GB2332072A

GB2332072A - Controlling heating element usable for flow or thermal conductivity measurement

Info

Publication number: GB2332072A
Application number: GB9825203A
Authority: GB
Inventors: Hartmut Stark
Original assignee: Draeger Medical GmbH
Current assignee: Draeger Medical GmbH
Priority date: 1997-12-06
Filing date: 1998-11-17
Publication date: 1999-06-09
Anticipated expiration: 2018-11-17
Also published as: GB2332072B; GB9825203D0

Abstract

A thermal element 2 (ie an electrically heated element which comprises a junction 7 between two dissimilar materials so that the element produces a DC voltage related to its temperature), is arranged in a gas atmosphere 2, and is kept by means of an AC voltage source 11 at a working temperature that is higher than the gas temperature. The thermal element voltage is separated from the AC voltage, as shown by a low pass filter 12, and compared at 14 with a predetermined reference voltage U b related to the desired working temperature of the thermal element. The difference signal controls the AC voltage source 11 to maintain the thermal element at the desired temperature. The AC voltage is rectified at 16, and displayed 18. In alternative embodiments, the AC current may be measured as the voltage across a series resistor, or the AC voltage may be disconnected from the element at intervals, and the DC voltage determined at that time. The element may be used to measure thermal conductivity of a static gas in channel 3, or the flow rate of a gas in channel 3, the measurement being determined in either case from the power required to maintain the thermal element at a constant temperature above the temperature of the gas atmosphere 2.

Description

2332072 Method of operating a thermal element The invention relates to a

method of operating a thermal element disposed in a gas atmosphere, which is kept by means of an a.c. voltage source at a working temperature above that of the gas atmosphere, and to an apparatus for carrying out such a method.

A thermal element of the general type is known from EP 187 723. A thermal element comprising two parts made of different materials (a thermocouple) is connected to an a.c. voltage source to heat it to a predetermined working temperature. The value to be measured is determined by evaluating the induced d.c. thermal element voltage. To remove the a.c. voltage portion superimposed on the thermal element voltage from the measurement circuit, an evaluation element comprising low pass filters is connected to the terminals of the thermal element. Specific physical features of the gas atmosphere surrounding the thermal element are to be determined from the thermal element voltage.

Other thermal measurement procedures are known which involve the introduction of a predetermined heat power into a gas medium, whilst simultaneously determining the temperature. A measurement procedure of this type is used, for instance, in a hot wire anemometer. Here the actual measuring element is in direct thermal contact with the medium to be measured, and the temperature difference between the measuring medium and the measuring element is regulated to be a constant value. In the case of throughflow measurement, with known gas parameters, the heat output necessary for temperature regulation is a direct measure of the flow speed. In order to be able to regulate the temperature difference between the measuring medium and the measuring element to the required constant value, it is usual to determine the temperatures of the measuring element and the measuring medium separately, and to calculate the excess temperature by taking the difference in an evaluation unit connected afterwards. Whilst the temperature coefficient of the heating conductor used is available for determining the temperature of the measuring element, any temperaturedependent resistance can be used to measure the temperature of the medium, for instance. In practice, however, for reasons of symmetry the use of two similar elements is accepted, one of which is the active element which is intended to be heated, while at the same time the second passive element is used with a minimum evaluation current for measuring the temperature of the medium.

This widely used arrangement, however, has some disadvantages which result from its principle. The actually relevant temperature difference, which has a direct effect on the measurement quality, is produced by the formation of the difference between two relatively large electrical signals. Since, in a procedure of this type, the relative errors in the difference quantity are increased, this difference signal will react in a very sensitive manner with respect to disturbances in the individual measurement values. To make matters worse, the individual temperature measurement values used to form the difference first have to be determined by conversion from the measured resistance values. These resistance values are also affected by contact resistances of the plug contacts located in the supply lines. Fluctuations in the contact resistances at the plug contacts affect in various ways the resistances of the heating conductor used and of the temperature measuring element, it also being possible for the contact resistances to change with time. If a resistance measuring bridge is used to form the difference, then a frequent rebalancing of the zero is necessary because of the time variation of the resistances.

The fact that the temperature is obtained from the difference between two individual temperatures, one of which is substantially higher than the other, makes the spatial separation of the temperature measuring points necessary. If this requirement is not met, then heat conduction through the medium, and convection effects, interfere considerably with the independent measurement of the individual temperatures, especially in the low flow speed ranges. However, the spatial separation of the temperature measuring points would mean a larger structure. Moreover, there is no guarantee in every case that with sufficiently distant temperature measuring points the measured temperature of the medium will reflect the conditions at the heated element, since inhomogenous temperature distributions in the flow cannot be excluded. Such a hot wire anemometer is described in US 3 913 379.

