Author: Daniel Träutlein, Product Manager Sensirion, Medical

Sensors are used to measure gas flows for various purposes in modern medical ventilation technology. A variety of measuring principles are used, depending on the manufacturer and the intended use. Many different sensor products are now available on the market. Medical ventilation technology has made rapid progress since the invention of the "iron long" about 80 years ago - the first device used for patient ventilation. Along with pressure sensors, flow sensing technology and gas flow sensors play a significant role today. The original iron lung used negative pressure to achieve patient ventilation, but modern devices use positive pressure in the patient's lungs for this purpose. Along with air, other gases - primarily oxygen - are also used for ventilation. The quantity and mixture of the various gases are precisely controlled and monitored using sensor systems. The requirements on sensor systems vary greatly, depending on where and how the ventilation device is used.

Medical ventilation devices can basically be divided into three application areas: devices for emergency treatment, devices for intensive care, and devices for home use. Naturally, each area and each individual device has its own specific features. The key considerations with emergency treatment devices are size and weight, since it must be possible to bring them to patients quickly and easily. They usually operate from battery power to make them independent of the AC power line. Ventilation devices for intensive care are used in hospitals and operated by medical staff. They have a significantly greater functional scope than devices for emergency treatment. The main consideration with these devices is their performance capability. Finally, ventilation devices for home use must ensure proper breathing for patients who are not able to breathe adequately on their own.

A variety of measuring principles are used for flow measurement in ventilation technology. Due to accuracy requirements and the progressive integration of electronic control systems, the floating-body flow gauges previously used on a wide scale are now only found in older devices.

Differential pressure sensors are used for flow measurement in many devices. This allows the differential pressure sensor to be located relatively far away from the patient and still determine the flow close to the patient. The accuracy of this measuring method does not depend directly on the sensor, but instead on the combination of the differential pressure sensor and the component used to obtain the pressure drop, which is usually an orifice or a linear flow element. The hose between the flow element and the sensor is also a significant factor. The hose basically has an attenuating effect, so kinks in the hose should be avoided.

There are also sensors available that use ultrasonic signals to make "time of flight" measurements. The actual sensor measures through a plastic wall. Easy reconditioning is one of the advantages of this method. However, it entails much higher manufacturing costs.

There are also thermal measuring principles. With the thermal methods, we can distinguish between conventional hot-wire anemometers and hot-film anemometers. A drawback of hot-wire anemometers is that only the magnitude of the gas flow is measured. It is not possible to indicate the direction. This drawback can be eliminated by using hot-film anemometers with several zones.