Compensating for frequency change in flowmeters
View Patent ↗Motion is induced in a conduit that contains a fluid. The motion is induced such that the conduit oscillates in a first mode of vibration and a second mode of vibration. The first mode of vibration has a corresponding first frequency of vibration and the second mode of vibration has a corresponding second frequency of vibration. At least one of the first frequency of vibration or the second frequency of vibration is determined. A phase difference between the motion of the conduit at a first point of the conduit and the motion of the conduit at a second point of the conduit is determined. A quantity based on the phase difference and the determined frequency is determined. The quantity includes a ratio between the first frequency during a zero-flow condition and the second frequency during the zero-flow condition. A property of the fluid is determined based on the quantity.
1. A flowmeter transmitter comprising:
at least one processing device; and
a storage device, the storage device storing instructions for causing the at least one processing device to:
induce motion in a conduit such that the conduit oscillates in a first mode of vibration and a second mode of vibration, the first mode of vibration having a corresponding first frequency of vibration and the second mode of vibration having a corresponding second frequency of vibration, wherein the conduit contains fluid;
determine at least one of the first frequency of vibration or the second frequency of vibration;
calculate a property of the fluid based on the first frequency of vibration and the second frequency of vibration, wherein the first frequency is determined based on the second frequency by calculating:
ω
1
2
=
ω
2
2
(
E
2
+
(
E
2
-
D
2
)
t
2
)
2
(
E
2
+
(
D
4
-
E
4
)
t
2
ω
2
2
)
(
D
2
+
(
D
4
-
E
4
)
(
1
+
t
2
)
ω
2
2
)
where ω 1 is the first frequency, ω 2 is the second frequency,
t
2
=
k
tan
2
(
φ
2
)
where φ is the phase difference k, D 2 , D 4 , E 2 , and E 4 are calibration constants related to physical properties of the conduit,
determine an augmented temperature, wherein the augmented temperature is calculated using:
T f *=T f −k td1 ( T f −T m ),
where T f , is the temperature of the fluid contained in the flowtube, T 0 is a reference temperature and the ambient temperature or flowtube temperature is T m , and k td2 is an empirically determined constant that is specific to the flowtube;
update calibration constants E 2 , and E 4 using augmented temperature, wherein E is calculated using:
E 2 =E 20 (1 +E ( T f *−T 0 ))
E 4 =E 40 (1 +F ( T f *−T 0 )),
determine a density compensated for effects of mass flow rate on frequencies by the calculation:
ρ
0
=
1
2
(
D
4
+
E
4
+
4
D
2
E
2
ω
1
2
ω
2
2
+
(
D
4
-
E
4
)
2
)
,
where D 2 , D 4 , E 2 , and E 4 are calibration constants, and ω 1 is the Coriolis mode frequency, and ω 2 is the driven mode frequency.
2. The transmitter of claim 1 , wherein the property includes a density of the fluid contained in the conduit.
3. The transmitter of claim 2 , wherein the storage device further stores instructions for causing the at least one processing device to:
determine density calibration constants based on physical properties of the conduit;
determine a reference temperature;
determine a first temperature that influences the first mode of vibration;
determine a second temperature that influences the second mode of vibration;
compensate the density calibration constants based on the first temperature, the second temperature, and the reference temperature;
wherein determining the property includes determining the density based the compensated density calibration constants.
4. The transmitter of claim 1 , wherein:
the first mode is a Coriolis mode, and
the second mode is a driven mode.