KR20230138724A - Hydrogen gas sensor apparatus and measurement method applied with manufacturing deviation compensation - Google Patents

Abstract

The hydrogen gas measurement method to which manufacturing deviation compensation of the sensor of the present invention is applied is a measurement method using a hydrogen sensor module that applies temperature and humidity compensation, comprising the steps of applying power to the hydrogen sensor module; Calculating the size of the initial measurement deviation included in the initial output measurement value of the hydrogen sensor module after power is applied; If the initial measurement deviation is within a predetermined standard range, updating it as a manufacturing deviation; And it may include the step of correcting the measurement value output by the hydrogen sensor module by reflecting the manufacturing deviation.

Description

Hydrogen gas sensor device and measurement method with manufacturing deviation compensation applied {HYDROGEN GAS SENSOR APPARATUS AND MEASUREMENT METHOD APPLIED WITH MANUFACTURING DEVIATION COMPENSATION}

The present invention is a hydrogen gas sensor device with manufacturing deviation compensation applied to accurately correct the deviation that occurs in the initial measurement value due to the manufacturing deviation of the hydrogen concentration output by the hydrogen sensor immediately after temperature/humidity compensation is completed immediately after the hydrogen sensor is turned on. and measurement methods.

In general, the outlook for fossil fuels is not bright. Recently, with the emergence of new large energy consumers such as China and India, the use period based on reserves is expected to shorten. Accordingly, research is being conducted worldwide to replace fossil fuels with hydrogen energy.

Hydrogen energy is receiving attention as a storage medium for various alternative energies to address energy supply and demand problems.

Hydrogen is being researched globally as a clean energy source, and is currently being used in various fields such as fuel cells and internal combustion engines, and its consumption is expected to increase in fields such as fuel cell vehicles and power generation within a few years.

However, hydrogen has a high flash point and has the property of exploding when its concentration exceeds a certain level, so the need for a hydrogen sensor that can detect hydrogen leakage in the process of handling hydrogen is becoming important.

On the other hand, facilities such as storage facilities for chemicals and/or radioactive products, geothermal drilling, warehouse sites and industrial tanks may be at risk of hydrogen emissions associated with the stored products, which are explosive and hazardous to human health. You may be placed in an environment that is harmful to you. There is also a need to control risks in the above-mentioned fields and to preventively detect any hydrogen emissions that may occur.

In particular, fuel electric vehicles (FCEV), which are expected to be the most widely used field of hydrogen energy, rely heavily on hydrogen sensors mounted on the fuel cell stack to increase energy efficiency and stability.

Accordingly, as a hydrogen sensor that detects leaks of hydrogen gas, for example, a sensor using an electrical method that utilizes the change in electrical conductivity of metal when hydrogen is adsorbed on metal such as platinum or palladium, a sensor using an oxide semiconductor type and a gas MOSFET, etc. Sensors using electrochemical methods and sensors using optical methods to detect hydrogen leaks using light have been developed.

Among them, in the case of a hydrogen detection sensor of the heat conduction type, which has excellent cost efficiency, that is, economic efficiency, the heat conduction value due to the hydrogen concentration in the atmosphere is affected by changing depending on the temperature / humidity value in the atmosphere. It is applied to determine hydrogen concentration by compensating for the heat conduction value.

Figure 1 shows the configuration of a hydrogen sensor device including a temperature and humidity sensor that measures temperature and humidity and a hydrogen sensor that measures hydrogen gas concentration by heat conduction. In process ①, the temperature/humidity value is transmitted, in process ②, temperature/humidity value compensation is performed, and in process ③, the hydrogen concentration with the temperature/humidity value compensated can be output.

In the case of a hydrogen sensor device for thermal conduction-type hydrogen gas sensing, when the atmospheric temperature/relative humidity reaches the extreme, deviations, especially individual product deviations depending on the manufacturing process, increase. Since heat conduction type sensors must physically measure heat conductivity, manufacturing deviations inevitably occur, which makes it difficult to determine the exact amount of leakage when hydrogen gas actually leaks. The manufacturing deviation may occur in the temperature and humidity sensor 10 and/or the hydrogen sensor 20, and an error value may be included in the final hydrogen concentration output in ③.

