Measuring the flow rate of liquids and gases
This year we began to represent IST in Russia - a Swiss manufacturer of thin-film sensors for temperature, relative humidity, fluid conductivity and flow rate.
IST products are not a mass market, they do not produce DHT22 analogues for millions of copies of cheap microcircuits for standard applications. Instead, emphasis is placed on special tasks: non-standard constructs and temperature ranges, increased accuracy, minimal response time, and so on.
Among the diverse products IST there is such an interesting thing as flow sensors - sensors for the flow of continuous media. Under the cut I tell how they work, what they look like and why they are needed. I think that it will be interesting not only to developers of flowmeters.
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So, various physical effects are used to measure the flow of liquids or gases. To measure the flow rate, mechanical, optical, electromagnetic, ultrasonic and other sensitive elements are used to determine the flow rate of the continuous medium passing through the pipe using indirect characteristics.
Since most IST products are platinum temperature sensors (thermal resistances), thermal effects are also used to determine the flow rate.
In thermal flow meters, measurements are made either by cooling a heated body placed in the flow (thermal anemometers) or by transferring thermal energy between two points along the flow (calorimetric flow meters). Let's see how both principles are used in real applications.
Flowmeters with thermo-anemometric converters IST are mainly used for gas flows. In the simplest case, they consist of a heating element and a temperature sensor. In fact, these are two thermal resistances, on the basis of which the following algorithm is implemented:
In the absence of flow, the temperature of the microheater remains unchanged, and in the presence of flow, the heater begins to give off heat to the external environment. The amount of heat that is given to the flow depends on several factors: the initial temperature difference between the heater and the medium, the parameters of the pipe and the actual flow velocity.
Since the temperature difference is determined by the flow sensor on circuit, and we consider the pipe parameters unchanged, the heat output of the heating element can be used to measure the flow velocity.
The heater and temperature sensor are included in the bridge circuit, which is balanced in the absence of flow and unbalanced when the resistance of the heater changes. As the flow rate increases, the heater cools, the bridge unbalances, and the unbalance signal goes to the amplifier. The output of the amplifier informs the heater of a higher temperature and brings the bridge back into equilibrium. The same signal is used as an output, i.e. as a function of flow rate.
With the known parameters of the pipe, the position of the sensor, the type of flow, as well as the unchanged thermophysical characteristics of the gas (composition, pressure, temperature), this function can be calculated using one of the well-known methods.
The figure shows an example of the flow sensor switch on circuit and a graph of the dependence of the voltage Uflow on the flow velocity.
According to this principle, the FS7 series sensors work. Conductive paths are applied on a ceramic zirconium dioxide substrate - a platinum microheater and a temperature sensor, between which connections are provided. The whole structure is covered with a thin insulating layer of glass.
Sensitive elements of this design can measure the flow velocity in the range from 0 to 100 m / s with a sensitivity of 0.01 m / s and an error of less than 3% of the measured value. However, the accuracy of measurements is determined not only by the sensitive element, but also by the scheme of its switching on, and the method of calibration of the final device.
The operating temperature range of the FS7 sensor is -20 ... 150 ° C for the standard version, however, IST practices the manufacture of sensors with a permissible temperature of up to +400 ° C.
The figure shows two versions of the FS7 sensors - in the case and without it.
It is important to note that the FS7 sensors, as well as the FS2 discussed below, are mainly used for gases, as well as for liquid media that do not contain water - during long-term operation in water, the upper insulating layer of the sensor gradually collapses and electrolysis occurs.
For the flow of water and other similar media, an Out Of Liquid module is provided - an anemometric sensor, the elements of which are isolated from the flow. Out Of Liquid is a small stainless steel tube, on the outer wall of which a microheater and a temperature sensor are placed.
The tube has a length of 40 mm and a diameter of 4 mm, the working temperature range of this solution is from -50 ° C to +180 ° C.
Hot-wire flow meters have some obvious limitations. In particular, they do not allow to determine the direction of flow and are not suitable for applications requiring high sensitivity of the sensor.
Calorimetric flow meters, on the other hand, are designed for relatively slow gas flows with a variable direction. The calorimetric sensor consists of three elements - a microheater and two sensors that measure the temperature before and after it. In the absence of flow, the heat spot emitted by the heater is motionless, therefore the continuous medium at the right and left of the heater has the same temperature. When a flow occurs, the heat spot “shifts” according to the direction and speed of the flow. Thus, with the known parameters of the pipe and the characteristics of the medium, the flow velocity can be measured by the difference in the readings of the temperature sensors.
