Mass Air Flow Sensor(AFM3000-200) for Gases – Medical Ventilator Solution

As the biomass in the human body and drugs used for treatment need to be detected and monitored online, sensors are particularly important in medical devices. Take a ventilator as an example, a ventilator with a total weight of no more than 1.5 kilograms involves more than 100 kinds of spare parts. The core hardware of the device is the motor and sensor, among which the airflow sensor is one of the most significant and widely used sensors.

But what is a mass air flow sensor?

What is a Mass Air Flow(MAF) Sensor?

A mass air flow sensor(MAF) is a sensor for determining air mass flow, and it is an important component in the ventilation system and engine.

MAF Sensor (AFM3000-200)

The AFM3000-200 MAF sensor is a digital air flow meter specifically designed for ventilator applications. This sensor is a key component of the ventilator and other medical equipment, which can be used to control the air flow. It measures the flow rate of air, oxygen and other non-aggressive gases with extremely high accuracy. Unlike other ordinary MAF sensors, this flow meter has a special design of the flow channel which results in a very low-pressure drop through the flow body of the sensor making it extremely suitable for very demanding applications, such as medical ventilation and respiratory applications.

This mass flow sensor combines a thermal sensor chip and a high-performance integrated 24-bit AD acquisition CMOS microprocessor. The flow rate of the gas is measured by a thermal sensor element which assures very fast signal processing time and bidirectional measurement with best-in-class accuracy.

The AFM3000 operates from a 5 Volts power supply and features a digital I2C interface. The measurement results are internally linearized, and temperature compensated.

Feature of MAF Sensor AFM3000-200:

  • Integrated thermal sensor chip with a high-performance 24-bit AD acquisition CMOS microprocessor
  • Standard I2C interface for convenient communication with the device
  • Ability to measure several gases such as air, oxygen, and other non-aggressive gases
  • Special design of the sensor channel resulting in a low-pressure drop
  • Very fast update time with excellent accuracy
  • Low zero drift
  • Fully calibrated
  • Temperature compensated by the on-chip temperature sensor
  • The gas flow through the sensor can be bidirectional

Typical Applications

  • Medical (e.g. Ventilation, Respiratory measurements, Anesthesia)
  • Process automation
  • Spectroscopy
  • Environment monitoring
  • Laboratory

MAF Sensor Working Principle

Inside the sensor packet, there is a thermal sensor and a heating source.  When gas starts flowing through the heating source, the flowing gas molecule transport the heat away, resulting in a change in temperature difference between the thermal sensor and the heating source. Thus, it is possible to calculate the energy required to maintain the sensor at the same temperature, then interpreted into a value and sent through the serial I2C communication interface to calculate the flow.

Flow Conversion Formula 

To find out the flow result, the following formula should be used:

Please note: the first measurement performed directly after chip initialization is invalid.

Typical Circuit Connection

AFM3000 sensor uses the standard I2C communication protocol to communicate with various devices. The serial data bus (SDA) and serial time bus (SCL) are the two data lines used by the I2C communication protocol. These two internal data lines are connected to VDD with pull-up resistors.

Please note, multiple sensor devices can share the bus, but only one master device is allowed.

Note: The recommended resistance value of R1 and R2 is 10k.

Medical Solutions with MAF AFM3000-200

The “sensory organ” is the key to realize automatic detection and automatic control. MAF is a detection device, which can convert the input into a signal that can be measured.

Ventilator System with MAF Sensor

Sleep apnea is a condition in which apnea occurs repeatedly during sleep. If it is not treated on time, it may cause many serious problems. The ventilator can be used for this situation. A ventilator equipped with an airflow sensor can monitor the breathing cycle, making patients more comfortable and safer.

MAF sensor is located between the flow valve and the patient, which is aim to measure the amount of gas entering the client body.

MAF sensor is an important part of the ventilator, it is responsible to convert the flow of inhaled and exhaled gas into electrical signals. These electrical signals are thus transmitted to the processing unit in order to detect and monitor inhaled and exhaled tidal volume, minute ventilation, and flow rate. Hence, based on this monitoring system, the inhaled and exhaled tidal volume, minute ventilation, and flow rate can be calculated. These data can be used as the basis for the monitoring system and medical staff to determine whether it is abnormal. Therefore, the performance of the MAF meter will directly affect the accuracy and reliability of the ventilator system.

Ventilator working principle diagram

Anesthesia Machine with MAF Sensor

The anesthesia machine is specially used to deliver anesthesia medicine to patients to reduce pain and other discomforts. General anaesthesia will relax the patient’s skeletal muscles. Without using a ventilator, the patient will be in a risk situation.

The continuous anesthesia machine can continuously supply the required medical gas (such as air, oxygen, nitrous oxide) and precise concentrations of anaesthetic gas(such as isoflurane), and deliver these mixed gases to the patient at an appropriate pressure and flow rate.

MAF sensor measures the flow of air, oxygen and nitrous oxide to make the gas composition suitable for the patient. At the same time, it also detects the concentration of anaesthetic gas. If it is low, there is no anaesthetic effect; if it is too high, the patient will be in danger.

Oxygen Machine with MAF Sensor

The role of the oxygen generator is to reduce the nitrogen in the air delivered to the patient while increasing oxygen. It is suitable for patients who cannot absorb oxygen smoothly into the blood. In this case, the MAF sensor must be able to measure ultra-low flow rate. With the use of the MAF sensor, the expiration time of the patient can be detected, and the result can be used to control the airflow rate from the oxygen machine.

Other Medical Machines with MAF Sensor

The flow sensor can also be used in various diagnostic instruments, including:

  • Blood analyzer
  • Mass spectrometer
  • Chromatograph (gas phase, liquid phase, high-performance liquid phase)
  • Clinical Chemistry Analyzer
  • Other laboratory automation equipment

Keen to build some projects based on MAF? Grab one from Seeed!

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May 2020