A thermal anemometer is known from DD 245 055 A1 which works in accordance with the constant temperature difference measurement principle and is used to measure the speed of a gas flow. A sensor comprising a hot wire and two thermal elements is placed in the gas atmosphere in such a way that one of the thermal elements measures the temperature of the hot wire and the other thermal element measures the temperature of the gas atmosphere. By means of a control loop the difference between the hot wire temperature and the gas temperature is kept constant. The electrical power supplied to the hot wire is a measure of the speed of the gas flow. With the known anemometer, different components are used for heating and temperature measurement, which is technically complicated.

A corresponding measuring device for determining the speed of a liquid in a channel can be seen in GB-A 729 180. With this device also different components are used for the heating and for the measurement of temperature difference.

is The aim of the invention is to provide a measurement procedure which makes it possible to determine the thermal output introduced into a fluid, with simultaneous temperature detection.

According to a first aspect of the invention there is provided a method of operating a thermal element arranged in a fluid, which is kept by means of an a.c. source at a working temperature which is higher than the fluid temperature, comprising the steps of separating the EMF of the thermal element, the thermal element voltage, from the alternating voltage, comparing the thermal element voltage with a predetermined reference voltage U,, corresponding to the working temperature, forming a difference signal between the thermal element voltage and the reference voltage U,, and controlling the a.c. source with the difference signal so that the thermal element voltage is kept at a constant value with respect to the reference voltage UB.

The first aspect of the invention also includes a system for carrying out the method comprising a thermal element, a controlled a.c. source for supplying a heating current to the thermal element, a separation element for separating the d.c. thermal e.m.f.

generated by the thermal element from the a.c. signal, and a subtraction element for comparing the separated d.c. thermal e.m.f. with a predetermined value to form a control input to the a.c. source.

is According to a second aspect of the invention there is provided a method of operating a thermal element disposed in a fluid, which is kept by means of an a.c. voltage source at a working temperature which is higher than the fluid temperature, comprising the steps of: measuring the EMF of the thermal element, the thermal element voltage, during time periods in which the a.c. voltage source is cut off by means of a switch from the thermal element, comparing the thermal element voltage with a predetermined reference voltage U. corresponding to the working temperature, to form a difference signal between the thermal element voltage and the reference voltage U,, and influencing the a.c. voltage source with the difference signal so that the thermal element voltage is kept at a value that is constant with respect to the reference voltage U..

The second aspect of the invention also includes a system for carrying out the method comprising a thermal element, an a.c. source for supplying a heating current to the thermal element, a switch arranged to disconnect the a.c. source from the thermal element so that the thermal e.m.f. of the thermal element can be obtained, and a subtraction element for comparing the obtained thermal e.m.f. of the thermal element with a predetermined value, wherein the output of the subtraction element is arranged to control the a.c. source.

The fluid may be gas or liquid.

An advantage of the invention is that in the case of a thermal element which is heated by means of an a.c. voltage source to a working temperature, the thermal element voltage produced on the thermal element is used to regulate the temperature of the thermal element. By using the thermal element voltage for temperature regulation a separate temperature measuring device is no longer necessary.

The thermal element voltage represents a voltage value which is proportional in value to the temperature difference between the heated junction point in the centre of the wire, and the two junction points on the supply wires. Since the junction points on the supply wires, which are also called comparative points, are essentially at the temperature level of the gas atmosphere, the separate temperature measuring device can be dispensed with. With respect to throughflow measurement with two measuring wires, the measurement procedure provided by the invention is marked by the elimination of the error-ridden determination of the temperature difference from two approximately equal measurement values. Instead of this, a signal proportional to the temperature difference is generated directly at the measurement point. Furthermore, the temperature difference signal as the thermal element voltage is already available as an electrical signal which can be directly further processed, rendering superfluous the disturbance-ridden conversion of a resistance signal into temperature values. Contact and line resistances have no effect on the conveyance of the measurement signal since it is a pure voltage signal. Since only an outgoing and a returning line are necessary for supplying the heat power and conducting the thermal element voltage, cabling is made easier with the simultaneous reduction of the plug connections. Apart from handling advantages this also means lower production costs.

