Abstract: the three factors influencing the electromagnetic flow meter
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One, the influence of the velocity distribution of measurement by fluid mechanics know, liquid flow in the pipe, the pipe cross section on flow at every point is not equal, but both laminar and turbulent flow, after a certain distance of straight pipe, the velocity differential can be axially symmetric.
More flow meter manufacturers choose model price quote you are welcome to inquire, here is the three factors influencing the electromagnetic flow meter scale for measuring the article details.
, the influence of the velocity distribution of measurement by fluid mechanics know, liquid flow in the pipe, the pipe of each point on the cross section flow rate is not equal, but both laminar and turbulent flow, after a certain distance of straight pipe, the flow velocity differential can be axially symmetric distribution, of velocity of each point on the cross section, only is a function of that point to the pipe center distance, velocity of flow in the pipe axis center for zui, on the wall is zero.
As long as the relative velocity distribution electromagnetic flowmeter measurement tube central axis of symmetry, the induced electromotive force on the electrode size and velocity distribution state of each point has nothing to do, but just is proportional to the measured average flow velocity of liquid.
As a result, the velocity distribution of axial symmetry is uniform magnetic field model of the electromagnetic flowmeter must meet one of the working conditions.
If the relative velocity distribution of pipe center axis of asymmetric, although the total flow rate is the same, but near the electrode induction electromotive force is big, so the measured signal is greater than the actual flow value;
On the contrary, with the electrode into 90 & deg;
Where the induction electromotive force is small, the signal is smaller than the actual flow rate value, cause measurement error.
Therefore, in order to make the relative velocity distribution measuring tube central axis of symmetry, transmitter
before it is necessary to add straight pipe.
Second, the edge effect of magnetic field on infinite length measurement assumes that the influence of the magnetic field distribution, the influence of the edge effect of magnetic field can be neglected.
In fact, this assumption of actual flow meter is difficult to do it.
Discussed below the edge effect will effect the performance of the instrument.
The wall is supposed to be insulated, field coil length is 2 l, sounding pipe radius for D / 2, central electrodes A and B in the magnetic field, the magnetic induction intensity B parallel to the x axis.
Middle section, the magnetic field near the electrode is roughly uniform, both ends is waning, form uneven edges, zui after drop to zero.
So, make the internal electric field E liquid is uneven, the results in the y -
Z plane will produce eddy current.
By eddy current secondary magnetic flux, in turn, change the magnetic field generated by the edges of the flux, further damage to the uniformity of magnetic field.
At this moment, on the electrode measured induced electromotive force and infinite magnetic field induction electromotive force is not the same size, thus produced the error.
Assuming that axial length is limited length of A magnetic field, electromagnetic flowmeter electrode produce inductive electromotive force between A and B as the UAB;
Axial length is infinite, the magnetic field induction electromotive force between the electrodes is BvD, with the ratio of S said they, namely UABS = -
BvD obviously, hope S value is close to 1, the better.
That is to say, hope to have the scene of the magnetic field produced by the induced electromotive force and an infinite long magnetic field produced by the induction electromotive force close, zui small at this time of traffic signal loss.
If the wall is conductive, edge effect of magnetic field should be more apparent, cause loss of electrode of the induced electromotive force increase, so the pipe wall is usually coated with insulation layer.
If the electrical conductivity of medium is extremely high, will produce very big eddy current magnetic field edge area, causing a magnetic flux, make work magnetic field on either side of the edge area is weakened and enhancement, respectively.
So high conductivity measurement medium unfavorable use ac excitation, and the application of dc excitation.
If measured medium containing guide magnetic materials, magnetic field edge effect is more complex.
As a result of the existence of permeable material, serious distortions in the magnetic field, caused by the nonlinear measurement.
Three, the influence of the conductivity of measured medium at present, the electromagnetic flowmeter converter input impedance has been improved, measuring the conductivity of liquid, generally will not caused by a slight change of conductivity of medium error, but for certain converter input impedance, the electrical conductivity of measured medium has a lower limit value, cannot be lower than the lower limit value.
The electrical conductivity of measured medium is too big is not allowed.
For example, when more than 10 - conductivity
, can reduce traffic signals, to change the indicated value, the instructions flow value less than the actual flow.
This is because in the electromagnetic flow meter, the magnetic field of finite length, were measured electrically conductive liquid, only through the limited magnetic field to produce inductive electromotive force.
So, on behalf of the traffic signal of induction electromotive force is the result of the magnetic field of conductive liquid cutting lines of magnetic force, magnetic field outside the conductive liquid has not made any contribution on both ends.
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