The principle of thermal mass flowmeter operation

The principle of thermal mass flowmeter operation.

The main elements of the gas flowmeter construction are the special element of the flow resistance that provides the ideal decomposition of the stream. Its main aim is to split the parts of the stream in the measuring cell channel. Such a cell provides temperature stability of the sensor. The element of the flow resistance is represented by the set of special disks made of the stainless steal with peceision-corroded channels. Each of the channels corresponds to the size of measuring capillary of sensor.

Structure of this element resistance of stream provides the stability factor dependence of the stream that passes through the sensor our outside of it. While such a dependence remains constant in all the range of measurement parameters of the device exploitation. The measuring cell sensor consists of the capillary measuring channel, 2 thermo resistances RT1 and RT2 and the heating element RH between them. The part of the gas that is sent by the resistance flow element passes through the capillary and heats with the help of the heating element.

Thermo resistance is used for the registration of the gas temperature under T1 and after T2 of the heating element. The temperature difference T2-T1 is directly proportional to the mass gas expense. The electronic part of the measurer provides the transformation of the signal from the measuring cell into the standard linear unified output signal OUTPUT 0 5V, 0 10 V, 0 20 mA, 4 20 mA . If the device is not only the measurer, but also the regulator, then the electronic scheme is based on the task signal difference SETPOINT and the measuring OUTPUT. It also works out the control signal with the help of the PID-controllers regulator laws for the following control of the constant expense with the high accuracy. Types of the gas flowmeter-counters. 1. Turbine gas counter It made in the form of the pipe, where the screw s-turbine small-turbineтурбинка, as a rule with a little overhead cover of the vans. In the flow-part of the case the deflectors are based.

They are used to overhead cover the most part of the pipeline section that is provided by the additional leveling of the speeds epure of the flow and increasing of the gas flow speed. Also the formation of the turbulent mode of the gas flow arises.

Due to this the linearity of the gas counter characteristics is provided in the great range. The height of the s-turbine as a rule doesn t exceed 25-30 of the radius.

On the counter input in the constructions set the additional flow jet-rectifier is used. It is produced in the form of the straight vans or in the form of the thick disk with the holes of the different diameters. Application of the net сетка on the input of the turbine counter, as a rule, isn t necessary, because of the pollution that decreases the passing area of the pipeline. Due to this the speed of the flow increases, that tends to the increasing of the counter readings. Transformation of the rotational speed in the s-turbines in the volumetric values of amount of the passed gas is performed due to transition of the s-turbine rotation through the magnetic clutch on the counting mechanism.

In this mechanism by the way of gear pairs selection the linear bond between the rotational speed of the s-turbine and passed gas quantity is performed. The next method of the quantity of the passed gas determination is the usage of the magnetic-inductive transformation. The vans of the s-turbine while passing near the transformer excites in them the electrical signal, due to this the rotational speed of the s-turbine and the signal frequency are proportional with the transformer.

Under using of such method the signal transformation is made up in the electronic block, as the calculation of the volume of the passed gas. For the explosion hazard of the counter the power supply of the block should be constructed with the explosion proof. But the usage of the electronic block simplifies the question of the measurement range of the counter.

This performs because of the non-linear characteristics of the counter, that appear on the low expense, and is simply excluded with the help of piece-vice approximation of the characteristics. This cannot be performed in the counter with the help of the mechanical counting head. 2. Rotational gas counters The principle of action of such counter is based on the wear smoothing of two rotors with the special constructed form it looks like number 8, each on other under the action of the gas flow. Synchronous of the rotor wear smoothing is performed due to the special gears that are connected with the rotor and between them. For more accurate measurements the rotor profile and internal surface of the counter case should be done with the high accuracy that is performed with the help of application of special technological methods of such surfaces processing.

It should be mentioned that there are some advantages of such type of counters comparing with turbine ones. The wide range of the measuring expense under 1160 and a little error during the measurement of the alternating flows.

The second property makes them irreplaceable in the gas expense measurement that is used by the roof boiler flue, and work in the pulse regime. Any direction of the gas through the counter, absence of the treatments to the presence of the linear zones before and after the counter. 3. Vortex flowmeter-counter The principal action of this type of flowmeter is based on the effect of the periodical vortexes appearance under the streaming by the gas flow of the streaming body. The vortex breakaway frequency is proportional to the flow speed, and due to this to the volume expense.

The vortex indication may be performed by the hot-wire anemometer or supersound. Under the measurement range counters occupy the intermediate value between the turbine and rotational under 150. Due to the fact that in this type of the counter the movable element is absent, there is no need in the system lubricate, that is necessary for the turbine and rotational counters.

So there arises the ability to use such counter for the quantity of the oxygen measurement, that cannot be measured by the turbine and rotational counters because of the oil combustion in the oxygen environment. The measurement range of the expense for this type of device is higher then in the previous ones. 4. Super sound gas flowmeter-counters The principle of action is based on the direction of the super sound ray in the direction of the flow and in the reverse way and the determination of the time difference of these rays passing.

The time difference is proportional to the speed of gas flow. 5. Membrane gas counter The principle of action of this type of counter is based on the displacement of the movable membranes partition of the cameras under the incoming gas in the counter. Input and output of the gas, which expense should be measured, causes the alternating displacement of the membranes and moves the counting mechanism through the arm system and reducer.

Membrane counter differs by the great measuring range under 1100, but they are used for under the low gas pressure, as a rule not higher then 0,5 kgcm Membrane counters are basically used for the gas measuring in houses, cottages. If the turbine and rotational gas counters follow by the noise, then membrane counters work silently. They don t require any lubricant while exploitation, however the turbine counters should be lubricated once at a quarter.

But under the great expense higher then 25 m3 h the sizes of the counter become very big. 6. Jet gas counters The principle of action of this type is based on the oscillation of the gas stream in the special stream generator. The gas stream alternating jumps from one steady state into the other and creates the pulses of the pressure and sound with the frequency that is proportional to the gas flow speed and due to this to the quantity of the missed gas. 7. Drum gas counter The principle of action of this counter is that under the action of the gas pressure drop the rotation of the drum happens, that is divided into several cameras, it s measuring volume is bounded by the closing liquid level.