Meanwhile, in an industrial cooperation system in which manufacturers of each component are independent, the contractor who uses the thermal conduction type hydrogen detection sensor product (i.e., hydrogen sensor device) that performs the above temperature and humidity compensation, or the process responsible department that final assembles it, In this case, setting or use of the MCU 40 is possible, but since the manufacturer is not the manufacturer of the hydrogen sensor 20 itself, the final hydrogen concentration measurement value of the ③ process of the hydrogen detection sensor can be obtained.

In the above-described situation, in fields where the heat conduction type hydrogen sensor is installed, a method is needed to accurately determine actual hydrogen gas leakage without missing it due to manufacturing deviation.

Republic of Korea Registration Publication No. 10-1490178

The present invention seeks to provide a hydrogen gas sensor device and measurement method with manufacturing deviation compensation that can eliminate manufacturing deviations by using values measured while learning about a heat conduction type hydrogen gas sensor.

A method of measuring hydrogen gas to which manufacturing deviation compensation of a sensor is applied according to an aspect of the present invention is a measurement method using a hydrogen sensor module that applies temperature and humidity compensation, comprising the steps of applying power to the hydrogen sensor module; Calculating the size of the initial measurement deviation included in the initial output measurement value of the hydrogen sensor module after power is applied; If the initial measurement deviation is within a predetermined standard range, updating it as a manufacturing deviation; And it may include the step of correcting the measurement value output by the hydrogen sensor module by reflecting the manufacturing deviation.

Here, the step of initially storing the manufacturing deviation as a value obtained by measuring the deviation range by flowing a master gas may be further included.

Here, the reference range may be determined by reflecting temperature-specific deviation information collected during the manufacturing process of the hydrogen sensor module.

Here, the lower limit of the reference range may be one of -1.2 to 0%vol, and the upper limit of the reference range may be one of 0 to 1.2%vol.

Here, the step of checking whether the initial measurement deviation is within a predetermined reference range includes checking whether the temperature value measured by the hydrogen sensor module is less than a predetermined boundary temperature; If the temperature value is less than the boundary temperature, checking whether the above-described initial measurement deviation falls within the reference range; And if the temperature value is greater than or equal to the boundary temperature, it may include checking whether the dew point temperature according to the measured temperature value and humidity value is less than a predetermined boundary dew point temperature.

A hydrogen gas sensor device to which manufacturing deviation compensation is applied according to another aspect of the present invention includes a hydrogen sensor module including a temperature and humidity sensor that measures temperature and humidity, and a hydrogen sensor that measures hydrogen gas concentration by heat conduction; a storage unit that stores manufacturing deviation of the hydrogen sensor module; a correction calculation unit that reflects the manufacturing deviation and corrects the measured value output by the hydrogen sensor module to calculate the corrected hydrogen gas concentration; And after power is applied, the size of the initial measurement deviation included in the first output measurement value of the hydrogen sensor module is calculated, and if the initial measurement deviation is within a predetermined reference range, the manufacturing deviation stored in the storage unit is updated. It may include a deviation update unit.

Here, it may further include a digital interface that determines the hydrogen concentration by compensating heat conduction values for temperature/relative humidity of the temperature/humidity sensor and the hydrogen sensor.

Here, the correction calculation unit, the storage unit, the manufacturing deviation update unit, and the digital interface may be internal calculation modules of a single MCU.

Here, the value obtained by measuring the deviation range by flowing the master gas in the storage unit may be initially stored as the manufacturing deviation.

Here, the reference range may be determined by reflecting temperature-specific deviation information collected during the manufacturing process of the hydrogen sensor module.

Here, the lower limit of the reference range may be one of -1.0 to 0%vol, and the upper limit of the reference range may be one of 0 to 1.0%vol.

When the hydrogen gas sensor device and/or measurement method with manufacturing deviation compensation applied according to the spirit of the present invention of the above-described configuration is implemented, manufacturing deviation is removed using the measured value while learning about the heat conduction type hydrogen gas sensor, There is an advantage in being able to perform more accurate measurements.

The hydrogen gas sensor device and/or measurement method to which manufacturing deviation compensation of the present invention is applied supports accurate hydrogen concentration measurement by continuously updating the manufacturing deviation for each sensor, but may not misrecognize the actual hydrogen leak point or manufacturing deviation. There is an advantage in preventing false detection by not updating the manufacturing deviation value in existing environments/conditions.