The regulation of temperature of a heat circuit proposed in accordance with the invention can be particularly advantageously used with a thermal infrared radiator, whose heating wire is composed of two parts of different materials. The two parts may then be wound in such a way that the junction point of the parts is approximately in the middle of the wire coil. The radiation properties can be further improved by providing the coil with a ceramic coating.

The apparatus according to the invention is particularly advantageous for measuring the flow speed of a gas, or, for the heat conductivities of a stationary gas.

The physical quantity to be measured, i.e. the speed or the heat conductivity, is determined by the a.c. voltage across the thermal element. Alternatively, the physical quantity can be determined by the amplitude of the current flowing through the thermal element- To measure the current a measurement resistor is connected in series with the thermal element and the voltage across the resistor measured.

A specific embodiment of the invention will now be described. purely by way of example, with reference to the accompanying drawings, in which:

Figure 1 shows a measuring device arranged in a gas flow channel, Figure 2 shows a schematic representation of a temperature-regulated infra-red radiator, 8- Figure 3 shows a thermal element in a gas flow channel, is Figure 4 shows a sectional representation of the thermal element according to Figure 3 along the line of intersection A A, Figure 5 shows a second measuring device arranged in a gas flow channel, and Figure 6 shows a third measuring device arranged in a gas flow channel.

Figure 1 shows schematically a first measuring device 1 in which a first thermal element 2 is arranged in a gas flow channel 3. Gas flows through the gas flow channel 3 along an axis 4 perpendicular to the drawing plane. The first thermal element 2 comprises a first part 5, of Chromel, and a second part 6 of Alumel which meet at a junction point 7 and are joined by means of further junction points 8, 9 to supply wires 10. The first thermal element 2 is connected to a d.c. free alternating voltage source 11 (a.c. source) and is heated by this to a working temperature which is higher than the temperature of the medium in the gas flow channel. With this wiring of the first thermal element 2, an alternating voltage signal is applied to the supply wires 10 and superimposed on it is a direct voltage, below referred to as the thermal element voltage, caused by the intrinsic EMF of the first thermal element 2. This thermal element voltage is filtered out of the alternating voltage signal by means of a low pass filter 12 connected to the supply wires 10 as an elimination element, is increased by an amplifier 13 to a higher signal level, and is compared with a reference voltage U. at a subtraction element 14. The reference voltage U,, corresponding to a desired is temperature value, is proportional to the desired working temperature of the first thermal element 2. The difference signal from the amplified thermal element voltage and the reference voltage U,, produced at the output 15 of the subtraction element 14, is supplied to the a.c. source 11 to control the output of the a.c- voltage source. The first thermal element 2, the low pass filter 12, the amplifier 13, the subtraction element 14 and the a.c. voltage source 11 together form a temperature regulation circuit, the thermal element voltage representing the actual temperature value. In addition to the low pass filter 12, a rectifier 16 is connected to the supply wires 10, and connected on the outlet side of this rectifier are a smoothing device 17 and an indicator 18. The indicator 18 provides a measurement value proportional to the effective value of the amplitude of the a.c. voltage signal applied at the supply wires 10.

The first measuring device 1 of the invention works as follows:

When there is a constant gas flow in the gas flow channel 3, a constant working temperature resulting from the reference voltage U,, is set up on the thermal element 2, and a read-out, also constant, can be read on the indicator 18 and determined as the zero point for the constant gas flow. In the gas of a gas flow in the gas flow channel 3 which is different from zero, the first thermal element 2 is initially cooled by the gas flow. The voltage of the a.c. voltage source 11 and hence the heat power of the first thermal element 2 is accordingly increased, until the original temperature has again been reached. According to the voltage increase in the a.c. voltage source 11, the read-out of the indicator 18 also increases, the read- out relative to the zero position being a measure of the speed of the flowing gas in the gas flow channel 3. In this way the gas flow in the gas flow channel 3 can be determined particularly easily. With a stationary gas flow the first measuring device 1 is suitable for determining the heat conductivity of the gas atmosphere surrounding the first thermal element 2.

is Figure 2 shows a schematic representation of a temperature-regulated infra-red radiator 20, having a second thermal element 21 which is composed of two parts 22, 23 which are made of different materials. The parts 22, 23 are welded together at a junction point 24 and joined to the supply wires 10 at further junction points 25, 26. Like components are given the same reference numerals as in Figure 1. The parts 22, 23 of the second thermal element 21 are designed in a coil shape. To increase the heat capacity of the infra-red radiator 20, the parts 22, 23 and the junction point 24 are embedded into a ceramic material 27.

