KR20200091570A - Measurement method of oxygen dissolved in fuel during aircraft climb at the ground conditions - Google Patents

Abstract

According to the present invention, a measurement method of oxygen dissolved in fuel during an aircraft′s rising from the ground surface comprises: a step (a) of injecting a certain volume of fuel into a fuel tank for test and forming an ullage, which accommodates air, on an upper space of the fuel tank for test; a step (b) of supplying air into the fuel tank for test and making the oxygen concentration of the air in the ullage an oxygen concentration at the atmospheric pressure; a step (c) of supplying nitrogen gas into the fuel tank for test and reducing the oxygen concentration in the ullage down to the nonflammable concentration or below, which is lower than the oxygen concentration at the atmospheric pressure; a step (d) of reducing the pressure in the fuel tank for test from the atmospheric pressure in the fuel tank for test down to the pressure corresponding to the aircraft rising up to a certain altitude; and a step (e) of measuring the oxygen-partial pressure in the ullage of the fuel tank for test and the oxygen-partial pressure dissolved in the fuel in accordance with the pressure changed from the atmospheric pressure to a certain altitude in the previous step. The measurement method of oxygen dissolved in fuel during an aircraft′s rising from the ground surface is able to measure the discharge volume of oxygen dissolved in the fuel by configuring a measuring system which is the same as the rising process of an actual aircraft from the ground surface.

Description

Measurement method of dissolved oxygen in fuel during aircraft ascent {MEASUREMENT METHOD OF OXYGEN DISSOLVED IN FUEL DURING AIRCRAFT CLIMB AT THE GROUND CONDITIONS}

The present invention relates to a method for measuring the dissolved oxygen in the ground during the flight of the aircraft, and more specifically, the fuel during the flight of the aircraft to measure the amount of dissolved oxygen discharged from the fuel contained in the fuel tank according to the altitude change of the rising aircraft. It's about my dissolved oxygen ground measurement method.

In order to suppress fire and explosion in the fuel tank during operation, technology for maintaining the oxygen concentration in the fuel tank below the ignition concentration has been applied.

For example, the United States' Federal Aviation Administration (FAA) has conducted a number of studies on ways to exclude or significantly reduce aircraft exposure to ignitable vapors.

Representatively, one of the U.S. Federal Aviation Administration's studies is evaluating gas separation technology using HFM (Hollow Fiber Membrane), which can supply low-cost inert gas, which is cost-effective onboard inert gas generation. This is being applied to possible civilian aircraft design standards.

Inside the fuel tank of the aircraft, an empty space inside the fuel tank known as ullage is created in addition to the space in which the fuel is accommodated. Since the oxygen concentration in the freezing area must maintain the ignition concentration, that is, the non-combustible concentration, as described above, nitrogen is supplied as an inert gas into the freezing area. The nitrogen is supplied into the freezing through an inboard inert gas generation system (OBIGGS) placed on the aircraft.

On the other hand, the oxygen concentration in freezing is changed according to the amount of dissolved oxygen dissolved in the fuel. For example, when the aircraft rises from take-off to a cruising altitude, the pressure inside the fuel tank may drop and the amount of dissolved oxygen in the fuel may be changed. During the aircraft's ascent, dissolved oxygen emissions at different altitudes increase with pressure drop, which can increase the concentration of oxygen in the freezing.

However, the method of measuring dissolved oxygen emissions according to altitude during aircraft ascension uses a method of substantially raising the aircraft after mounting fuel tanks of various sizes and/or shapes, but these measurement methods are cost and time consuming. There is a problem in.

Republic of Korea Patent Registration No. 10-1041994: dissolved oxygen measurement sensor probe, its manufacturing method and dissolved oxygen sensor using the same

An object of the present invention is to measure the dissolved oxygen in the fuel during aircraft ascending by measuring the dissolved oxygen in the fuel during aircraft ascending process of the actual aircraft ascending process.

