JPH0547066B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Industrial Application The present invention can be operated both by the real heat method and by the heat conduction method, and below a predetermined reference value for the measurement signal, the real heat method is used. It relates to a method for detecting the proportion of combustible gases in an air mixture with the aid of a measuring device adapted to an indicator unit which only takes readings.

The invention also relates to a measuring device suitable for this method.

PRIOR ART For the detection and measurement of combustible gases, the gas is catalyzed on the surface of the sensor at a certain temperature (e.g. 500°C), i.e. consuming the oxygen fraction present in the gas to be measured. It is desirable to have the gas sensor heated to such a temperature that it burns at a temperature that increases the temperature of the sensor.
The actual calorific effects occurring during the combustion reaction are analyzed and expressed by the sensor temperature rise as a measurement signal regarding the concentration of the air-mixed combustible gas in which the test is carried out. The measuring devices used for this are generally active (detector) and passive (compensator) sensors in a half-arm bridge arrangement. This bridge is powered either by constant current or constant voltage. This type of measuring device works well when the concentration of flammable gases in the air is above the 100% LEL (Least Explosive Limit). For example, for methane, this limit is 5
%.

Above this value, the temperature rise in the detector,
Residual activity in the compensator, oxygen depletion and heat conduction in the gas mixture all lead to an increase in the nonlinear measurement signal in gas concentration.

For gas concentrations in the range higher than 100% LEL, the bridge output signal remains unambiguous, ie, two gas concentrations are found for each output voltage value.

To achieve unambiguous measurements of concentrations above 100% LEL, a Wheatstone bridge, for example, is provided in a separate arm. This will also only measure the change in the thermal conductivity of the gas mixture around the sensor in a bridge arrangement with a reduced operating temperature, for example 200°C.

When a certain threshold value for heat transfer in the range of 100% LEL is reached, the indicator connected to the zero arm of the actual heat Wheatstone bridge automatically switches to full scale reading through the appropriate control element. This provides an indication that the gas sample contains a combustible gas concentration greater than 100% LEL.

A type of measuring device and method with switching between heat conduction and actual calorimetry is described in DE-05 16 73 306.

In the case of already known measuring devices, for example to measure methane in the air, one sensor can
At an operating temperature around 500°C, the actual heat value is installed in the Wheatstone bridge, and another sensor is e.g.
Installed in a thermally conductive Wheatstone bridge with an operating temperature of 2000°C.

In the measurement range up to 100% LEL, the sensor in the real heat Wheatstone bridge gives a clear signal.

When the 100% LEL limit is exceeded, the measuring instrument in the zero arm of the real heat Wheatstone bridge actually switches to a full scale reading, but the current or voltage supply to the real heat sensor continues.

If the methane content in the air mixture is 100% LEL
If increased significantly beyond , the combustion heating on the surface of the catalytic calorific value sensor may result in thermal breakdown of the catalytic bed, or at least limit the sensor's use for other measurements. Expand until it looks like this.

At these high concentrations, catalytic combustion is spontaneous and will continue until the gas concentration decreases or the catalyst becomes fouled and/or the sensor is damaged, depending on the operating temperature of the sensor. There is no point to switch off the heating current to reduce the heating current.

After the indicator in the zero arm of the real heat Wheatstone bridge switches to full scale reading, a practically detectable concentration measurement of harmful substances present in the air mixture is no longer possible.

In already known measuring devices, the two Wheatstone bridges must be supplied with power simultaneously and continuously, and in particular:
Both sensors in the Wheatstone Bridge are
200℃ or 500℃ to ensure constant measurement readiness
It is necessary to keep heating to ℃.

This means that power consumption increases because both sensors need to be maintained at their respective operating temperatures at the same time, but in actual measurements or Only one of them is needed to set the time to switch to full-scale reading. In order to provide the measuring equipment with the necessary charge time, this increased power demand requires suitably increased capacity power supplies and, in the case of portable measuring instruments, for example, batteries that are heavy and therefore difficult to handle. That will happen.

OBJECT OF THE INVENTION The problem that the present invention seeks to solve is to reduce power consumption as low as possible, from 0 to 100% by volume.
It is an object of the present invention to improve known methods of detection of combustible gases so as to allow unambiguous measurements of combustible gases in air such that the total concentration is in the range of . In addition, the accuracy of the measurements in the low measurement range near the lowest explosive limit is particularly high. Measuring instruments capable of operating in a model with an improved method are:
It is small, light, easy to maintain, and has a small number of parts. The life of the sensing element is also longer.

