ASME MFC-16-2007 pdf – Measurement of Liquid Flow in Closed Conduits With Electromagnetic Flowmeters
ASME MFC-16-2007 pdf – Measurement of Liquid Flow in Closed Conduits With Electromagnetic Flowmeters.
3.1 DefinitIons From ASME MFC-IM
accuncy: the degree of freedom from error; the degree of conformity of the indicated value to the true value of the measured quantity
pn’iswn: the cloness of agreement between the results obtained by applying the experimental procedure several times under prescribed conditions. The smaller the random part of the experimental errors that affect the results, the more precise is the pnxedure.
rangeabilitv flurudown.): Ilowmeter rangeability is the ratio of the maximum to minimum flow rates or Reynolds number in the range over which the meter meets a specified uncertainty (accuracy).
ptatabihty: Ihc cksseneei of agreement among a series of results obtained with the same method on identical test material, under the same conditions (same operator. same apparatus, same bhorator and short intervals of time).
unarlaintV (rnisurrnwnti: the range within which the true value of the measured quanti can be expected to lie with a specified probability and confidence level.
3.2 DefinitIons for ASME MFC-16
bias: the systematic errors (i.e., tho that cannot he reduced by increasing the number of measurements taken under fixed flow conditions).
flowmefrrpri.nary: includes the flowtube, process conneclions, electromagnetic coils, and electrodes. flowmeter primary is also known by other names such as: flow- meter primary device, primary device, primary etc.
ftoumctL-r svondary: includes the electronic transmitter. measurement of the emfr, and in most cases the power for the electromagnet coils of the llowmeter primary,
?irrearit linearity refem to the constanc of the meter factor over a specified range, defined by either the pipe Reynolds number or the flow rate.
meter fiwtor: the number, determined by liquid calibration, that enables the output flow signal to be related to the volumetric flow rate under defined reference conditions.
3.3 SymboLs See Table 1.
4.2 ELectrochemical Electromotive Force
In addition to the flow-related electromotive force, emf,, an electrochemical electromotive force, eiiif, is present between the electrodes. The e’mf is an electrochemical emf produced in the flowmeter primary similar to that generated in a battery. It can be similar in magnihide to emf. and changes slowly. In order to reduce emf, which would be a measurement bias, an alternating electromagnetic field is used. There exist a number of variations of the basic AC and DC fields shown in this Standard. See para. A-2.1 for additional information. This electrochemical voltage, which varies slowly in time, is substantially reduced in magnitude by utilizing an alternating electromagnetic field.
The manner in which the electromagnetic field is varied includes the following:
(a) AC — field is varied in a sinusoidal fashion [see Fig. 3, illustration (a)]
(b) DC — field is varied in a stepwise fashion [see Fig. 3, illustrations (b) and (c)]
4.3 Types of Electrodes
An alternating electromagnetic field generates an alternating emf1,. Two types of electrodes can be used with an alternating electromagnetic field
(a) wetted electrodes that protrude through the pipe wall/liner into the flow stream [see Fig. 4, illustration (a)]
(b) nonwetted (capacitive) electrodes located behind or within the tube wall/liner [see Fig. 4, illustration (b)]
4.4 Calculation of Volumetric Flow Rate
From eq. (1) the flow velocity is given by.