According to the present invention, a solution for solving the above problems is (a) injecting a certain amount of fuel into a test fuel tank to form an ullage through which air is accommodated in an upper space of the test fuel tank, and (b) ) Supplying air inside the fuel tank for the test to configure the oxygen concentration for the air in the air to an oxygen concentration in the atmospheric pressure state, and (c) supplying nitrogen gas into the fuel tank for the test to allow the air to flow within the freeze. Reducing the oxygen concentration below a relatively low inflammable concentration than the oxygen concentration in the atmospheric pressure state, and (d) the fuel tank for the test at a pressure corresponding to an aircraft rising from the atmospheric pressure inside the test fuel tank to a predetermined altitude. Decompressing the interior, and (e) measuring the partial pressure of oxygen in the freezer and the partial pressure of dissolved oxygen in the fuel of the test fuel tank according to the pressure varying from atmospheric pressure to a predetermined altitude in step (d). It is made by a method for measuring the dissolved oxygen in the fuel during aircraft ascension.

Here, the partial pressure of oxygen in the ridge and the partial pressure of dissolved oxygen in the fuel measured in step (e) may be converted to the concentration of oxygen in the ridge and the concentration of dissolved oxygen in the fuel, respectively.

In addition, the method for measuring the level of dissolved oxygen in fuel while the aircraft is rising is (f) measuring the amount of dissolved oxygen discharged from the fuel into the ice in accordance with the dissolved oxygen concentration value corresponding to the pressure change of the aircraft rising to a certain altitude. It may further include a step.

The step (a) may include opening a ventilation valve connected to the fuel tank for testing to maintain the inside of the fuel tank for testing at atmospheric pressure, and an interface for forming the ridge within the fuel tank for testing. It may include the step of injecting fuel to the (interface).

In step (b), compressed air may be supplied inside the fuel to generate air bubbles.

The step (c) may include supplying nitrogen into the freezing at atmospheric pressure, and closing a ventilation valve connected to the test fuel tank and supplying nitrogen into the fuel to generate nitrogen bubbles.

It is preferable that the inflammable concentration of oxygen in the freezing is 12% or less.

It is preferable that the inflammable concentration of oxygen in the freezing of the military aircraft among the aircraft is 9% or less.

The steps (a) to (c) may include the aircraft ground conditions corresponding to the preparation for takeoff of the aircraft.

In step (d), the inside of the test fuel tank may be decompressed while the ventilation valve connected to the test fuel tank is closed.

In each of the steps (a) to (d), the oxygen partial pressure in the ridge and the dissolved oxygen partial pressure in the fuel may be measured and converted into the oxygen concentration in the ridge and the dissolved oxygen concentration in the fuel.

The fuel tank for testing may be connected to a low pressure tank having a pressure lower than the pressure inside the fuel tank for testing to measure the partial pressure of oxygen in the ridge.

Details of other embodiments are included in the detailed description and drawings.

The effect of the method for measuring the dissolved oxygen in the fuel during the aircraft ascending process according to the present invention is as follows.

First, it is possible to know the change in the oxygen concentration in the early stage by measuring the amount of dissolved oxygen in the fuel according to the altitude change during the aircraft ascending process from the ground, so that the design value for maintaining the oxygen concentration in the early stage below the inflammable concentration can be calculated. .

Second, since it is possible to measure the dissolved oxygen emissions in the fuel according to the altitude change during the aircraft ascending process from the ground, it is possible to actually reduce the measurement cost and the time required to measure the dissolved oxygen emissions by flying the aircraft.

(A) and (b) of Figure 1 is a flight graph for the time-altitude of military aircraft and civil aircraft,
2 is a block diagram of a system for measuring dissolved oxygen in a fuel ground during an aircraft rise according to an embodiment of the present invention;
Figure 3 is a first operation configuration of the dissolved oxygen ground measurement system in the fuel during the flight of the aircraft according to an embodiment of the present invention,
Figure 4 is a second operational configuration of the dissolved oxygen ground measurement system in the fuel during the flight of the aircraft according to an embodiment of the present invention,
Figure 5 is a third operation configuration of the dissolved oxygen ground measurement system in the fuel during the flight of the aircraft according to an embodiment of the present invention,
Figure 6 is a fourth operational configuration of the dissolved oxygen ground measurement system in the fuel during the flight of the aircraft according to an embodiment of the present invention,
7 is a flow chart of a method for measuring dissolved oxygen in a fuel ground during an aircraft rise according to an embodiment of the present invention.

Hereinafter, a method for measuring ground level of dissolved oxygen in fuel during an aircraft rise according to an embodiment of the present invention will be described in detail with reference to the accompanying drawings.