Configuring the Invention This problem is solved by a method characterized by the use of a single sensor element that is catalytically active at higher temperatures. At the beginning of the measurement, the sensor is kept constant at an initial temperature T ₁ , which is suitable for heat transfer measurements, and the resulting measurement signal is compared with the reference value R ₁ , and if it is lower than the reference value R ₁ . If it is below the temperature of the sensor is a second temperature that allows measurement of the actual calorific value.
The measured signal emitted by the sensor, which is increased to T ₂ and kept constant there, is compared with a load limit value G, which corresponds to an even higher gas concentration than R ₁ , and whose temperature is increased until the measured signal reaches the limit value. As soon as the temperature exceeds G, the temperature is lowered to _T1 and kept constant.

When the sensor is in operation, the first step in the process is to check if any combustible gas is present.

This is done by setting the sensor to take heat transfer measurements at a low operating temperature T ₁ using very little power.

If the measurement result signal is a combustible gas,
If it is shown to be present in a concentration smaller than the value corresponding to R ₁ , the operating mode of the sensor is switched to real calorimetry and its operating temperature is increased to T ₂ .

Actual calorimetry can be performed more accurately than conductivity measurements, and the concentration of combustible gases in the air mixture is more accurately indicated. The reference value R ₁ is thus:
For more accurate actual calorific value measurements that indicate the presence of combustible gas and do not damage the sensor when such low concentrations of flammable gas are present before the minimum explosive limit of the gas mixture is reached. It can be viewed as a threshold value for switching the sensor at an appropriate time.

When the sensor enters the actual heat measurement mode of operation, the measurement signal generated is compared with the load limit value G.

As long as the measurement signal is below the limit value G, the sensor remains in the actual calorimetry operating mode. For example, when the concentration of the gas increases and reaches this limit value G, the sensor switches to the thermal conductivity measuring mode of operation and to the lower operating temperature T ₁ . Even if the gas concentration increases further, measurements continue to be carried out in this mode and the measured values are checked without ambiguity under the corresponding gas concentration. The purpose of fixing the limit value G is
When the gas concentration rises, a lower temperature T ₁ is required before self-continuation of catalytic combustion occurs and destroys or damages the sensor despite the removal of any power supply.
This is because I want to switch to In more accurate actual calorific value measurement operation, in the presence of combustible gas,
It is possible that the sensor is not subjected to uncontrolled heating, but is kept constant at the desired temperature T ₂ . Switching to the thermal conductivity measurement operating mode at the correct time is necessary for actual calorific value measurement when the concentration of combustible gas increases. This will protect the sensor's catalyst surface from damage. In this way, the lifetime of the sensor is extended and also the amount of power consumption required to maintain operating temperature is limited.

Reference values R ₁ and G are 100% LEL and 140%
It is reasonable to locate it in the range between LEL.
Thus, all measurements below this range are
This was done using the actual calorific value measurement method.

The varying method taken to perform the actual calorimetry is carried out only when the measurement signal is within the window between the reference values R ₁ and R ₂ .

In this way, a lower threshold value is determined, below which energy-saving heat conduction measurements are carried out, and only when the reference value R ₂ is exceeded, more accurate but energy-consuming actual heat measurements are carried out. , the sensor is switched.

This method is particularly advantageous when the appearance of combustible gases in real concentrations above the reference value R ₂ over long periods is not expected.

The reference value R ₁ or load limit value G is provided around 100-140% of the lowest explosive limit (LEL), and the second reference value R ₂ is provided in the range between 2% and 5% of the LEL.

Thus, in the process, in _the range below the minimum explosive limit, the sensor is The control is rationally adjusted to operate with a thermal conductivity measurement method at a lower operating temperature T ₁ .

As soon as the concentration of flammable gas falls below the minimum explosive limit, the sensor automatically switches back to the active calorimetry method. When increased concentrations above the lowest explosive limit appear, this will protect the sensor from damage or destruction due to catalytic combustion when operating in real calorimetry mode, as long as its operating temperature is adequate. This is because sometimes it is switched to a lower thermal conductivity measurement level.

Thus, it is also intended to provide accurate measurements again in the actual calorimetry mode when high concentrations of combustible gases are dissipated.

It may be desirable to trigger a full scale reading indicator in the upper measurement range to identify when a hazard is present.