In the production of a colorimetric sensor, platinum tracks and the connections between them — a microheater and two temperature sensors — are also applied to the ceramic substrate.
Since, in the presence of flow, the heating element is cooled, and this process is no longer used for measurements, an additional compensation temperature sensor is provided on the flow sensor.
According to this principle, the sensors of the FS2 series are built. With their help, you can determine both the direction and flow rate. In the range from 0 to 2.5 m / s, the sensor has a sensitivity of 0.001 m / s.
The measurement range of calorimetric sensors is limited by the principle of its operation — at a certain flow rate, the heat spot “shifts” too far and the difference between the right and left sensors does not allow us to judge the flow rate.
This annoying property of calorimetric sensors is quite easy to manage. When the flow reaches a certain speed, you can "switch" to work in a hot-wire mode - start using a pair of heater + compensating temperature sensor according to the thermo-anemometric principle already known to us.
When using a combination of two methods of measurement, the magnitude of the flow rate over most of the range is determined by a quadratic function of the voltage Uflow (lower graph), and the direction of flow is determined by the voltage from a full bridge circuit consisting of a pair of sensors and a microheater.
If the task does not involve working with streams at a speed exceeding 1.5 m / s and we are talking about a gaseous medium, then MFS02 (Micro Flow Sense) sensors can be used. MFS02 has a maximum sensitivity (0.0003 m / s) and a response speed (response time less than 10 ms).
Structurally, the MFS02 sensor is similar to FS2 and consists of a microheater, a pair of temperature sensors and an additional compensating sensor. However, MFS02 is manufactured according to a different technological process: in the glass-ceramic substrate of the sensor, an area is distinguished, which is a membrane. It is assumed that only the membrane is immersed in the flow; therefore, components for calorimetric measurements are located exactly on it, and a compensating temperature sensor is installed outside the membrane.
The MFS02 sensor is only 3.5 x 5.1 mm in size, and it is quite difficult to solder to the contact pads, so the MFS02 is also available as part of the expansion cards that provide access to the terminals of the element.
In conclusion, I will thank the reader for your attention and remind you that questions about the use of products, about which we write on Habré, can also be asked by email, indicated in my profile.
upd: all mentioned sensors and modules are available from stock. More information on efo-sensor.ru
IST products are not a mass market, they do not produce Among the diverse products IST there is such an interesting thing as flow sensors - sensors for the flow of continuous media. Under the cut I tell how they work, what they look like and why they are needed. I think that it will be interesting not only to developers of flowmeters.
')
So, various physical effects are used to measure the flow of liquids or gases. To measure the flow rate, mechanical, optical, electromagnetic, ultrasonic and other sensitive elements are used to determine the flow rate of the continuous medium passing through the pipe using indirect characteristics.
Here we note that by flow rate can be meant both the flow volume (liters per minute or cubic meters per minute), and the mass flow (kilogram per minute) or its speed (meters per second). Assuming that in most applications both the characteristics of the medium and the characteristics of the pipe in which the flow moves are known, we will consider all the concepts listed above to be identical.
Since most IST products are platinum temperature sensors (thermal resistances), thermal effects are also used to determine the flow rate.
In thermal flow meters, measurements are made either by cooling a heated body placed in the flow (thermal anemometers) or by transferring thermal energy between two points along the flow (calorimetric flow meters). Let's see how both principles are used in real applications.
Thermoanemometric sensors
Flowmeters with thermo-anemometric converters IST are mainly used for gas flows. In the simplest case, they consist of a heating element and a temperature sensor. In fact, these are two thermal resistances, on the basis of which the following algorithm is implemented:
In the absence of flow, the temperature of the microheater remains unchanged, and in the presence of flow, the heater begins to give off heat to the external environment. The amount of heat that is given to the flow depends on several factors: the initial temperature difference between the heater and the medium, the parameters of the pipe and the actual flow velocity.
Since the temperature difference is determined by the flow sensor on circuit, and we consider the pipe parameters unchanged, the heat output of the heating element can be used to measure the flow velocity.
The heater and temperature sensor are included in the bridge circuit, which is balanced in the absence of flow and unbalanced when the resistance of the heater changes. As the flow rate increases, the heater cools, the bridge unbalances, and the unbalance signal goes to the amplifier. The output of the amplifier informs the heater of a higher temperature and brings the bridge back into equilibrium. The same signal is used as an output, i.e. as a function of flow rate.
With the known parameters of the pipe, the position of the sensor, the type of flow, as well as the unchanged thermophysical characteristics of the gas (composition, pressure, temperature), this function can be calculated using one of the well-known methods.
