We start the gas flow rate sensor

    Almost a year ago, an article was published with an overview of gas and liquid flow velocity sensors manufactured by IST-AG.

    Last time, I had the opportunity to explain only on my fingers the basic principle of operation of these elements, but now I am publishing a quite informative story about the FS7 series hot-wire anemometer.

    We will start with a theoretical base, and end with a video where a prototype measuring device based on FS7 is demonstrated using a bicycle pump and tape.



    So, all IST production flow sensors use the thermal measurement principle - the flow rate is calculated either from the amount of heat that the heated body gives off to the flow, or from the difference in the readings of two temperature sensors located symmetrically relative to the heated body along the flow.

    In the first case, the flow sensor is called hot-wire and does not allow to determine the direction of flow, and in the second case, the sensor is called calorimetric and allows to determine both the speed and direction of flow.

    The principle of operation of the hot-wire sensor


    Today we are talking about the sensitive elements of the simplest design - hot-wire anemometric sensors. The hot-wire anemometer consists of a temperature sensor and a heating element.

    In the absence of flow, the temperature of the heater remains unchanged,

    and in the presence of flow, the heater begins to give off its heat to the environment.


    The amount of heat that is given to the flow by the heated element depends on the thermophysical characteristics of the medium, on the parameters of the pipe and on the flow rate. For applications where media characteristics and pipe dimensions are known, the heat transfer from the heater can be used to calculate the flow rate.

    Both the temperature sensor and the heater are platinum resistance thermometers - elements whose resistance is almost linearly dependent on the temperature of the medium.

    All you need to know about thermal resistances is in the articles “ Thermal Resistance: Theory ” and “ Thermal Resistance: Production Process

    Both thermistors are included in the bridge circuit, which is balanced in the absence of flow. When the flow rate increases, the heater cools, its resistance changes and the bridge is unbalanced. The unbalance signal is fed to the amplifier, the output signal of the amplifier tells the heater a higher temperature and brings the bridge back to equilibrium. The magnitude of the voltage required to balance the bridge is a function of flow rate.



    Sensor structure


    The manufacturing process of IST flow rate sensors is very similar to the production of conventional thermistors (temperature sensors). I refer to an article devoted to the production of thin-film thermoresistance a little higher.

    On a ceramic substrate with low thermal conductivity, platinum meanders - conductive paths, of which two thermal resistors are formed, are sprayed.

    The first thermal resistance - the heater - has a nominal resistance of R0 = 45 Ohms, the second - the temperature sensor - has a nominal resistance of R0 = 1200 Ohms.



    The necessary connections and contact pads for fixing the terminals are also applied to the substrate. The structure on both sides is covered with a passivation layer of glass, after which the terminals are attached to the sensor.

    Formula for calculating the flow rate


    I see no reason to delve into physics and analyze the derivation of the formula for calculating the flow velocity; I will only note the basic laws on which this formula is based.

    1. The equation of heat balance - the dependence of the amount of heat$ {\ displaystyle Q} $that the heater gave to the medium, from the temperature difference between the heater and the medium $ {\ displaystyle \ Delta T} $surface area of ​​the heater $ {\ displaystyle A} $ and heat transfer coefficient of the heater $ {\ displaystyle h} $.

    $ {\ displaystyle Q = hA \ Delta T} $

    2. King's law relating the amount of heat to the instantaneous flow rate $ {\ displaystyle v} $

    $ {\ displaystyle Q_ {h} = I_ {h} ^ {2} R_ {h} = (A + Bv ^ {n}) \ Delta T} $where $ {\ displaystyle n = 0.3 .. 0.5} $

    The formula for calculating the flow velocity in which the FS7 element is placed is the result of transformations and simplifications of King's law. The formula is as follows:

    $ U = U_ {0} \ sqrt {1 + kv ^ {n}}, where $

    $ {\ displaystyle U} $ - output voltage of the circuit
    $ {\ displaystyle U_ {0}} $ - voltage in the absence of flow (value $ {\ displaystyle U_ {0}} $ reflects $ {\ displaystyle \ Delta T} $ - the initial difference between the temperature of the heater and the temperature of the medium)
    $ {\ displaystyle k} $- coefficient, which depends on the flow profile and on the position of the sensor; value$ {\ displaystyle k} $ belongs to the range (0.9 ... 0.93)
    $ {\ displaystyle n} $ - coefficient, for FS7 sensors equal to 0.51
    $ {\ displaystyle v} $- desired flow rate.

    The inverse formula is also used in the work.$ {\ displaystyle v = \ frac {[(U-U_ {0}) (U + U_ {0})] ^ {{1} / {n}}} {(k ^ {{1} / {n} }) U_ {0} ^ {{2} / {n}}}} $.

    Odds$ {\ displaystyle n} $ and $ {\ displaystyle k} $ are selected during the calibration of the sensor (see below).

    Sensor switching circuit


    FS7 sensor has three outputs: heater contact, temperature sensor contact, ground.