The hydrogen gas sensor device and/or measurement method to which manufacturing deviation compensation of the present invention is applied is not used in an environment where manufacturing deviation update increases due to relative humidity/ambient temperature, so manufacturing deviation/extreme temperature deviation/extreme humidity deviation It has the advantage of being able to distinguish only manufacturing deviations.

Figure 1 is a block diagram showing the configuration of a hydrogen sensor device including a temperature and humidity sensor that measures temperature and humidity and a hydrogen sensor that measures hydrogen gas concentration by heat conduction.
Figure 2 is a flow chart showing an embodiment of a hydrogen gas measurement method to which compensation for manufacturing deviation of a sensor is applied according to the spirit of the present invention.
Figure 3 is a block diagram showing an embodiment of a hydrogen gas sensor device to which manufacturing deviation compensation is applied according to the spirit of the present invention.
Figure 4 is a graph showing the relationship between temperature and the relative humidity measurement value of hydrogen using heat conduction method as a manufacturing deviation correction learning condition.
Figure 5 is a flowchart showing an embodiment embodying step S140 of Figure 2.
Figure 6 is a graph showing trends in thermal conductivity/air temperature/relative humidity.
FIG. 7A is a graph showing the relationship between temperature and state humidity applied in the case of low temperature in the flowchart of FIG. 5.
FIG. 7B is a graph showing the relationship between temperature and state humidity applied in the case of high temperature in the flowchart of FIG. 5.
8A is a graph showing a voltage pattern as a hydrogen sensor measurement value within manufacturing deviation correction value update conditions.
8B is a graph showing a voltage pattern as a hydrogen sensor measurement value outside of manufacturing deviation correction value update conditions.

In describing the present invention, terms such as first and second may be used to describe various components, but the components may not be limited by the terms. Terms are intended only to distinguish one component from another. For example, a first component may be referred to as a second component, and similarly, the second component may be referred to as a first component without departing from the scope of the present invention.

When a component is mentioned as being connected or connected to another component, it can be understood that it may be directly connected to or connected to the other component, but other components may exist in between. .

The terms used herein are only used to describe specific embodiments and are not intended to limit the invention. Singular expressions may include plural expressions, unless the context clearly indicates otherwise.

In this specification, terms such as include or have are intended to designate the presence of features, numbers, steps, operations, components, parts, or combinations thereof described in the specification, including one or more other features or numbers, It can be understood that the existence or addition possibility of steps, operations, components, parts, or combinations thereof is not excluded in advance.

Additionally, the shapes and sizes of elements in the drawings may be exaggerated for clearer explanation.

The MCU provided in the hydrogen sensor device of the present invention learns the manufacturing deviation and removes the deviation in the measured thermal conductivity (hydrogen concentration) value to enable more accurate measurement. A hydrogen gas measurement method with manufacturing deviation compensation of the sensor is applied. Perform. At this time, the measured thermal conductivity may include manufacturing deviation + extreme temperature deviation + extreme relative humidity deviation, so it is necessary to set conditions for the MCU to learn the manufacturing deviation correction value.

Figure 2 is a flowchart showing an embodiment of a hydrogen gas measurement method to which compensation for manufacturing deviation of a sensor is applied according to the spirit of the present invention.

The hydrogen gas measurement method to which manufacturing deviation compensation of the sensor is applied is a measurement method for a hydrogen sensor module that applies temperature and humidity compensation, and includes the steps of applying power to the hydrogen sensor module (S110); Calculating the size of the initial measurement deviation included in the initial output measurement value of the hydrogen sensor module after power is applied (S120); If the initial measurement deviation is within a predetermined standard range (S140), updating it as a manufacturing deviation (S150); And it may include a step of correcting the measured value output by the hydrogen sensor module by reflecting the manufacturing deviation (S160, S180).

Although not shown, a step of initially storing the manufacturing deviation as a value obtained by measuring the deviation range by flowing a master gas may be further included. The step of initially storing the manufacturing deviation is performed during the manufacturing process of the hydrogen sensor device, and is not included from the perspective of operation of the hydrogen sensor device. In the case of a simple manufacturing process in which it is difficult to measure the deviation, a fixed method inferred statistically/empirically The values may also be applied as original manufacturing deviations.

