GB/T 18659-2023 Measurement of fluid flow in closed conduits - Guidance for the use of electromagnetic flowmeters
1 Scope
This document applies to industrial electromagnetic flowmeters used for the measurement of flowrate of a conductive liquid in a closed conduit running full. It covers flowmeter types utilizing both alternating current (AC) and pulsed direct current (DC) circuits to drive the field coils and meters running from a mains power supply and those operating from batteries or other sources of power.
This document is not applicable to insertion-type flowmeters or electromagnetic flowmeters designed to work in open channels or pipes running partially full, nor does it apply to the measurement of magnetically permeable slurries or liquid metal applications.
This document does not specify safety requirements in relation to hazardous environmental usage of the flowmeter.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
——IEC Electropedia: available at http://www.electropedia.org/
——ISO Online browsing platform: available at http://www.iso.org/obp
3.1
electromagnetic flowmeter
flowmeter which creates a magnetic field perpendicular to the direction of flow, so enabling the flowrate to be deduced from the induced voltage, Uv , produced by the motion of a conducting fluid through the magnetic field
Note: The electromagnetic flowmeter consists of a sensor (3.2) and a transmitter (3.3).
3.2
sensor
device containing at least the following elements:
——an electrically insulating meter tube through which the conductive fluid to be measured flows;
——one pair of electrodes across which the signal generated in the fluid is measured;
——an electromagnet for producing a magnetic field in the meter tube (3.4)
Note 1: The sensor produces a signal proportional to the flowrate and, in some cases, a reference signal (3.9). See 6.2.
Note 2: For a sensor, the wording primary device or flowtube has previously been used.
Note 3: In some cases, further electrodes are used such as grounding electrodes, full pipe detection electrodes (empty pipe detection) (see 3.5).
3.3
transmitter
equipment which contains the circuitry which drives the field coils and extracts the flow signal
Note 1: This equipment may be mounted directly onto the sensor (3.2) or remotely, connected to the sensor by a cable.
Note 2: For a transmitter, the wording secondary device, converter or electronic unit has previously been used.
3.4
meter tube
pipe section of the sensor (3.2) through which the liquid flows, at least part of whose inner surface is electrically insulating
3.5
measuring electrodes
one or more pairs of electrical contacts or capacitor plates by means of which the induced voltage is detected
3.6
lower range value
lowest value of the measured variable that a device is set to measure
3.7
upper range value
highest value of the measured variable that a device is set to measure
3.8
span
difference between the upper and lower range values (3.6)
3.9
reference signal
signal which is proportional to the magnetic flux created in the sensor (3.2) and which is compared in the transmitter (3.3) with the flow signal
3.10
output signal
signal from the transmitter (3.3) which is a function of the flowrate
3.11
Reynolds number
dimensionless parameter expressing the ratio between the inertial and the viscous forces
Note: For closed pipe flow through an electromagnetic flowmeter (3.1), Reynolds number should be based on the nominal diameter of the meter and corresponding mean velocity through a section of that size.
3.12
accuracy
closeness of the agreement between the result of a measurement and the (conventional) true value of the measurement
Note 1: The quantitative expression of accuracy should be in terms of uncertainty (see Annex E).
Note 2: The use of the term precision for accuracy should be avoided.
3.13
uncertainty
range within which the true value of the measured quantity can be expected to lie with a specified value and confidence level
Note: See Clause 11.
3.14
calibration factor
number, determined by liquid calibration, that enables the output signal (3.10) to be related to the volumetric flowrate
3.15
calibration
operation that, under specified conditions, in a first step, establishes a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties and, in a second step, uses this information to establish a relation for obtaining a measurement result from an indication
3.16
verification
means of verifying that an electromagnetic flowmeter (3.1) is operating correctly, normally with a poorer uncertainty than under controlled laboratory conditions
3.17
calibration validation
number of runs (one or more) at flowrates between zero and the upper range value (3.7) in order to verify that the flowmeter does perform in the expected way and within the manufacturer's specification
3.18
measuring window
period of time during which the voltage representing the flow velocity is measured
3.19
ideal flow conditions
conditions that exist when a pipe is infinitely long and straight with no internal disturbances
Note: For electromagnetic flowmeters (3.1), it may, in addition, also be assumed that the metering liquid has a viscosity and density similar to water. Under these conditions, the flow is axisymmetric and will be fully developed and turbulent at flowrates and pipe sizes most often found in industry.
4 Symbols