The method of operation of the infra-red radiator 20 is based on the fact that the thermal element voltage of the second thermal element 21, which is proportional to the temperature of the second thermal element 21, is separated with the low pass filter 12 from the heating a.c. voltage, amplified by the amplifier 13 and then further processed for temperature regulation. The amplified thermal element voltage is compared at the subtraction element 14 with the reference voltage U,, the reference voltage U. being proportional to the temperature to be set.

Figure 3 shows schematically the structure of a third thermal element 28, in which a chromium nickel wire 31 extending between two junction points 29, 30 is provided in one part 32 with a nickel coating 33. The nickel coating 33 extends approximately from the centre 34 of the wire 31 to the junction point 30. Like components are given the same reference numerals as in Figure 1. The nickel coating 33 can be made, for instance, in such a way that the chromium nickel wire 31 is clamped between the supply wires 10 and is then coated up to a half with pure nickel, for instance by galvanising.

In Figure 4, for the sake of clarity, a sectional representation of the chromium nickel wire 31 with the nickel coating 33 is shown along a line of intersection A-A, Figure 3.

The second measuring device 100 shown in Figure 5 is an alternative embodiment to the first measuring device 1, Figure 1. Like components are provided with the same reference numerals as in Figure 1. By means of a switch 19, which has first switch contacts 191 in the wiring path between the amplifier 13 and the first thermal element 2 and second switch contacts 192 in a line extending from the a.c. voltage source 11 to the first thermal element 2, the heat flow is cyclically interrupted. The thermal element voltage is only measured during the time periods in which the a.c. voltage source 11 is separated from the first thermal element 2. For the cyclical interruption of the heat flow, the switch 19 is controlled by a square wave generator 193 with reversing pulses. In Figure 5, the time of temperature measurement with the second measuring device 100 is shown. The first switch contacts 191 are closed here, and the second switch contacts 192 are open. The cyclical interruption of the heat flow,means that it is no longer necessary to filter-out the thermal element voltage from the a.c. voltage signal by means of the low pass filter 12, of the embodiment of Figure 1.

Figure 6 shows schematically a third measuring device 200 which differs from the first measuring device 1 according to Figure 1 in that the amplitude of the alternating current flowing through the thermal element 2 is evaluated instead of the amplitude of the alternating voltage. For this purpose, a measurement resistance 35 is located in a power line leading to the supply wires 10, on which resistance in voltage proportional to the alternating current occurs. This alternating voltage is rectified and conveyed via the smoothing device 17 to the indicating instrument 18.

is

Claims

1. A method of operating a thermal element arranged in a fluid, which is kept by means of an a.c. source at a working temperature which is higher than the fluid temperature, comprising the steps of separating the EMF of the thermal element, the thermal element voltage, from the alternating voltage, is comparing the thermal element voltage with a predetermined reference voltage UB corresponding to the working temperature, forming a difference signal between the thermal element voltage and the reference voltage U, and controlling the a.c. source with the difference signal so that the thermal element voltage is kept at a constant value with respect to the reference voltage U,

2. A method of operating a thermal element disposed in a fluid, which is kept by means of an a.c. source at a working temperature which is higher than the gas temperature, comprising the steps of:

measuring the EMF of the thermal element, the thermal element voltage, during time periods in which the a.c. voltage source is cut off by means of a switch from the thermal element, comparing the thermal element voltage with a predetermined reference voltage U,, corresponding to the working temperature, to form a difference signal between the thermal element voltage and the reference voltage U, and influencing the a.c. source with the difference signal so that the thermal element voltage is kept at a value that is constant with respect to the reference voltage US.

is

3. A method according to claim 1 or 2, wherein a measurement value proportional to the flow speed of the fluid is determined from the amplitude of the a.c. voltage across the thermal element or from the amplitude of the a.c. current flowing through the thermal element.

4. A method according to claim 1 or 2, wherein the fluid is stationary and a measurement value proportional to the heat conductivity of the fluid is determined from the amplitude of the a.c. voltage across or a.c. current through the thermal element.

5. A method according to claim 3 or 4, wherein the measurement value is formed by rectification of the a.c. voltage or a.c. current.

6. A method according to any of claims 1 to 3 claim wherein the reference voltage U. is set to a value such that the thermal element assumes a temperature suitable for emitting infra-red radiation.