Apart from this, an alarm system can also be activated when the second limit value is exceeded.

A suitable measuring device for carrying out this method is
It includes a single sensor maintained at a predetermined temperature by a controllable current source, which temperature is continuously measured, checked, and balanced against the ambient temperature. The computer converts the sensor signal to a predetermined reference value.
R ₁ , R ₂ or the load limit value G is compared and depending on the size of the signal it is determined whether the sensor should operate in the actual calorimetry mode or in the thermal conductivity measurement mode. A suitable signal is applied to the current source.

Thus, two modes for measuring the concentration of combustible gases by only a single sensor are possible.
This can be done by using one measuring circuit for both operating modes.

The operation of embodiments of the invention is illustrated in the drawings,
It will be explained in more detail below.

EXAMPLE In FIG. 1, a sensor 1, suitable both for heat conduction and for measuring the actual amount of heat, is connected to a controllable current source 2 and a load resistor 3.

The sensor resistance across the sensor 1 is measured with the aid of an amplifier 4 and converted into a temperature value in a computer 5,6. The sensor measurement signal reaches the positive input point of the measuring amplifier 8 through the instrument lead 7, and its output point is connected to the computer 5 through the signal lead 9. Through leads 10, 11, computer 5 continuously supplies reference values R ₁ and R ₂ or load limit value G to comparator 6.

Depending on the height of the temperature of the sensor 1 and the position of the measured value with respect to the reference values R ₁ and R ₂ or G, the current source 2 is controlled by the comparator 6 through a positive feedback 12. The current source provides a current of the required strength either to maintain the temperature of sensor 1 or to switch between the operating temperatures for thermal conductivity measurements and actual calorimetry measurements. is controlled so that it is supplied to sensor 1. The desired value is collected through a temperature probe that depends on the ambient temperature. The measured values determined by the computer 5 are represented by the display unit 13. If the measured signal from the signal lead 9 exceeds the load limit value G, the alarm 14 is operative to provide an audible or visual indication when the limit value is exceeded.

In FIG. 2 and in an enlarged detail in FIG. 3, two measurement curves for methane measured by sensor 1 are shown. The first measuring curve 21 is the course of the measuring signal _US as a function of the concentration of combustible gas in the air mixture, expressed as a percentage of the volume.

This applies to the actual calorific value measurement operating mode of sensor 1. Its intersection with the abscissa is the origin and the volume
and the point at 100%. Between those two points, the curve passes through a maximum value. The second measurement curve 22 shows the linear signal course of the measurement signal of the sensor 1 in the heat transfer measurement operating mode. It starts at the origin of the coordinates and shows a flat, negative slope. The reference values R ₁ , R ₂ and the limit value G are entered on the ordinate. Their intersections with the corresponding measurement curves are 23 and 24 or 25.

To start the measuring device, sensor 1
is initially regulated by a current source 2 to an operating temperature T ₁ suitable for heat transfer measurements. The measured value lies on the measurement curve 22. If the measured value is received by the computer 5 as lower than the limit value _R1 , the computers 5, 6
identifies that a lower concentration of combustible gas is present and moves to an operating temperature T ₂ where the sensor can operate in real calorimetry mode.

All measurement results obtained by actual calorimetry are above the measurement curve 21. As long as the measurement signal _US remains below the limit value G, the sensor 1 remains in the actual heat measurement mode of operation.

However, as soon as the measurement signal exceeds the limit value G, the computers 5, 6 determine that the concentration of combustible gas is
For example, it is determined that the volume ratio has reached more than 5%,
Therefore, the current source 2 is made to lower the operating temperature of the sensor 1 to reach the lower operating temperature T ₁ required for the thermal conduction measurement mode of operation.

If the concentration of combustible gas increases, subsequent measurements are performed along the measurement curve 22 in the darkened range.

If the concentration of combustible gas decreases further below 5% by volume, the sensor 1 switches back to the actual calorific value measurement operation mode, and the subsequent measurement results are in the range drawn by the thick line of the measurement curve 21. move along.

With this method, it is possible to switch alternately from one operating mode to another depending on the concentration of combustible gas.

If a further value R ₂ is exceeded, the operation is changed such that the measuring device is simply switched to the actual calorific value measuring mode of operation, the comparator 6 detects that the measurement result signal is equal to R ₁ and R ₂ . Checks if it came from within the window between.

