Thermal Mass Air Flow Sensor – Constant Temperature Anemometer
The CTA mode (Constant Temperature Anemometer) consists of a simple feedback circuit for temperature regulation of the heater on the flow sensor and, as flow speed changes the thermal energy loss by the heater changes. By adapting controllers, a constant temperature difference between the heater and the temperature sensor can be achieved. The supplied electrical power, which controls the temperature difference, is a function of the flow speed. The power is converted into a voltage output signal with a bridge circuit and can be easily read out. Knowing the temperature of the medium, the flow rate can be determined from the amount of voltage compensation needed to maintain a constant temperature differential.
The medium temperature variation is compensated by the temperature sensor on the chip (PT1200). The resistors R1 to R6 can be chosen as shown in the circuit below. The temperature difference (DT) between the heater (RH) and the medium (RS) is set by resistor R4, e.g. DT=30 K for air. The resistor PR2 should be adjustable within ±10 % for calibration. The R8 resistor is placed for the stability of the anemometer circuit. It is important to use high-grade components for accuracy and output stability, all resistors 0.1 to 1%. The below electronic circuits and curve progression are examples. Each application linked to the accuracy level requires an individual calibration of the system.
Sensor No Included , Purchase Separately
Features
- Power Supply 10V DC @ 80mA
- On Board Power LED
- Trimmer Potentiometers for Calibration
- Header Connector for Sensor
- Header Connector for Power and Output
- PCB Dimensions 36.99 x 18.73 mm
Hot-Film Flow Measurement Principles
A mass airflow (MAF) sensor converts the amount of air into a voltage signal. Units used are usually kilogram per second (kg/s). For example, airflow mass is a necessary parameter in engine operation in order to determine how much fuel to inject, ignition timing, and when to shift the gears. Used in conjunction with an oxygen sensor, the mass air flow sensor measurement can enable very accurate control of the fuel ratio. There are different types of MAF sensors. The hot-film type sensor is more common. It consists of a heating element (typically a temperature-dependent, low-ohm platinum resistor) and an additional temperature sensor. Flow direction can also be determined with specific sensor configurations. The temperature sensor (Pt in Figure 1) is used as a reference for the heating element to maintain a constant reference to the ambient temperature, which can be provided by an electronic control circuit such as a constant temperature anemometer (CTA) as shown in Figure 1. The voltage needed to heat the element in order to maintain thermal equilibrium with the ambient temperature is proportional to the air mass flow through the sensor. Higher flow increases the cooling of the element, which is compensated by raising the voltage of the heating element. This measuring principle covers large operational ranges with high accuracy and a wide temperature operating range.
The air density can vary, which changes the thermal capacity of the air. The density is a function of the ambient temperature, altitude, and pressure, which makes mass flow meters more appropriate for determining the quantity of the air. Another factor that should be taken in account is the air humidity. An increase in the humidity decreases the air density. This is because the molecule mass of water is less than the molecule mass of air, and for any gas at a
given pressure and temperature, there is a constant number of molecules for a particular volume. This results in a decrease in the gas mass per unit volume. MAF sensors feature a quick response time, small overall package, less sensitivity to mounting and orientation, durability, and lower costs. Care should be taken to prevent contamination of the sensor, which reduces the accuracy. Possible thermocouples between the heating element and the temperature sensor should be taken into account as well.
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The CTA mode (Constant Temperature Anemometer) consists of a simple feedback circuit for temperature regulation of the heater on the flow sensor and, as flow speed changes the thermal energy loss by the heater changes. By adapting controllers, a constant temperature difference between the heater and the temperature sensor can be achieved. The supplied electrical power, which controls the temperature difference, is a function of the flow speed. The power is converted into a voltage output signal with a bridge circuit and can be easily read out. Knowing the temperature of the medium, the flow rate can be determined from the amount of voltage compensation needed to maintain a constant temperature differential.
The medium temperature variation is compensated by the temperature sensor on the chip (PT1200). The resistors R1 to R6 can be chosen as shown in the circuit below. The temperature difference (DT) between the heater (RH) and the medium (RS) is set by resistor R4, e.g. DT=30 K for air. The resistor PR2 should be adjustable within ±10 % for calibration. The R8 resistor is placed for the stability of the anemometer circuit. It is important to use high-grade components for accuracy and output stability, all resistors 0.1 to 1%. The below electronic circuits and curve progression are examples. Each application linked to the accuracy level requires an individual calibration of the system.
Sensor No Included , Purchase Separately
Features
- Power Supply 10V DC @ 80mA
- On Board Power LED
- Trimmer Potentiometers for Calibration
- Header Connector for Sensor
- Header Connector for Power and Output
- PCB Dimensions 36.99 x 18.73 mm
Hot-Film Flow Measurement Principles
A mass airflow (MAF) sensor converts the amount of air into a voltage signal. Units used are usually kilogram per second (kg/s). For example, airflow mass is a necessary parameter in engine operation in order to determine how much fuel to inject, ignition timing, and when to shift the gears. Used in conjunction with an oxygen sensor, the mass air flow sensor measurement can enable very accurate control of the fuel ratio. There are different types of MAF sensors. The hot-film type sensor is more common. It consists of a heating element (typically a temperature-dependent, low-ohm platinum resistor) and an additional temperature sensor. Flow direction can also be determined with specific sensor configurations. The temperature sensor (Pt in Figure 1) is used as a reference for the heating element to maintain a constant reference to the ambient temperature, which can be provided by an electronic control circuit such as a constant temperature anemometer (CTA) as shown in Figure 1. The voltage needed to heat the element in order to maintain thermal equilibrium with the ambient temperature is proportional to the air mass flow through the sensor. Higher flow increases the cooling of the element, which is compensated by raising the voltage of the heating element. This measuring principle covers large operational ranges with high accuracy and a wide temperature operating range.
The air density can vary, which changes the thermal capacity of the air. The density is a function of the ambient temperature, altitude, and pressure, which makes mass flow meters more appropriate for determining the quantity of the air. Another factor that should be taken in account is the air humidity. An increase in the humidity decreases the air density. This is because the molecule mass of water is less than the molecule mass of air, and for any gas at a
given pressure and temperature, there is a constant number of molecules for a particular volume. This results in a decrease in the gas mass per unit volume. MAF sensors feature a quick response time, small overall package, less sensitivity to mounting and orientation, durability, and lower costs. Care should be taken to prevent contamination of the sensor, which reduces the accuracy. Possible thermocouples between the heating element and the temperature sensor should be taken into account as well.
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