Method for determining a physical and/or chemical, temperature dependent, process variable
A method for determining a physical and/or chemical, temperature dependent, process variable of process automation technology utilizing a resistance thermometer (R Pt1000 ), wherein the resistance thermometer (R Pt1000 ) is installed in an electrical circuit, comprising the steps of: measuring a first voltage (U 1 ) across at least a first precision resistor (R 1 ); measuring a second voltage (U 2 ) across at least a second precision resistor (R 2 ); measuring a third voltage (U 3 ) across at least a third precision resistor (R 3 ), wherein cyclically or continuously a constant electrical current (I) is sent through the at least first precision resistor (R 1 ), the at least second precision resistor (R 2 ) or the at least third precision resistor (R 3 ); determining temperature coefficients (a, b, c) characteristic for the electrical circuit by means of the first voltage (U 1 ), the second voltage (U 2 ) and the third voltage (U 3 ); sending the constant electrical current (I) through the resistance thermometer (R Pt1000 ) and measuring a voltage (U) across the resistance thermometer (R Pt1000 ); and determining the temperature (T) by means of the temperature coefficients (a, b, c) and the measured voltage (U). An electrical circuit and a sensor of process automation technology, especially a conductivity sensor, comprising such an electrical circuit are also discussed.
1. A method for autocorrecting a temperature-dependent, process variable of a process medium, the method comprising:
providing an electrical circuit including:
a first precision resistor having a known first resistance;
a second precision resistor having a known second resistance, the second resistance different than the first resistance;
a third precision resistor having a known third resistance; and
a resistance thermometer having a temperature-dependent resistance;
measuring a first voltage across the first precision resistor using a constant electrical current;
measuring a second voltage across at least the second precision resistor using the constant electrical current;
measuring a third voltage across at least the third precision resistor using the constant electrical current, wherein cyclically or continuously the constant electrical current is passed through said first precision resistor, said second precision resistor and said third precision resistor;
calculating temperature coefficients characteristic for the electrical circuit using said first voltage, said second voltage and said third voltage;
passing the constant electrical current through said resistance thermometer and measuring a fourth voltage across said resistance thermometer, the fourth voltage proportional to a temperature;
autocorrecting the temperature using the calculated temperature coefficients and the measured fourth voltage, wherein the autocorrected temperature is calculated using the equation:
T
=
-
b
2
a
-
(
b
2
a
)
2
-
c
a
+
U
a
,
wherein T is the autocorrected temperature, U is the fourth voltage, and a, b and c are the temperature coefficients;
measuring the process variable using a sensor of process automation; and
adjusting the measured process variable using the autocorrected temperature to correct for the temperature dependency of the process variable.
2. The method as claimed in claim 1 , wherein:
the temperature coefficients are calculated by solving the linear system of equations:
U 1= α·R 1 2 +b·R 1+ c,
U 2= α·R 2 2 +b·R 2+ c, and
U 3= α·R 3 2 +b·R 3+ c,
wherein U 1 is the first voltage, U 2 is the second voltage, U 3 is the third voltage, R 1 is a resistance of the first precision resistor, R 2 is a resistance of the second precision resistor, and R 3 is a resistance of the third precision resistor.
3. The method of claim 1 , wherein the second precision resistor is arranged in series with the first precision resistor, the third precision resistor is arranged in series with the second precision resistor and the first precision resistor, and the resistance thermometer is arranged in parallel with the first precision resistor, second precision resistor and third precision resistor.
4. The method of claim 1 , wherein the process variable is a conductivity, a pH, a turbidity, an oxygen concentration, an ammonia concentration or a chlorine concentration.
5. An electrical circuit for autocorrecting a temperature-dependent, process variable of a process medium, comprising:
a resistance thermometer having a temperature-dependent resistance;
a first precision resistor having a known first resistance;
a second precision resistor having a known second resistance, the second resistance different than the first resistance;
a third precision resistor having a known third resistance;
a constant current source configured to cyclically or continuously pass a constant electrical current through the first precision resistor, the second precision resistor, the third precision resistor and/or the resistance thermometer;
a voltage meter configured to measure a first voltage across the first precision resistor, a second voltage across the second precision resistor, a third voltage across the third precision resistor and a fourth voltage across said resistance thermometer, wherein, in each case, the constant electrical current is passed through said first precision resistor, said second precision resistor, said third precision resistor and/or the resistance thermometer; and
a data processing unit configured to:
calculate, using said first voltage, said second voltage and said third voltage, temperature coefficients characteristic for the electrical circuit;
autocorrect a process temperature using the temperature coefficients and said fourth voltage; and
adjust the process variable determined using a sensor of process automation based on the autocorrected process temperature to correct for temperature dependency of the process variable,
wherein the autocorrected process temperature is calculated using the equation:
T
=
-
b
2
a
-
(
b
2
a
)
2
-
c
a
+
U
a
,
wherein T is the process temperature, U is the fourth voltage, and a, b and c are the temperature coefficients.
6. The electrical circuit as claimed in claim 5 , wherein:
said data processing unit calculates the temperature coefficients by solving the linear equation system:
U 1= α·R 1 2 +b·R 1+ c,
U 2= α·R 2 2 +b·R 2+ c, and
U 3= α·R 3 2 +b·R 3+ c,
wherein U 1 is the first voltage, U 2 is the second voltage, U 3 is the third voltage, R 1 is a resistance of the first precision resistor, R 2 is a resistance of the second precision resistor, and R 3 is a resistance of the third precision resistor.
7. The electrical circuit as claimed in claim 5 , wherein:
the electrical circuit includes a switch.
8. The electrical circuit as claimed in claim 7 , wherein:
said switch is configured to switch the electrical current between said first precision resistor, said second precision resistor, said third precision resistor and/or the resistance thermometer.
9. The electrical circuit as claimed in claim 5 , wherein:
the electrical circuit includes a demultiplexer, wherein:
said demultiplexer is configured to switch the electrical current to said first precision resistor, said second precision resistor, said third precision resistor and/or the resistance thermometer; and
the electrical circuit includes a multiplexer, said multiplexer is configured to select said first voltage across said first precision resistor, said second voltage across said second precision resistor, said third voltage across said third precision resistor or the fourth voltage across the resistance thermometer that said voltage meter measures said first voltage, said second voltage, the third voltage or the fourth voltage across said resistance thermometer.
10. The electrical circuit as claimed in claim 5 , wherein:
the electrical circuit further includes at least one analog to digital converter and/or amplifier.
11. A conductivity sensor of process automation technology, comprising an electrical circuit as claimed in claim 5 .
12. The electrical circuit as claimed in claim 7 , wherein:
the switch is a MOS analog switch.
13. The electrical circuit of claim 5 , wherein the second precision resistor is arranged in series with the first precision resistor, the third precision resistor is arranged in series with the second precision resistor and the first precision resistor, and the resistance thermometer is arranged in parallel with the first precision resistor, second precision resistor and third precision resistor.
14. The electrical circuit of claim 5 , wherein the process variable is a conductivity, a pH, a turbidity, an oxygen concentration, an ammonia concentration or a chlorine concentration.