In FIG. 2, an example of methane measurement is shown, rather than a constant ratio, where a concentration of 5% by volume corresponds to a 100% minimum explosive limit. The maximum explosive limits are for information only.
And this is also shown not as a constant ratio.

Measurements of other combustible gases are performed in a similar manner.

Effects of the Invention It is possible to provide a combustible gas concentration measuring device with low power consumption and a small number of constituent elements. Moreover, according to the present invention, concentrations near the explosive limit can be measured with high precision, and the life of the sensor can be extended.

[Brief explanation of drawings]

Figure 1 is a block diagram of a measuring device using the method of the present invention, Figure 2 is a diagram showing actual heat quantity and heat conduction measurement, and Figure 3 is an enlarged view of the vicinity of the origin of Figure 2. . 1...Sensor, 2...Current source, 3...Resistor, 4...
Amplifier, 5...Computer, 6...Comparator,
7... Lead wire, 8... Measuring amplifier, 9, 10, 1
1...Lead wire, 12...Feedback circuit, 13
...Display unit, 14...Alarm, 21...Actual calorific value measurement curve, 22...Measurement curve, 23, 24, 25
...The intersection of the reference value, limit value and measurement curve.

Claims (1)

[Scope of Claims] 1. A method for detecting the proportion of combustible gas in an air mixture using a measuring device compatible with an indicator unit that operates both by the actual heat amount and by the heat conduction method, comprising: , the indicator unit uses one single sensor element 1 which is catalytically activated at high temperatures, in such a way that it only sends out the actual calorific value when the measurement signal falls below a preset reference value. steps
(a) to (d), i.e. (a) at the beginning of the measurement the sensor 1 is kept constant at an initial temperature T ₁ sufficient for the heat transfer measurement, (b) the measurement result signal is at the reference value R ₁ (c) If it is lower than the reference value R ₁ , the temperature of the sensor 1 is increased to a second temperature T ₂ at which the actual calorimetry can be started;
(d) The measurement signal originating from sensor 1 is compared with a load limit value G, which corresponds to a gas concentration even higher than R ₁ , and as soon as the measurement signal exceeds the limit value G, the sensor temperature increases. lowered to T ₁ ,
and a detection method characterized by having a constant step. 2 Both the standard value _R1 and the load limit value G are approximately 100% or 100% of the lowest explosive limit (LEL).
140%. 3 In step (b), the measurement result signal is the reference value.
A reference value to which the measurement signal is added, which is compared to a second reference value R ₂ , corresponding to a gas concentration lower than R ₁
When the temperature of sensor 1 exceeds R ₂ , the temperature of sensor 1 reaches step (c)
2. A method according to claim 1, with the addition of being raised according to. 4. For the measurement of methane, the reference value R ₁ or the load limit value G is approximately 100-140% of the flammable gas.
4. A method according to claim 3, wherein the LEL is at LEL and the second reference value _R2 is in the range between 2% and 5% LEL. 5. A method according to claim 1 or 3, wherein in step (d) the sensor 1 causes the indicator unit 13 on the measuring device to take a full scale reading. 6. The method of claim 5, wherein the alarm system 14 is activated. 7. A method for detecting the proportion of combustible gas in an air mixture using a measuring device compatible with an indicator unit that operates both by the actual heat amount and by the heat conduction method, wherein the indicator unit In a measuring device for carrying out a method in which only the actual heat value is delivered when the signal falls below a preset reference value, one single sensor 1 is connected to a controllable current source 2, The output signal is sent to the measuring amplifier 8
through a positive feedback unit 12 to a computer 5, 6, which is connected to a current source through a positive feedback unit 12 and transmits the control signal to the sensor resistance measured by the amplifier 4 and the reference values R ₁ , R stored in the comparator 6. ₂ or a load limit value G, the current source supplying a first initial temperature sufficient to perform a thermal conduction measurement on said sensor 1 at the beginning of a measurement.
If the measurement signal obtained during the measurement at the first temperature T ₁ is lower than the reference value _{R 1} , the sensor 1
supplying a current of an intensity to raise and maintain a second temperature T ₂ sufficient to perform an actual calorific value measurement on the gas, such that the measurement signal generated by said sensor 1 is the gas represented by the reference value R ₁ . If the load limit value G corresponding to a higher gas concentration is exceeded,
the sensor 1 is lowered to the second temperature T ₁ ;
A measuring device that is maintained at the temperature.