Glove Boxes: Leaks and Leak Detection
The most important thing to consider when building and maintaining a glovebox is to ensure the inner atmosphere is exposed to as little moisture and oxygen as possible. Leaks within a glove box are inevitable. However with effective detection, sources of leaks can be identified - even prevented - to ensure the inert glove box atmosphere isn't compromised. On this page, we have discussed how and where leaks occur, and ways to spot and deal with them.
- Why must leaks be avoided in gloveboxes?
- What causes leaks in a glovebox?
- How are gloveboxes built to prevent leaks?
- What to do if you suspect a leak?
- Where can leaks form in a glove box?
- References & Further Reading
Why must leaks be avoided in gloveboxes?
Leaks in a glove box may result in contaminants entering which may compromise the inert environment inside. However, it is important to remember that there is no such thing as a perfectly sealed system. Therefore, a glovebox will always have leaks and it is the management of these leaks that is important. Managing leaks when working within a glove box comes down to a few simple rules:
- Using appropriate seals for joints between surfaces.
- Introducing standard operating procedures when using glove boxes to reduce occurrences of human error
- Monitoring gas usage so that leaks can be detected early
- Scheduled tests and inspections of seals
- Have procedures in place to reduce the impact of containment failure
Glove boxes are typically held at a positive pressure of a few millibar (mb) relative to atmosphere. This means if there is a leak anywhere, the pressure difference will cause gas to flow outwards and a low O2/H2O atmosphere can be maintained within. A glove box with a small leak will be able to maintain overpressure by replenishing the gas within the leaking chamber, so initially the contamination levels will not increase substantially. However, leaks can have other impacts on glove boxes. Increased gas flow into a chamber can result in turbulent airflow within the glove box, for systems with HEPA filtration this can result in stagnation points within the glove box leaving areas unfiltered. The increased gas usage for maintaining an over pressure can often result in depletion of the nitrogen source, the internal and external pressures equalising and water and O2/H2O entering the system. This shows how, while initially a small leak is no cause for alarm, identifying a leak, locating its origin, and fixing it are critical in maintaining the atmosphere inside the glove box.
Small leaks can also develop into larger leaks which can be dangerous not only for the glove box functionality but for the users of the system. Often glove boxes are used to isolate materials that can be dangerous if exposed to air or harmful to the user. Therefore, large leaks in a glove box can result in users being exposed to harmful materials unknowingly. Additionally, if the glove box is continually purging to maintain an overpressure, this constant outflow can affect the oxygen levels in the room. Inert gasses, such as nitrogen, cannot be detected in the air by sight or smell and the effects of oxygen depletion mean that it is difficult for those exposed to know they are in imminent danger. For this reason, it is important that gloveboxes are used in rooms that are well ventilated and fitted with oxygen-level monitors.
What causes leaks in a glovebox?
The most common contributor to leaks in glove boxes is that of human error. This can be from either following procedures incorrectly, or through accidental damage. The transferring of samples in and out of the glove box via an antechamber is one of the most frequent causes of accidental leaks. Incorrect cycling or not purging the antechamber results in a large volume of atmospheric air mixing with that of the glove box causing a large spike in O2 and H2O. The most frequent damage by users is from holes punctured in the glove. This can be from sharp objects manipulated in the glove box – or ripped by a fingernail or jewellery catching when inserting or removing arms.
Another common cause of leaks is where seals are present. Frequently used seals are those that pose the greatest risk of leaking as they are more likely to become damaged and develop a fault. Repeated opening and closing of seals can easily result in debris falling across a sealing surface, even small pieces of debris such as a human hair can result in failure of a seal. In addition, repeated expansion and contraction of any rubber or polymer seals will eventually cause fatigue resulting in less elasticity to the material and poorer sealing performance. There are also other mechanisms where damage to the seals can occur resulting in formation of micro cracks. For example, ozone cracking of rubbers can occur at surfaces exposed to air, however this can be mostly eliminated through correct material choice. Frequently used seals should be checked often to ensure that no visible damage has occurred, and they should be replaced periodically to ensure a high-quality seal is maintained.