Prior to the description, the nitrogen supply used in the method for measuring the dissolved oxygen in the fuel during aircraft ascent according to an embodiment of the present invention is a nitrogen-enriched generation (NEA) using 90-98% nitrogen in compressed air in an actual aircraft. It is noted in advance that an onboard inert gas generation system (OBIGGS) may be used or a nitrogen cylinder supplying 100% nitrogen may be used.

1(a) and 1(b) are operational graphs for the time-altitude of military aircraft and civil aircraft.

As shown in (a) of FIG. 1, a taxi section (T) including a ground taxi and a ground taxi taxi in which the aircraft moves by itself on the ground, an ascending section (C), and a first altitude ascending to the first altitude Cruising section (Cr) operated by (A ₁₎ , descending section (D) descending to the second altitude (A ₂ ) descending for close air support, and an attack that performs a mission such as dropping a bomb on the ground at the second altitude interval (or dash interval) (Da), back to the first height (a ₁₎ up section (C), the first height (a ₁₎ falling period for cruising region (Cr) and landing flying from rising to (D ) And the landing section (Lo) to land on the runway.

On the other hand, the civilian aircraft shown in Figure 1 (b), unlike the military aircraft shown in Figure 1 (a), the taxi section (T), the first altitude (A ₁ ) to the elevation section (R), the first altitude It is divided into a cruising section (Cr) operated by (A ₁ ), a descending section (D) for landing and a landing section (Lo).

The first altitude A _{1 in} FIGS. 1A and 1B is 36,000 feet (ft), approximately 10,000 meters to 11,000 meters (m), and the second altitude shown in FIG. 1A ( A ₂ ) is about 600 meters (m). The aircraft is at atmospheric pressure, ie, 1.0 atmosphere at the runway, but at 0.1 altitude (A ₁ ). During the decompression process until the outside pressure of the aircraft is at O.1 atmosphere, the inside of the fuel tank of the actual aircraft in operation is pressurized to +5.5 psig.

The method for measuring the dissolved oxygen in fuel during the aircraft ascent according to an embodiment of the present invention measures the amount of dissolved oxygen in the fuel in the rising section (C) of the aircraft shown in FIGS. 1(a) and (b) from the ground. will be.

Next, FIG. 2 is a block diagram of a system for measuring ground level of dissolved oxygen in fuel during aircraft ascending according to an embodiment of the present invention, and FIG. 3 is a first operation of a system for measuring ground level of dissolved oxygen in fuel during aircraft ascending according to an embodiment of the present invention Configuration diagram, Figure 4 is a second operation configuration of the dissolved oxygen ground measurement system in the fuel during the aircraft rise in accordance with an embodiment of the present invention, Figure 5 is a dissolved oxygen ground measurement system in the fuel during aircraft rise according to an embodiment of the present invention 3 is a configuration diagram of a third operation, and FIG. 6 is a configuration diagram of a fourth operation of the ground measurement system for dissolved oxygen in fuel during aircraft ascent according to an embodiment of the present invention.

2 to 6, the dissolved oxygen in the fuel ground measurement device (hereinafter referred to as a measurement device) 1 during the aircraft rise according to an embodiment of the present invention is a fuel tank 10 for testing, freezing ( ullage (30), ventilation valve (ventilation valve) 50, compressor 70, gas tank 90, heat exchanger 110, bubble generator 130, nitrogen supply 150, vacuum generator 170 ), a first oxygen measurement unit 190, a second oxygen measurement unit 210 and a low pressure tank 230. In addition, the measuring device 1 according to an embodiment of the present invention further includes a plurality of valves 250 and a plurality of pressure regulators 270.

Test fuel tank 10 is injected with a fluid (F) to measure the dissolved oxygen discharge. Here, as the fluid F, aviation oil used in an aircraft may be used. Early ridge 30 is a space formed in the upper portion in the test fuel tank 10 after the fluid F is injected into the test fuel tank 10. Air is filled in the freezing 30.

The ventilating valve 50 opens and closes the flow path so that the fuel tank 10 for testing is selectively ventilated. In detail, as shown in FIGS. 3 and 4, the ventilating valve 50 has a fuel tank 10 for testing when supplying compressed air inside the fluid F and when supplying nitrogen gas in the freezing 30. The flow path is opened to maintain the atmospheric pressure. On the other hand, as shown in Figures 5 and 6, the ventilation valve 50 closes the flow path when depressurizing the inside of the fuel tank 10 for supplying and testing nitrogen gas into the fluid F.