    There is no universal scheme for switching on the sensor, as well as detailed recommendations for its installation. The reason is obvious - the ratio of flow velocity to voltage depends not only on the geometry of the sensitive element, but also on the parameters of the medium (temperature, composition, pressure, presence of mechanical particles), as well as on the geometry of the pipe, the position of the sensor in the pipe and on the flow profile. In each specific task, this set of parameters will differ, therefore, the selection of ratings for the switching circuit and the calculation of the coefficients for calculating the flow rate are selected for each task separately.

    However, it is always necessary to build on something, in this case it is best to push off from the scheme given in the FS7 documentation:



    An example of the dependence of the output voltage on the flow rate:



    Three points are used to calibrate the sensor - zero speed, maximum flow rate and a point in the middle.

    In the absence of flow, the value is fixed$ {\ displaystyle U_ {0}} $. Let be$ {\ displaystyle U_ {0} = 3.6} $B.

    When$ {\ displaystyle U = 6,6} $ In and $ {\ displaystyle v = 6} $ m / s formula $ {\ displaystyle U = U_ {0} \ sqrt {1 + kv ^ {n}}} $ takes the form $ {\ displaystyle 6.6 = 3.6 \ sqrt {1 + k * 6 ^ {n}}} $.

    At$ {\ displaystyle U = 7.5} $ At $ {\ displaystyle v = 12} $ m / s formula $ {\ displaystyle U = U_ {0} \ sqrt {1 + kv ^ {n}}} $ takes the form
    $ {\ displaystyle 7.5 = 3.6 \ sqrt {1 + k * 12 ^ {n}}} $

    We get a system of two equations with two unknowns, from which we find $ {\ displaystyle n = 0.50} $ and $ {\ displaystyle k = 0.96} $.

    Substituting Values$ {\ displaystyle n} $, $ {\ displaystyle k} $ and voltage $ {\ displaystyle U_ {0}} $ into the formula $ {\ displaystyle v = \ frac {[(U-U_ {0}) (U + U_ {0})] ^ {{1} / {n}}} {(k ^ {{1} / {n} }) U_ {0} ^ {{2} / {n}}}} $, we obtain a simple expression for calculating the flow rate.

    FS7 sensor types and FS-flowmodul


    Three standard versions of the FS7 sensor are available, which differ from each other by the presence of a round plastic case and an operating temperature range.
     FS7.0.1L.195FS7.0.4W.015FS7.A.1L.195
    Measuring range0 ... 100 m / s
    Resolution0.01 m / s
    Response time~ 200 ms
    Operating temperature range−20 ... +150 ° C−20 ... +400 ° C−20 ... +150 ° C
    Item Dimensions6.9 x 2.4 mm
    conclusionsinsulated length 195 mmnot insulated 15 mm longinsulated length 195 mm
    Case Dimensions-Ø 6 mm, length 14 mm
    Retail price21.29 EUR
    UPD from July 4, 2017:
    16 EUR
    21.29 EUR25,44 EUR




    At the stage of acquaintance with the FS7 series sensors, you can also use the ready-made FS-Flowmodul module, on which the switching circuit is implemented.



    The FS-Flowmodul board has three pins for connecting the FS7 sensor on one side and the Power, Ground, and Output pins on the other. Among other things, the board is equipped with a potentiometer to adjust the output voltage (see resistor R2 in the connection diagram).

    It is important to note that the module is not intended for use in serial devices. The fee can be used only at the prototyping stage, when it’s easier for someone to assemble the circuit yourself, and for someone it’s more convenient to pay me an extra 108 euros and get a ready-made debugging board :)

    Demonstration


    Naturally, the simplest way was chosen to demonstrate the operability of the sensor. The sensor is connected to the FS-Flowmodul, and the module output is connected to the ADC input on the control board.
    The debug board is built on the base of the SiLabs microcontroller and connected to the Riverdi TFT touch screen.

    The process of creating a program for displaying information on the screen was devoted whole five articles on the Habre . Now, a module for measuring flow velocity has been added to the previously described prototype for measuring temperature and humidity.


    By the way, when we show this prototype live, then to demonstrate the operation of the sensors on them, it is enough to simply blow out - from breathing, both humidity, temperature, and flow rate simultaneously increase. Unfortunately, this process does not work out beautifully in the video, therefore, the operation of the HYT-271 sensor was demonstrated on a mug of boiling water, and for FS7 it was necessary to construct a makeshift duct from a tube for cleaning the aquarium, into which air was supplied using a bicycle pump.

    Important: the sensor must be installed in the center of the pipe diameter, with the working surface exactly along the direction of flow.

    Notes


    1. I admit some simplifications when describing the description of physical phenomena that, in practice, work with flow sensors should be taken into account. The purpose of today's publication is to demonstrate the basic principles of operation of FS7 sensing elements. However, if there are commentators who are ready to reveal the physics of the process in more detail, then such explanations will be accepted by the author with gratitude expressed in the discount on the purchase of FS7.

    2. All the information that can be found on the Internet for the flow sensor FS5 is also relevant for the FS7 sensor. First of all, I recommend Application Note FS5 and an article in which, among other things, there is a description of the stream profile.

    Conclusion


    In conclusion, I traditionally thank the reader for your attention and remind you that questions about the use of the products that we write about in the Habré can also be sent to the email address specified in my profile.

    upd: all mentioned sensors and modules are available from stock. More information on efo-sensor.ru

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