The figure shows an example of the flow sensor switch on circuit and a graph of the dependence of the voltage Uflow on the flow velocity.
According to this principle, the FS7 series sensors work. Conductive paths are applied on a ceramic zirconium dioxide substrate - a platinum microheater and a temperature sensor, between which connections are provided. The whole structure is covered with a thin insulating layer of glass.
Sensitive elements of this design can measure the flow velocity in the range from 0 to 100 m / s with a sensitivity of 0.01 m / s and an error of less than 3% of the measured value. However, the accuracy of measurements is determined not only by the sensitive element, but also by the scheme of its switching on, and the method of calibration of the final device.
The operating temperature range of the FS7 sensor is -20 ... 150 ° C for the standard version, however, IST practices the manufacture of sensors with a permissible temperature of up to +400 ° C.
The figure shows two versions of the FS7 sensors - in the case and without it.
About water and corrosive environments
It is important to note that the FS7 sensors, as well as the FS2 discussed below, are mainly used for gases, as well as for liquid media that do not contain water - during long-term operation in water, the upper insulating layer of the sensor gradually collapses and electrolysis occurs.
For the flow of water and other similar media, an Out Of Liquid module is provided - an anemometric sensor, the elements of which are isolated from the flow. Out Of Liquid is a small stainless steel tube, on the outer wall of which a microheater and a temperature sensor are placed.
The tube has a length of 40 mm and a diameter of 4 mm, the working temperature range of this solution is from -50 ° C to +180 ° C.
Determining the direction of flow
Hot-wire flow meters have some obvious limitations. In particular, they do not allow to determine the direction of flow and are not suitable for applications requiring high sensitivity of the sensor.
Calorimetric flow meters, on the other hand, are designed for relatively slow gas flows with a variable direction. The calorimetric sensor consists of three elements - a microheater and two sensors that measure the temperature before and after it. In the absence of flow, the heat spot emitted by the heater is motionless, therefore the continuous medium at the right and left of the heater has the same temperature. When a flow occurs, the heat spot “shifts” according to the direction and speed of the flow. Thus, with the known parameters of the pipe and the characteristics of the medium, the flow velocity can be measured by the difference in the readings of the temperature sensors.
In the production of a colorimetric sensor, platinum tracks and the connections between them — a microheater and two temperature sensors — are also applied to the ceramic substrate.
Since, in the presence of flow, the heating element is cooled, and this process is no longer used for measurements, an additional compensation temperature sensor is provided on the flow sensor.
According to this principle, the sensors of the FS2 series are built. With their help, you can determine both the direction and flow rate. In the range from 0 to 2.5 m / s, the sensor has a sensitivity of 0.001 m / s.
The measurement range of calorimetric sensors is limited by the principle of its operation — at a certain flow rate, the heat spot “shifts” too far and the difference between the right and left sensors does not allow us to judge the flow rate.
This annoying property of calorimetric sensors is quite easy to manage. When the flow reaches a certain speed, you can "switch" to work in a hot-wire mode - start using a pair of heater + compensating temperature sensor according to the thermo-anemometric principle already known to us.
When using a combination of two methods of measurement, the magnitude of the flow rate over most of the range is determined by a quadratic function of the voltage Uflow (lower graph), and the direction of flow is determined by the voltage from a full bridge circuit consisting of a pair of sensors and a microheater.
About working with “micro flows”
If the task does not involve working with streams at a speed exceeding 1.5 m / s and we are talking about a gaseous medium, then MFS02 (Micro Flow Sense) sensors can be used. MFS02 has a maximum sensitivity (0.0003 m / s) and a response speed (response time less than 10 ms).
Structurally, the MFS02 sensor is similar to FS2 and consists of a microheater, a pair of temperature sensors and an additional compensating sensor. However, MFS02 is manufactured according to a different technological process: in the glass-ceramic substrate of the sensor, an area is distinguished, which is a membrane. It is assumed that only the membrane is immersed in the flow; therefore, components for calorimetric measurements are located exactly on it, and a compensating temperature sensor is installed outside the membrane.
The MFS02 sensor is only 3.5 x 5.1 mm in size, and it is quite difficult to solder to the contact pads, so the MFS02 is also available as part of the expansion cards that provide access to the terminals of the element.
Conclusion
In conclusion, I will thank the reader for your attention and remind you that questions about the use of products, about which we write on Habré, can also be asked by email, indicated in my profile.
upd: all mentioned sensors and modules are available from stock. More information on efo-sensor.ru
Source: https://habr.com/ru/post/280031/
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