The compressor 70 compresses air to supply compressed air to the fuel tank 10 for testing, and the gas tank 90 stores compressed air compressed by the compressor 70. Then, the heat exchanger 110 removes the moisture of the compressed air supplied to the fuel tank 10 for testing in the gas tank 90. The bubble generator 130 is disposed inside the fuel tank 10 for testing in a state immersed in the fluid F. The bubble generator 130 forms bubbles by supply of compressed air or bubbles by supply of nitrogen gas.

3 to 5 show that the aircraft not only constituted the state before taking off, that is, the ground, but also applied the fuel tank state before taking off the aircraft to the test fuel tank 10.

As shown in FIG. 3, the ventilating valve 50 opens the flow path to maintain the inside of the test fuel tank 10 in an atmospheric pressure state. The valve 250 of the first line L ₁ is opened to supply compressed air from the gas tank 90 to the bubble generator 130. The bubble generator 130 bubbles compressed air. The first oxygen measurement unit 190 and the second oxygen measurement unit 210, which will be described later, measure the oxygen partial pressure in the freezing 30 and the dissolved oxygen partial pressure in the fluid, respectively. In addition, the first oxygen measurement unit 190 and the second oxygen measurement unit 210 convert the oxygen partial pressure and the dissolved oxygen partial pressure into respective oxygen concentrations and dissolved oxygen concentrations.

The oxygen concentration and dissolved oxygen concentration in the freezing 30 can be converted from the oxygen partial pressure and the dissolved oxygen partial pressure using the following <Equation 1> and <Equation 2>, respectively.

<Equation 1>

(PO _2-ullage ullage is the oxygen partial pressure, CO ₂ is a _{freeze-Ullage} the oxygen concentration, P is absolute pressure within the ullage _-ullage)

<Equation 2>

(P0 _2-DO is the partial pressure of dissolved oxygen in the fluid, CO _2-DO is the concentration of dissolved oxygen in the fluid, P is the absolute pressure in the fluid)

In the bubble generator 130, the bubbling of compressed air is 21%, the oxygen concentration in the freezing 30 equal to the oxygen concentration in normal atmospheric air (general air consists of 79% nitrogen and 21% oxygen ??excluding other traces of gas) ).

When the oxygen concentration in the freezing 30 reaches 21%, as shown in FIG. 4, the valve 250 of the second line L ₂ is opened to supply nitrogen gas into the freezing 30. At this time, the oxygen partial pressure in the freezing 30 and the dissolved oxygen partial pressure in the fluid F are measured. When the oxygen concentration in the freezing 30 becomes equal to or less than the inflammable concentration, supply of nitrogen gas is stopped. Here, the nonflammable concentration is designated as 9% or less for military aircraft and 12% or less for civil aircraft. In one embodiment of the present invention, as applicable to the military aircraft, nitrogen gas is supplied until the oxygen concentration in the freezing 30 becomes 9% or less.

As shown in Figure 5, the ventilation valve 50 closes the flow path. Then, the valve 250 of the third line L ₃ is opened to supply nitrogen gas to the bubble generator 130. At this time, nitrogen gas is supplied to the bubble generator 130 in a very short time to be bubbled. The reason for supplying nitrogen gas to the bubble generator 130 for a very short time after closing the flow path with the ventilation valve 50 is to compensate for a small amount of nitrogen gas that can be discharged to each line.

The vacuum generator 170 is arranged to depressurize the fuel tank 10 for testing. The vacuum generator 170 decompresses the inside of the fuel tank 10 for testing to 0.1 atm, similar to the pressure at the first altitude A ₁ .

The first oxygen measurement unit 190 is connected to the freezing 30 to measure the partial pressure of oxygen in the freezing 30 and converts it into an oxygen concentration. The second oxygen measurement unit 210 is connected to the fluid F receiving space of the test fuel tank 10 to measure the partial pressure of dissolved oxygen in the fluid F and converts it to dissolved oxygen concentration.

