications, this list is comprised of Ceramic, Glass, Kovar®, Au/Sn, and Silicon. These materials are compatible with the autoclave’s heated steam and it is the sensors’ harsh media compatibility, together with a wide temperature range, that allows for the heated steam media to be measured directly.
For example, sensors in the Bourns BPS130 family of pressure sensors are designed specifically for harsh environments. They are constructed using only inert silicon, glass, Au/Sn, and ceramic materials, which are resistant to many aggressive liquids and gases.
Pressure sensors that are based on MEMS technology have been shown to provide extremely accurate condition readings in a miniature package size. The sensing element in MEMS-based sensors is comprised of embedded piezoresistive elements bonded to a chemically etched silicon diaphragm. The output is amplified, compensated, and calibrated using an ASIC, which provides low TEB (total error band).
Using a piezoresistive element construction makes these pressure sensors more sensitive to temperature variations, however. To compensate for temperature errors, the sensors can be optimized for specific application temperature requirements. By comparison, a standard pressure sensor without a piezoresistive element that delivers 0.1 percent accuracy at room temperature, would not be able to deliver the same degree of accuracy when used in an autoclave with temperatures of up to 134 °C.
Direct media sensing is the most accurate way of measuring the pressure in a system and the trend in many designs is toward greater accuracy and integration.
Critical Components for Data Collection
Sensors are critical components for accurate collection information in autoclave steam applications. The need for “real-time” data analytics is driving even further evolution of these sensors. Features such as self-diagnostics, network compatibility, small form factors and self-calibration will also help drive usage. It is safe to say that dependable sensors for every type of pressure and environment that are also compatible with high temperature operation will continue to be in high demand in applications across multiple market segments.