Therefore, it is important that correct training is given to users of the glove box on how to work with and maintain it. It is also important to have standard glove box operating procedures for commonly performed tasks available and a plan of action in the event of issues occurring. Even with thorough procedures and careful users, leaks can still happen, therefore it is also important to ensure that in the event of atmospheric exposure that samples are safe and that a plan of action is in place to get the system back to inert conditions quickly.
How are gloveboxes built to prevent leaks?
At the interface between two hard surfaces there will always be a pathway for gas to pass through. On a microscopic level, the two mating surfaces have a high degree of roughness relative to the size of gas molecules. To get the two surfaces to a level of flatness that would allow for a gas tight seal is not economically feasible. For this reason, a softer material is compressed between the two surfaces to fill these gaps and reduce leaks - see Figure 1.
For stationary and permanent joins, silicone sealants can be used along with mechanical fixtures to ensure a tight fit. Alternatively, metals can be welded together to create a seamless join. However, there are a several parts of a glove box that require non-permanent seals, such as doors for pass throughs and feedthroughs for services. For this, gaskets are extremely useful. A gasket is a piece of material that can be placed between two surfaces to form a seal. In the context of a glovebox, their main purpose is to create an air-tight seal against gas/liquids entering and leaving the system. There are many types of gasket each suited for different situations. Here, we will discuss the three most used gaskets in glove boxes, these are shown in Figure 2, they are O-rings, flat gaskets, and knife edge gaskets.
Flat gaskets are used to form a seal between two surfaces as shown in Figure 1. Flat gaskets can be cut to a wide variety of shapes, they can also be made from many types of materials with the most common being rubber and expanded rubber foams. Flat gaskets are often used for washers to help make seals, or rectangular joins such as windows and doors. They are especially beneficially when low clamping forces are used.
O-rings are doughnut-shaped mechanical gaskets that sit within a groove milled into one of the surfaces. These are usually made of harder elastomeric materials such as nitrile or Viton. Compression of O-ring creates air-tight seal that can withstand high pressures and temperatures. These are often used in regions where high-pressure differentials occur such as inlet piping, or doors of vacuum chambers integrated into glove box systems.
Knife-edge metal gaskets are often used in situations where a seal needs to withstand very low vacuum pressures1. Knife-edge seals are nearly perfect seals as the materials used have extremely low ingress rates and can withstand extremely high clamping forces. To achieve this, there are "knife-edge" indentations that are on the surfaces of the two joining parts. The gasket itself is made of a much softer metal such as copper. When these two surfaces are pressed together, the knife-edges of the hard metal indent the softer metal gasket in between. This is shown in Figure 2. Knife-edge gaskets creates a flawless metal-metal seal. However, this results in a permanent denting of the gasket, limiting the amount of times the seal can be broken and resealed. In addition, the production of the knife-edge is expensive and so is limited to smaller surface couplings. In glove box systems these are typically reserved for integrated vacuum chambers for connecting components.
What to do if you suspect a leak?
K. Curtis specifies a leak rate of 0.05 to 0.5% of box volume per hour to be acceptable in a glovebox2. However, each glove box will be built to a specified leak rate with most being below 1% of the total volume per hour. Often this is quoted as a classification rating with Class I glove boxes having less than 0.05% per hour, Class II having between 0.05% and 0.25%, and Class III being between 0.25% and 1%. If you suspect that your system is not achieving the value stated by the manufacturer, the first thing to do is a leak test.
There are many versions of the leak test that can be performed depending on your system. Standard methods of testing are outlined in the ISO 10648-2 standard. The most used leak rate test is the positive pressure test which involves increasing the internal pressure of the glove box and monitoring how it changes over time. A general overview of the process is shown in Figure 3. Each manufacturer will specify a way of testing the glove box. The Ossila Inert Atmosphere Glove Box has a built-in automated test which calculates the leak rate of the system. The automated test follows the procedure outlined below:
- Pressurises the system to a set overpressure value between 5 mbar and 10 mbar based upon the user settings.
