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26 MagCheck
5 Interpretation of verification results
Check in the trend analysis which parameters are exceeding their limits. Most common causes are
outlined below.
5.1 Accuracy data ADC, field current, mA or frequency output
ADC is the primary signal processing. It includes input instrumentation amplifier and analogue to
digital conversion of the flow signal voltage. Accuracy of ADC and field current directly influence
accuracy of any output of the electromagnetic flowmeter. If accuracy of ADC, mA or frequency
output, or field current is wrong: exchange converter electronics.
5.2 Coil resistance
Coil resistance value is dependent of coil temperature during verification. Coil temperature again is
dependent on ambient temperature and on medium temperature in the flow tube of the magnetic
flowmeter. Temperature coefficient is 0.4% per °C (0.2% per °F). Two failure modes can be
detected:
•
Resistance is zero or far below plausible value: Check cabling, connectors, terminals to field
coils on short circuits.
•
Resistance is high: Check cabling, connectors, terminals to field coils on interruptions or poor
contact.
Check connectors / connections below the terminal board. If these are ok and coil resistance
remains infinite or zero, interruption or short-circuit of an internal connection is possible (but
extremely rare). In that case, the flowhead must be exchanged.
5.3 Electrode resistance with full pipe
In general: Electrode resistance with full pipe depends on conductivity (
σ
) of the liquid. Conductivity
depends on liquid type, concentration, temperature. Change of electrode resistance between two
verifications only is significant as long as precisely the same liquid with same concentration and
temperature will flow through the flowmeter without creating any deposits. Electrode resistance
values may be estimated from electrode diameter “d” and conductivity (
σ
) according to following
formula:
[ ]
[ ] [ ]
100% / 50% : Tol.
/
1000
+ −
×
= Ω
cm S cm d
k R
Electrode
µ σ
This formula may be used for approximate plausibility checks if the results of electrode resistance
measurements are outside of the measuring range of MagCheck. The results of this formula may
show a deviation to true value of electrode resistance as indicated by the tolerance data.
Electrode resistance with full pipe very high: Check cabling to electrodes (signal cable, connector in
remote flowhead connection box) on interruption. Remove meter from line, clean electrodes and
grounding rings.
Asymmetric electrode resistance: Differences in electrode 1- and 2 resistance values by more than
±
50% may be caused by:
•
Heavy coating (contamination) on one of the electrodes (Countermeasure: Clean electrodes)
•
Beginning short circuit of one electrode, e.g. by contamination with highly conductive matter
(Action: Clean electrodes and liner carefully) or by electrode leakage.
Electrode leakage and conductive deposits can be confirmed by verification with empty pipe.
Select “Pipe Empty” in Menu 1.1.