Siemens MASS 2100 Operating Instructions Manual page 137

Hysteresis is used to adjust the tolerance by undershooting or overshooting the limit as
described below.
Process value derivations
The front-end of the device measures time and derives the values of certain process variables
from those measurements. The time period of vibration of the two measuring tubes is inversely
proportional to their frequency, which is used to determine density. The average difference in
phase of the two measuring tubes is dependent upon the mass flowrate of the process medium.
In this measurement context, phase difference is expressed not in degrees of rotation but as
an absolute time measurement. For this reason the result of zero offset correction is displayed
in µs, being the unit of the true measurement.
The process variables are interrelated and derived in the following fashions:
● Mass flow: proportional to the phase difference between pickup 1 and pickup 2, with
compensations for changes in the metal characteristics due to tube and frame metal
temperatures
● Volume flow: derived directly from the ratio of massflow and media density.
● Standard volume flow: derived from the ratio of massflow and standard density
● Density: derived from the average frequency of sensor tube vibration with compensation
for changes in the metal characteristics with tube temperature. The relationship between
density and vibration frequency is an inverse square-law curve which can be fitted to 3
reference points being the densities of air, hot water and cold water.
● Process media temperature: derived from the tube metal temperature. This is a legitimate
measurement outcome since the tube walls are thin and they are within a sealed, protected
environment, thereby giving similar sensitivity as an insertion thermometer.
● Frame temperature: Derived from the sensor frame termperature
● Fraction A (massflow or volumeflow): derived from the combination of media density and
temperature, and compared with a stored table of fraction percentage against a wide range
of both process values through a fifth-order polynomial
● Fraction B (massflow or volumeflow): ditto but fraction B is "Flow – A"
● Fraction A %: as for fraction A quantity but A% is the ratio between Fraction A flow and
Total flow
● Fraction B %: ditto but B% is "100% – A%"
Metal temperatures are measured using precision Pt1000 sensors. The accuracy of the
1)
temperature measurement is ±0.5 °C.
Standard density is the density of the media at reference conditions, normally atmospheric
2)
pressure and 20 °C. Standard density can be programmed into the flowmeter menu in two
forms, either as a fixed reference or with a selection of linear or square-law temperature
dependence. The choice of fixed or calculated standard density and of linear or square-law
temperature dependency is according to the application and user preferences.
The customer-specified density/temperature tables may be derived from the mass fraction
3)
or volume fraction of any two-part mixture. Fraction calculations are naturally performed in the
ratio provided, or in mass ratio when using the built-in tables. Volume or mass ratios derived
from the fraction table are calculated through the composite media density.
MASS 2100 & FC300 (FCT030)
Operating Instructions, 06/2017, A5E39789040-AA
.
1)
8.1 Process values
3)
Functions
.
2)
137