Zero check of common flowmeter

Micro flowmeter calibration flowmeter has been verified according to the verification procedures before delivery.For example, vortex flow meter, electromagnetic flow meter, turbine flow meter and coriolis force mass flow meter are calibrated on the standard device one by one, and the scale is calibrated.However, if the flowmeter after factory verification is installed on the site, the error will be too large if the appliance is improperly selected, the measuring range is improperly selected, the matching between the appliances is improper, the installation is unreasonable, and the environment is poor.So it is an important work to calibrate the flowmeter on site.The flowmeter is verified on the spot, which generally includes zero check and indicator check beyond zero. Usually, zero check is performed first. After zero is normal, indicator check of other points is performed only when conditions are met.Therefore, zero check of flowmeter is particularly important.

Zero test, first make the flow through the flowmeter to zero, and then read the flow meter display value.Zero check is easy to implement, but note the following points.

(1) ensure that the flow of fluid through the flowmeter is indeed zero.

(2) the measuring channel of the flowmeter must be filled with the measured medium.This point is particularly important for electromagnetic flowmeter, because the measurement tube empty tube, the electrode open circuit, so that the value is more than full, resulting in most electromagnetic flowmeter in the empty tube will point to the full value.

(3) small signal removal problem.For the flowmeter output by analog signal, the analog circuit inevitably has some drift, resulting in a slight deviation of the zero point.Usually with small signal removal method to solve the problem, but the following small flow signal is also cut out, so the cut point can not be set too high.Many flow meters can now be programmed to set a cut point.Some transmitters (such as differential pressure transmitters) due to the installation position has a certain tilt, or because of the mechanical stress, resulting in zero drift, can not be solved by small signal cutting method, can only be solved by zero calibration method.

(4) the influence of vibration on the zero point of vortex street flowmeter.When the measuring tube is filled with the measured medium, if the zero value is high, the vortex street flowmeter can generally make the output return to zero through noise balance adjustment and trigger level adjustment.But if severe vibration on installation site, often can't use instrument adjustment method to solve the problem, even would trigger level too high, or the amplifier gain is too small, will improve the measurable minimum flow values, even at large flow vortex street below triggered signals generated by the threshold value, and treated as noise filtering.At this time, it is necessary to find another way, such as reducing the vibration, replace the better vibration resistance of the meter, and so on.

(5) vortex street flowmeter is easy to introduce interference at zero flow, mainly because of the variable gain characteristic of its sensor preamplifier.Weak signal into the preamplifier of vortex flowmeter, the amplifier to amplify the gap of amplitude frequency signals approximately equal to the amplitude of the signal, the variable gain amplifier is adopted, namely the high velocity at high input frequency, small gain, low velocity of low input frequency, big gain, of course, the input frequency is equal to zero, gain maximum, at this time, all kinds of interference is magnified many times, but higher than the threshold value of the trigger, eventually be used as a signal to the output terminal.