- Monitors the internal temperature waiting until this value has stabilised before measuring the pressure drop. Once this value varies less than 0.3 C inside the chamber monitoring of the pressure begins.
- Measures the internal and external pressure of the system updating the current leak rate value every 5 minutes for the user to evaluate.
- The test can run for a maximum of one hour before allowing the overpressure to return to the normal overpressure value.
In some glove box systems, a negative pressure leak test can be performed. It is possible that leaks will be present at negative pressures that are not seen under positive pressure – however, it is unusual. For this a procedure like that describes above is used, but an under pressure is used of between -5 mbar and –10mbar. If there is a leak in the system, the pressure will increase towards atmospheric pressure and the rate of this increase can be used to determine the leak rate of the system.
These leak tests can identify if there a significant leak within the system but are not thorough enough to provide exact leak rate or tell you where the leak is coming from. The exact leak rate values require specific conditions to be met around variations in external pressure, and internal and external temperature which are hard to control in laboratory conditions. In addition, leak rate tests require that glove ports be blanked off and glove box classification values stated are always in situations where gloves are not used.
To test to see where a leak is coming from, a simple bubble test using a leak detecting fluid can be done. This involves putting soapy water at suspected leaky points, for example at the join between the glove and the main chamber. If this soap begins to create large bubbles then there is gas escaping from the glovebox.
Where can leaks form in a glove box?
There are a number of places where leaks can be found in a glovebox, such as:
- Small puncture holes/tears in gloves.
- Seal on door between the antechamber and the main chamber
- Seal on door between the antechamber and ambient atmosphere.
- Breach in gas lines/ gas inlet/exhaust connectors.
- Glove/main chamber join.
- Feedthroughs for power.
- Window seals/wall seals
The most common source of leaks in a glovebox is through small (or large) holes in the gloves. The gloves are the thinnest, most rubbery element of gloveboxes, and are therefore the most vulnerable to punctures. They are also the most heavily used part of the glovebox so the most likely to undergo significant stress. Small holes can be fixed with electrical tape3 but spare gloves should always be kept handy in case a complete replacement is needed.
The seals on the doors between the main chamber, the antechamber and the outside environment can be another source of leaks. The doors between the antechamber and the main chamber will be the most heavily used, so may be the first seals to deteriorate. It is therefore very important that a tight seal is maintained between these two chambers. If the problem is between either the outside door/antechamber or antechamber/main chamber, then the antechamber will struggle to reach vacuum when cycling. Additionally, the pressure within the glove box could be affected when the antechamber is being evacuated. Check whatever seal is used between these doors (most likely O-ring) for damage or dirt that may compromise the seal. Clean them and coat with vacuum grease if necessary.
If neither of these places is the cause of the leak, check every join in the glovebox. Ensure the sealant material is not damaged, clean any seals and ensure that any bolts holding them in place are tight – vibration of the vacuum pump may cause bolts to come loose over time.
Every seal will begin to degrade over time, so it is very important that seals are regularly checked, and replaced when needed. As previously mentioned, some seals, like knife-edge gaskets, will need to be replaced after use as they are not built for repeat usage. Other gaskets, such as rubber O-rings, are more resilient to multiple uses. However, these are usually made from flexible materials so are less resistant to chemical stress or extreme temperatures or pressure. Therefore, it is important to continually assess the quality of your seals to reduce the likelihood of serious leaks. Even as simple an act as looking at the seals each time a door is opened can prevent a serious compromise in your glovebox environment.
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- Mechanism And Optimum Shape Of Knife Edge For Metal Sealing, Y. Matsuzaki et al., Tribol. Int., 25 (6), 397–403 (1992); DOI: 10.1016/0301-679X(92)90077-Z.
- A Review Of Glove Box Construction And Experimentation, C. J. Barton, (1961); DOI: 10.2172/4043015.
- Experimental Methods And Techniques: Basic Techniques, D. A. Vicic et al., Compr. Organomet. Chem. III, 1 197–218 (2007); DOI: 10.1016/B0-08-045047-4/00008-X.