The reference range is preferably determined by reflecting temperature-specific deviation information collected during the manufacturing process of the hydrogen sensor module. Considering the above-described collection information and the purpose of use of the hydrogen sensor device, it is advantageous for the reference range to be -0.2 to 0.1% vol. More generally, the reference range can be defined as having a lower limit of -1.0 to 0%vol and an upper limit of 0 to 1.0%vol.

According to the hydrogen gas measurement method with manufacturing deviation compensation applied to the sensor shown, the first hydrogen concentration with temperature/humidity correction applied after the power is turned on is measured (S120), and whether to update the manufacturing deviation compensation value in the EEPROM is determined. (S140), when updating (S150), after the update, the corresponding value is applied by erasing the deviation value from the next measurement value (S180), and if the update is not possible outside the conditions, the manufacturing deviation value previously saved in the EEPROM is loaded and the corresponding value is changed. Used as a correction value for manufacturing deviation.

Figure 3 is a block diagram showing an embodiment of a hydrogen gas sensor device to which manufacturing deviation compensation is applied according to the spirit of the present invention.

The hydrogen gas sensor device 100 to which manufacturing deviation compensation is applied includes a hydrogen sensor module including a temperature and humidity sensor 110 that measures temperature and humidity, and a hydrogen sensor 120 that measures hydrogen gas concentration by heat conduction; a storage unit 146 that stores manufacturing deviation of the hydrogen sensor module; A correction calculation unit 144 that corrects the measured value output by the hydrogen sensor module by reflecting the manufacturing deviation and calculates the corrected hydrogen gas concentration; And after power is applied, the size of the initial measurement deviation included in the first output measurement value of the hydrogen sensor module is calculated, and if the initial measurement deviation is within a predetermined standard range, the manufacturing deviation stored in the storage unit 146 is calculated. It may include a manufacturing deviation update unit 145 that updates.

In addition, as shown, it further includes a digital interface 140 that determines the hydrogen concentration by compensating the heat conduction value for the temperature/relative humidity of the temperature and humidity sensor 110 and the hydrogen sensor 120.

The manufacturing deviation update unit 145 may be implemented as an internal block of the correction calculation unit 144, and may correct the manufacturing deviation stored in the storage unit 146 by performing learning.

In the case of the hydrogen gas sensor device 100 shown, the correction calculation unit 144, the storage unit 146, the manufacturing deviation update unit 145, and the digital interface 142 are located inside a single MCU 140. It was implemented as a computation module.

The storage unit 146 may be a memory such as an EEPROM (implemented as an MCU-embedded EEPROM in the drawing) to store the manufacturing deviation that is updated from time to time. Depending on the implementation, the storage unit 146 flows a master gas to detect the deviation. The values obtained by measuring the range may initially be stored as the manufacturing deviation.

The hydrogen gas sensor device 100 shown is a hydrogen gas sensor and is comprised of a temperature and humidity sensor 110/hydrogen sensor 120/MCU 140. At this time, the MCU 140 acquires air temperature/relative humidity information from the temperature and humidity sensor 110. When the air temperature/relative humidity is sent to the hydrogen sensor 120, the hydrogen sensor 120 corrects the thermal conductivity using the received air temperature/relative humidity and outputs a hydrogen concentration value. Here, the hydrogen sensor 120 is a sensor that detects hydrogen leakage and is exposed to an environment in which general use conditions are 0% hydrogen.

At this time, under the usage conditions [specific conditions for learning], the MCU 140 stores the manufacturing deviation correction value in the non-volatile memory (EEPROM) 146 as an internal storage unit of the MCU 140.

According to the spirit of the present invention, when the MCU 140 outputs the final hydrogen concentration of the hydrogen gas sensor device 100, a more precise value can be output using the manufacturing deviation correction value stored in the EEPROM 146.

Figure 4 is a graph showing the relationship between temperature and the relative humidity measurement value of hydrogen using the heat conduction method as a manufacturing deviation correction learning condition of the manufacturing deviation update unit.

Since the daily use conditions of the hydrogen sensor device will not exceed the dewpoint of 45 degrees, the dry bulb temperature/relative humidity standard with a dewpoint of 45 degrees can be applied as shown in the graph in FIG. 4. Here, since it should not be mistaken as a manufacturing deviation in an environment where actual hydrogen leaks, if the measured hydrogen concentration is within a range that can be recognized as a manufacturing deviation, it is recognized as a manufacturing deviation and the hydrogen concentration value (i.e. voltage value) at that point is stored. Store it in unit 146 (EEPROM).