7. A method according to claim 6, wherein the thermal element is used as an infra-red radiator.

8. A system for carrying out the method according to claim 1, comprising thermal element, controlled a.c. source for supplying a heating current to the thermal element, a separation element for separating the d.c.

-is- thermal e.m.f. generated by the thermal element from the a.c. signal, and a subtraction element for comparing the separated d.c. thermal e.m.f. with a predetermined value to form a control input to the a.c. source.

9. A device according to claim 8, further comprising a demodulator to rectify the a.c. voltage developed across the thermal element.

is

10. A device according to claim 8, further comprising a demodulator to rectify an a.c. voltage proportional to the a.c. current flowing through the thermal element.

A system for carrying out the method according to claim 2, comprising a thermal element, an a.c. source for supplying a heating current to the thermal element, a switch arranged to disconnect the a.c. source from the thermal element so that the thermal e.m.f. of the thermal element can be obtained, and a subtraction element for comparing the obtained thermal e. m.f. of the thermal element with a predetermined value, wherein the output of the subtraction element is arranged to control the a.c. source.

12. A method of operating a thermal element substantially as herein described with reference to Figure 1, Figure 2, Figures 3 and 4, Figure 5 or Figure 6 of the accompanying drawings.

13. A system substantially as herein described with reference to and as shown in Figure 1, Figure 2, Figure or Figure 6 of the accompanying drawings.

13. A system substantially as herein described with reference to and as shown in Figure 1, Figure 2, Figures 3 and 4, Figure 5 or Figure 6 accompanying drawings.

of the - CIAmendments to the claims have been filed as follows Claims is 1. A method of operating a thermocouple arranged in a fluid, which is kept by means of an a.c. source at a working temperature which is higher than the fluid temperature, comprising the steps of separating the EMF of the thermocouple, the thermocouple voltage, from the alternating voltage, comparing the thermocouple voltage with a predetermined reference voltage U. corresponding to the working temperature, forming a difference signal between the thermocouple voltage and the reference voltage U, and controlling the a.c. source with the difference signal so that the thermocouple voltage is kept at a constant value with respect to the reference voltage UB.

2. A method of operating a thermocouple disposed in a fluid, which is kept by means of an a.c. source at a working temperature which is higher than the gas temperature, comprising the steps of:

measuring the EMF of the thermocouple, the thermocouple voltage, during time periods in which the a.c. voltage source is cut off by means of a switch from the thermocouple, -I%- comparing the thermocouple voltage with a predetermined reference voltage UB corresponding to the working temperature, to form a difference signal between the thermocouple voltage and the reference voltage U, and influencing the a.c. source with the difference signal so that the thermocouple voltage is kept at a value that is constant with respect to the reference voltage UB 3. A method according to claim 1 or 2, wherein a measurement value proportional to the flow speed of the fluid is determined from the amplitude of the a.c. voltage across the thermocouple or from the amplitude of the a.c. current flowing through the thermocouple.

4. A method according to claim 1 or 2, wherein the fluid is stationary and a measurement value proportional to the heat conductivity of the fluid is determined from the amplitude of the a.c. voltage across or a.c. current through the thermocouple.

6. A method according to any of claims 1 to 3 claim wherein the reference voltage U. is set to a value such that the thermocouple assumes a temperature suitable for emitting infra-red radiation.

7. A method according to claim 6, wherein the thermocouple is used as an infra-red radiator.

is 8. A system for carrying out the method according to claim 1, comprising thermocouple, controlled a.c. source for supplying a heating current to the thermocouple,.

a separation element for separating the d.c. thermal e.m.f. generated by the thermocouple from the a.c. signal, and a subtraction element for comparing the separated d.c. thermal e.m.f. with a predetermined value to form a control input to the a.c. source.

9. A device according to claim 8, further comprising a rectifier to rectify the a.c. voltage developed across the thermocouple.

10. A device according to claim 8, further comprising a rectifier to rectify an a.c. voltage proportional to the a.c. current flowing through the thermocouple.

11. A system for carrying out the method according to claim 2, comprising a thermocouple, an a.c. source for supplying a heating current to the thermocouple, a switch arranged to disconnect the a.c. source from the thermocouple so that the thermal e.m.f. of the thermocouple can be obtained, and a subtraction element for comparing the obtained thermal e.m.f. of the thermocouple with a predetermined value, wherein the output of the subtraction element is arranged to control the a.c. source.

12. A method of operating a thermocouple substantially as herein described with reference to Figure 1, Figure 2, Figure 5 or Figure 6 of the accompanying drawings.