As shown in FIG. 6, the fourth line L ₄ is opened to reduce the pressure inside the fuel tank 10 for testing to 0.1 atm. At this time, the first oxygen measurement unit 190 and the second oxygen measurement unit 210 respectively measure the oxygen partial pressure in the freezing 30 and the partial pressure of dissolved oxygen in the fluid F in real time, and each of them is within the freezing 30. It converts to oxygen concentration and dissolved oxygen concentration in fluid (F). Since the partial pressure of oxygen in the freezing 30 measured in real time during the decompression process of the fuel tank 10 for testing and the resulting oxygen concentration and the dissolved oxygen partial pressure in the fluid F and thus the dissolved oxygen concentration can be measured, the fluid ( The amount of dissolved oxygen change in F) can be measured, and accordingly, the change in oxygen concentration in freezing 30 can also be measured.

Meanwhile, in the measurement process of FIGS. 3 to 6, the low pressure tank 230 maintains a lower pressure than the fuel tank 10 for testing. In this way, the low pressure tank 230 must maintain a lower pressure than the fuel tank 10 for testing so that the gas of the freezing 30 can be moved to the first oxygen measurement unit 190.

The valve 250 and the pressure regulator 270 are each formed in a line. The valve 250 selectively opens a line in the measurement process, and the pressure regulator 270 maintains a constant pressure in each line in the measurement process.

7 is a flow chart of a method for measuring dissolved oxygen in a fuel ground during an aircraft rise according to an embodiment of the present invention.

Due to this configuration, the method for measuring the dissolved oxygen in the fuel during aircraft rise according to an embodiment of the present invention is as follows.

First, a certain amount of fuel F is injected into the test fuel tank 10 and the freezing 30 is formed on the test fuel tank 10 (S100). The compressed line is supplied to the test fuel tank 10 by opening the connection line between the gas tank 90 and the test fuel tank 10 (S300). At this time, compressed air is supplied to the bubble generator 130 immersed in the fluid F to be bubbled. When the compressed air is supplied to the test fuel tank 10, the oxygen partial pressure in the freezing 30 is continuously measured and converted into an oxygen concentration, and the partial oxygen dissolved in the fluid F is measured to be converted into the dissolved oxygen concentration. When the oxygen concentration in the freezing 30 reaches the oxygen concentration in the air at atmospheric pressure, the supply of compressed air is stopped.

Nitrogen gas is supplied to reduce the oxygen concentration in the freezing 30 to be less than the inflammable concentration (S500). Nitrogen gas is supplied into the freezing 30 and nitrogen gas is supplied to an oxygen concentration of 9% or less or 12% or less according to the standards of military aircraft or civil aircraft. Then, in order to measure the amount of dissolved oxygen discharged during the flight of the aircraft, a flow path connected to the fuel tank 10 for testing is closed using a ventilation valve 50. Thereafter, a small amount of nitrogen gas is supplied to the bubble generator 130 to compensate for the nitrogen gas. Steps S100 to S500 are a step-by-step method of measuring an aircraft's readiness before takeoff.

The vacuum generator 170 is operated to decompress the fuel tank 10 for testing to 0.1 atm corresponding to the first altitude A ₁ (S700). Here, step S700 decompresses the inside of the test fuel tank while maintaining the pressurization schedule (+5.5 psig) according to the elevation of the aircraft. The oxygen partial pressure in the freezing 30 and the dissolved oxygen partial pressure in the fluid are measured in real time with respect to the pressure change of the test fuel tank 10 ranging from atmospheric pressure of 1 atm to 0.1 atm corresponding to the first altitude A ₁ ( S900). The measured oxygen partial pressure in the freezing 30 and the dissolved oxygen partial pressure in the fluid F are converted into oxygen concentration and dissolved oxygen concentration (S1100). Using the oxygen concentration and dissolved oxygen concentration converted in real time in step S1100, the discharge amount of dissolved oxygen discharged into the freezing 30 in the fluid F may be measured.

Accordingly, since the change in the oxygen concentration in the early age can be known by measuring the amount of dissolved oxygen in the fuel according to the altitude change during the aircraft ascending process from the ground, the design value for maintaining the oxygen concentration in the early age below the inflammable concentration can be calculated. .

In addition, since it is possible to measure the dissolved oxygen emissions in the fuel according to the altitude change in the course of the aircraft ascending from the ground, it is possible to actually reduce the measurement cost and time required to measure the dissolved oxygen emissions by flying the aircraft.