The deviation range of the hydrogen concentration itself is measured by flowing a master gas with 0% hydrogen concentration in the sensor production line. In the case of the vehicle hydrogen sensor device exemplified in the present invention, it was found by analyzing information on the manufacturing process that it is advantageous to determine the manufacturing deviation range for 0% gas to be -0.2 to +0.1% vol.

Figure 5 is a graph showing trends in thermal conductivity/air temperature/relative humidity.

When the air temperature is lower than the standard of 45℃, the change in thermal conductivity due to changes in relative humidity is not significant. When the air temperature becomes greater than the standard of 45℃, the change in thermal conductivity due to changes in relative humidity increases. Therefore, a different method of manufacturing deviation compensation according to the spirit of the present invention can be applied based on the temperature condition of 45°C.

FIG. 6 is a flowchart showing an embodiment embodying step S140 of FIG. 2.

FIG. 7A is a graph showing the relationship between temperature and state humidity applied in the case of low temperature in the flowchart of FIG. 5, and FIG. 7b is a graph showing the relationship between temperature and state humidity applied in the case of high temperature.

As seen, in the case of an implementation that applies a different manufacturing deviation compensation method based on the temperature condition of 45°C, step S140 can be specified as shown in the flowchart of FIG. 6.

That is, the step of checking whether the initial measurement deviation shown is within a predetermined reference range (S140) includes the step of checking whether the temperature value measured by the temperature and humidity sensor is less than a predetermined boundary temperature (45°C in the drawing) (S142). ; If the temperature value is less than the boundary temperature, checking whether the above-mentioned initial measurement deviation falls within the reference range (S146); And if the temperature value is greater than or equal to the boundary temperature, it may include a step (S143, S144) of checking whether the dew point temperature according to the temperature and humidity values measured by the temperature and humidity sensor is less than a predetermined boundary dew point temperature.

If the initial measurement deviation described above in step S146 is determined to be within the reference range, the initial measurement deviation is updated as a manufacturing deviation (S147).

On the other hand, if the dew point temperature is above a predetermined boundary dew point temperature, or if the first measurement deviation described in step S146 is outside the reference range, the first measurement deviation obtained according to the corresponding power-on is ignored, and in the previous process The updated and stored manufacturing deviation is used to correct hydrogen gas measurement values.

In the drawings, both the boundary temperature and the boundary dew point temperature are applied as 45°C, but more generally, the boundary temperature and/or the boundary dew point temperature can be applied as one of 30°C to 60°C, and The boundary temperature and the boundary dew point temperature can be applied as the same value or as different values.

In the manufacturing deviation correction learning conditions shown in FIG. 6, which are the conditions for finally updating the manufacturing deviation correction value in the storage unit (EEPROM), when the air temperature is 45°C or lower, the entire relative humidity of 0 to 100% can be corrected, so the relative humidity condition is not included, and only the area for hydrogen concentration is added. On the other hand, if the air temperature is above 45℃, the relative humidity value for each dry bulb temperature, which becomes the dew point of 45℃, is applied by formulating (S143).

FIG. 8A is a graph showing a voltage pattern as a hydrogen sensor measurement value within manufacturing deviation correction value update conditions, and FIG. 8A is a graph showing a voltage pattern as a hydrogen sensor measurement value outside of manufacturing deviation correction value update conditions.

In the above graphs, the value output up to 2 seconds after turn-on is the value that appears when the circuit is initialized and is unrelated to the initial output measurement value. The value output at a flat level in the section from 2 to 3 seconds after turn-on is the initial output. It becomes a measured value. In a specific case, the level representing 0% is, for example, 0.5V as a voltage level, and this 0% level may be a fixed value depending on the type of hydrogen gas sensor device product. In Figure 8a, if the measured value is not output close to the voltage level indicating 0% (i.e., 0.4V or less), clamping processing as a false detection is also shown.

Figure 8a shows a case where the output is within the reference range in the section from 2 to 3 seconds after turning on, whereas in Figure 8b, it can be seen that it significantly deviates from the reference range and reaches an increased level.

In the case of FIG. 8A, the output value calculated as a % based on the 0% level is updated as a manufacturing deviation according to the spirit of the present invention, and the corresponding output value is considered to be 0% hydrogen gas.

