Method and device for measuring fat in milk
Disclosed is a method for determining of fat content of milk having variable solids fractions and flowing with variable gas content in a pipeline. The method includes ascertaining a velocity of sound and an average density value for the milk based on eigenfrequencies of at least two bending oscillation wanted modes of measuring tubes of a densimeter arranged in the pipeline. The method further includes ascertaining a static pressure in the pipeline; a gas volume fraction based on the velocity of sound; the average density; the pressure; a density of the milk without gas content based on the average density and the gas volume fraction; and a permittivity of the milk based on a propagation velocity and/or an absorption of microwaves in the milk. The fat fraction is calculated based on the density of the milk without gas content and on the effective permittivity.
1. A method for determining a fat content of milk, wherein the milk has variable solids fractions and flows with a variable gas content in a pipeline, the method comprising:
arranging a densimeter in the pipeline and flowing the milk through the densimeter;
determining a first eigenfrequency of a first bending oscillation mode of the densimeter and a second eigenfrequency of a second bending oscillation mode of the densimeter;
calculating a velocity of sound in the milk and calculating an average density for the milk based on the first and second eigenfrequencies;
measuring a static pressure in the pipeline using a pressure sensor connected to the pipeline;
calculating a gas volume fraction of the milk based on the velocity of sound in the milk, the average density, and the static pressure;
calculating a density of the milk without gas content based on the average density and based on the gas volume fraction;
measuring a propagation velocity and/or an absorption of microwaves in the milk using a microwave sensor arranged in the pipeline and calculating an effective permittivity of the milk from the propagation velocity and/or the absorption; and
calculating a fat fraction of the milk based on the density of the milk without gas content and on the effective permittivity of the milk.
2. The method as claimed in claim 1 , further comprising:
modeling the milk as a three component system of fat, water, and fat-free solids.
3. The method as claimed in claim 2 , wherein the fat-free solids include proteins and carbohydrates.
4. The method as claimed in claim 3 , further comprising:
calculating a concentration of each of the three components of fat, water, and fat-free solids:
modeling the density of the milk without gas content as a function of the concentration of each of the components of fat, water, and fat-free solids using density values of the respective component as weighting factors;
modeling the effective permittivity of the milk using the gas volume fraction of the milk, the concentration of each of the components of fat, water, and fat-free solids, and permittivity values of the respective components.
5. The method as claimed in claim 4 , further comprising:
measuring a temperature of the milk; and
calculating temperature dependent values for density and/or permittivity of the components contained in the milk.
6. The method as claimed in claim 4 , wherein the densimeter includes a Coriolis mass flowmeter, the method further comprising:
calculating at least one of the following: a mass flow of the milk, a volume flow of the milk, a fat flow of the milk, a fat-free solids flow of the milk, and a water flow of the milk in the pipeline.
7. The method as claimed in claim 1 , wherein the microwaves include at least one frequency above 1 GHz.
8. A measuring arrangement for determining a fat content of milk in a pipeline, comprising:
a densimeter having at least one oscillatable measuring tube and configured to determine a first eigenfrequency of a first bending oscillation mode and a second eigenfrequency of a second bending oscillation mode;
a pressure sensor for measuring a static pressure in the pipeline;
a microwave sensor for ascertaining a propagation velocity and/or an absorption of microwave signals in the milk; and
a computer unit for calculating the fat content of the milk, wherein the computer unit is configured to:
calculate a velocity of sound in the milk and calculate an average density for the milk based on the first and second eigenfrequencies;
read the static pressure in the pipeline using the pressure sensor;
calculate a gas volume fraction of the milk based on the velocity of sound in the milk, the average density, and the static pressure;
calculate a density of the milk without gas content based on the average density and based on the gas volume fraction;
measure the propagation velocity and/or the absorption of microwaves in the milk using the microwave sensor;
calculate an effective permittivity of the milk from the propagation velocity and/or absorption; and
calculate a fat fraction of the milk based on the density of the milk without gas content and on the effective permittivity.
9. The measuring arrangement as claimed in claim 8 , wherein the densimeter, the pressure sensor, and the microwave sensor are installed in the pipeline.
10. The measuring arrangement as claimed in claim 8 , wherein the densimeter includes a Coriolis mass flowmeter.