Although the embodiments of the present invention have been described with reference to the accompanying drawings, those skilled in the art to which the present invention pertains may be implemented in other specific forms without changing the technical spirit or essential features of the present invention. You can understand that. Therefore, it should be understood that the embodiments described above are illustrative in all respects and not restrictive. The scope of the present invention is indicated by the following claims rather than the above detailed description, and it should be construed that all changes or modified forms derived from the meaning and scope of the claims and their equivalent concepts are included in the scope of the present invention. do.

1: Ground measurement device for dissolved oxygen in fuel during aircraft ascent
10: fuel tank for test 30: ullage
50: ventilation valve 70: compressor
90: gas tank 110: heat exchanger
130: bubble generator 150: nitrogen supply
170: vacuum generator 190: first oxygen measurement unit
210: second oxygen measurement unit 230: low pressure tank

Claims (12)

(a) injecting a predetermined amount of fuel into a fuel tank for testing to form an ullage in which air is received in an upper space of the fuel tank for testing;
(b) supplying air inside the fuel tank for testing to configure the concentration of oxygen for air in the freezing to an oxygen concentration in an atmospheric pressure state;
(c) supplying nitrogen gas into the fuel tank for the test to reduce the oxygen concentration in the freezing to be less than a relatively low nonflammable concentration than an atmospheric oxygen concentration;
(d) decompressing the inside of the test fuel tank to a pressure corresponding to an aircraft that is raised to a predetermined altitude from atmospheric pressure in the test fuel tank;
(e) measuring the oxygen partial pressure in the ridge and the dissolved oxygen partial pressure in the fuel of the test fuel tank according to the pressure varying from atmospheric pressure to a predetermined altitude in the step (d). How to measure the dissolved oxygen in fuel.
According to claim 1,
The oxygen partial pressure in the ridge and the dissolved oxygen partial pressure in the fuel measured in the step (e) are respectively converted to the oxygen concentration in the ridge and the dissolved oxygen concentration in the fuel, respectively.
According to claim 2,
The method of measuring the dissolved oxygen in the fuel during the aircraft ascent,
(f) measuring the discharge amount of dissolved oxygen discharged from the fuel into the freezing area according to the dissolved oxygen concentration value corresponding to the pressure change of the aircraft rising to a predetermined altitude. Dissolved oxygen ground measurement method.
According to claim 1,
Step (a) is,
Opening a ventilation valve connected to the test fuel tank to maintain the inside of the test fuel tank at atmospheric pressure;
And injecting fuel to an interface that forms the ridge in the fuel tank for testing, a method for measuring dissolved oxygen in the ground during aircraft ascending.
According to claim 1,
The step (b) is a method for measuring the dissolved oxygen in a fuel during an aircraft rise, characterized in that air bubbles are generated by supplying compressed air inside the fuel.
According to claim 1,
Step (c) is,
Supplying nitrogen in the freezing at atmospheric pressure;
And closing the ventilation valve connected to the fuel tank for testing, and supplying nitrogen into the fuel to generate a nitrogen bubble.
The method of claim 6,
The method for measuring the dissolved oxygen in fuel during aircraft ascending, characterized in that the inflammable concentration of oxygen in the freezing is 12% or less.
The method of claim 7,
Method for measuring dissolved oxygen in fuel during aircraft ascending, characterized in that the inflammable concentration of oxygen in the early air relative to the military aircraft in the aircraft is 9% or less.
According to claim 1,
The steps (a) to (c) include a method for measuring the dissolved oxygen in fuel during aircraft ascending, characterized in that it includes the aircraft ground conditions corresponding to the preparation for takeoff of the aircraft.
According to claim 1,
The step (d) is a method for measuring the dissolved oxygen in the fuel during aircraft ascending, characterized in that the ventilating valve connected to the fuel tank for test is depressurized inside the fuel tank for test.
According to claim 1,
In each of the above steps (a) to (d), the oxygen partial pressure in the ridge and the dissolved oxygen partial pressure in the fuel are measured, and the oxygen concentration in the ridge and the dissolved oxygen concentration in the fuel are converted into fuel during aircraft ascending. How to measure my dissolved oxygen on the ground.
The method of claim 11,
The fuel tank for testing is connected to a low pressure tank having a pressure relatively lower than the pressure inside the fuel tank for testing to measure the partial pressure of oxygen in the ullage.

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