On the other hand, in the case of Figure 8b, the output value calculated as a % based on the % level is accepted as the concentration of hydrogen gas detected due to hydrogen leakage, etc.

Those skilled in the art to which the present invention pertains should understand that the present invention can be implemented in other specific forms without changing its technical idea or essential features, and that the embodiments described above are illustrative in all respects and not restrictive. Just do it. The scope of the present invention is indicated by the claims described later rather than the detailed description, and all changes or modified forms derived from the meaning and scope of the claims and their equivalent concepts should be construed as being included in the scope of the present invention. .

110: Temperature and humidity sensor
120: Hydrogen sensor
140: MCU
142: digital interface
144: Correction calculation unit
146: storage unit

Claims (11)

A measurement method using a hydrogen sensor module applying temperature and humidity compensation,
Applying power to the hydrogen sensor module;
Calculating the size of the initial measurement deviation included in the initial output measurement value of the hydrogen sensor module after power is applied;
If the initial measurement deviation is within a predetermined standard range, updating it as a manufacturing deviation; and
A step of correcting the measurement value output by the hydrogen sensor module by reflecting the manufacturing deviation.
Hydrogen gas measurement method with compensation for manufacturing deviation of the sensor including.
According to paragraph 1,
A step of first storing the manufacturing deviation as a value obtained by measuring the deviation range by flowing a master gas.
A hydrogen gas measurement method with manufacturing deviation compensation of the sensor applied, further comprising:
According to paragraph 1,
The above standard range is,
A hydrogen gas measurement method applying manufacturing deviation compensation of the sensor, which is determined by reflecting temperature-specific deviation information collected in the manufacturing process of the hydrogen sensor module.
According to paragraph 1,
The lower limit of the reference range is one of -1.0 to 0%vol,
A hydrogen gas measurement method with compensation for manufacturing deviation of a sensor in which the upper limit of the reference range is one of 0 to 1.0% vol.
According to paragraph 1,
The step of checking whether the initial measurement deviation is within a predetermined standard range is,
Confirming whether the temperature value measured by the hydrogen sensor module is less than a predetermined boundary temperature;
If the temperature value is less than the boundary temperature, checking whether the above-described initial measurement deviation falls within the reference range; and
If the temperature value is above the boundary temperature, checking whether the dew point temperature according to the measured temperature value and humidity value is below a predetermined boundary dew point temperature.
Hydrogen gas measurement method with compensation for manufacturing deviation of the sensor including.
A hydrogen sensor module including a temperature and humidity sensor that measures temperature and humidity, and a hydrogen sensor that measures hydrogen gas concentration by heat conduction;
a storage unit that stores manufacturing deviation of the hydrogen sensor module;
a correction calculation unit that reflects the manufacturing deviation and corrects the measured value output by the hydrogen sensor module to calculate the corrected hydrogen gas concentration; and
After power is applied, the size of the initial measurement deviation included in the first output measurement value of the hydrogen sensor module is calculated, and if the initial measurement deviation is within a predetermined standard range, the manufacturing deviation stored in the storage is updated. update department
A hydrogen gas sensor device with manufacturing deviation compensation applied, including a.
According to clause 6,
of the temperature and humidity sensor and the hydrogen sensor
Digital interface that determines hydrogen concentration by compensating heat conduction value for temperature/relative humidity
A hydrogen gas sensor device to which manufacturing deviation compensation is applied, further comprising:
In clause 7,
The correction calculation unit, the storage unit, the manufacturing deviation update unit, and the digital interface are,
Hydrogen gas sensor device with manufacturing deviation compensation, which is an internal calculation module of a single MCU.
According to clause 6,
A hydrogen gas sensor device with manufacturing deviation compensation applied in which the value obtained by measuring the deviation range by flowing the master gas in the storage unit is first stored as the manufacturing deviation.
According to clause 6,
The above standard range is,
A hydrogen gas sensor device with manufacturing deviation compensation determined by reflecting temperature-specific deviation information collected during the manufacturing process of the hydrogen sensor module.
According to clause 6,
The lower limit of the reference range is one of -1.0 to 0%vol,
A hydrogen gas sensor device with manufacturing deviation compensation applied, wherein the upper limit of the reference range is one of 0 to 1.0% vol.