Glove Box Leaks

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Inert-atmosphere glove boxes ensure low levels of oxygen and moisture to create an isolated working environment. They do this by purging an inert gas (usually nitrogen or argon) through the internal chambers. It is important to note that it is impossible to create a perfectly sealed system, so there will always be some level of contamination into or out of your inert environment. Most glove boxes will automatically purge more gas through the system as contamination levels increase, but to reduce the amount of gas needed and keep running costs low, it is important to limit this atmospheric contamination as much as possible.

The three main sources of atmospheric contamination are:

1. Moisture and oxygen passing through the glove box walls (ingress)
2. Outgassing of moisture and oxygen from the contents of the glove box
3. Physical leaks in the system caused by either damage or user error

Leaks are the most common source of contamination. When a leak occurs, it is important to find and fix it as soon as possible.

Inert atmosphere glove boxes run at an overpressure, so if the leak is small, the glove box may still be able to maintain the integrity of the internal atmosphere. If the leak rate is below the fill rate of the glove box, it could do this indefinitely (assuming that the leak does not get worse). However, during this time, the glove box will use a much larger amount of inert gas than normal as the overpressure in the glove box will result in a flow of gas from the inside of the system to the outside.

Causes of Glove Box Leaks

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Potential sources of glove box leaks include:

• Imperfect seals, particularly around the doors
• Damage to the glove box or glove box gloves
• Human error when using the glove box

The most common cause of glove box leaks is human error, either as a result of not following operating procedures correctly or from accidental damage. In terms of physical damage, the most likely puncture points will be the glove box gloves or the seals around the antechamber doors. These materials are the most malleable, and will experience the most ingression, so always check these components first if you suspect a hole.

Using the Glove Box Antechamber

You are most likely to accidentally expose the glove box chamber to the outside atmosphere when moving samples in and out of the glove box through the antechamber. One of the most frequent causes of accidental leaks is simply failing to correctly purge the antechamber during every day use. This results in a large volume of atmospheric air mixing with the air inside the glove box, which causes a large spike in O₂ and H₂O levels.

To avoid this happening, only open the internal door between the main chamber and antechamber when the antechamber itself is filled with inert gas. To avoid mistakes, always follow a standard process when opening the internal door, but if you are not certain, cycle the antechamber again.

Damage to the Glove Box Gloves

You are more likely to damage the glove box gloves than the glove box iself (the Ossila Glove Box is made from steel and glass). Butyl rubber is flexible, which is important for working in a glove box, but is relatively susceptible to puncture holes. Take extra are when manipulating sharp objects in the glove box, and remove any jewellery that could catch when you either insert or remove your arms from the glove box.

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Failed Gaskets and Seals

Gaskets and seals can also be a source of leaks, particularly those that are in frequent use. Repeatedly opening and closing the glove box doors, for example, could easily result in debris falling across a sealing surface. Even something small like a human hair can become a point of failure. In addition, repeated expansion and contraction of any rubber or polymer seals will eventually cause fatigue, resulting in less elasticity and a poorer sealing performance. For this reason, check all frequently used seals on a regular basis to ensure that no visible damage has occurred. We also recommend periodically replacing the antechamber door O-rings to ensure a high-quality seal.

Glove Box Leak Tests

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If you suspect a leak, the first thing you should do is perform a leak test.

Leak tests raise the glove box to an extreme positive or negative pressure and then measure the rate of change of pressure over a period of time. The resulting leak rate indicates how well-sealed your glove box is.

The Ossila Glove Box has an automated leak test programme to make this process as easy as possible. This calculates the leak rate of the system via the following process:

1. The system is pressurised to a set over pressure value between 5 mbar and 10 mbar
2. The internal temperature is monitored until its value has stabilised, at which point the pressure drop is measured
3. Once the temperature varies by less than 0.3 °C inside the chamber, the system begins to monitor the pressure
4. Every 5 minutes, the internal and external pressure of the system is measured and the current leak rate is updated

The test can run for a maximum of one hour before allowing the over pressure to return to the normal over pressure value.

The leak rate your system calculates may not match those listed in the glove box specifications. Finding an exact leak rate requires specific conditions to be met around variations in external pressure, and internal and external temperature. These are hard to control in laboratory conditions. In addition, glove box classification values are taken (by convention) with the gloves removed and the glove box ports blanked.

How to Perform a Leak Test

The video above shows how to perform a leak test using the automated Leakage Test for the Ossila glovebox.

You can also follow the instructions described below. To perform a leak test on your glove box, go to the "Leakage Test" section of the Menu.

Here, you can define how long the leak test is performed for and at what pressure it will be held. We recommend holding the glove box at 5 mBar overpressure for at least 5 minutes. Once the settings are defined, press "Start Purge" to begin the leak test. 

The glove box will reach the desired overpressure then measure pressure of the system over the defined period of time. There may be a leak in the system if the leakage rate is over 0.25% volume per hour (vol/hour). This data can be extracted using via a USB connection for further analysis.

A successful leak test indicates that there are no significant leaks in your glove box. If a leak is identified, the next step is to find the source so that it can be fixed or mitigated.

Finding and Fixing Leaks

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The easiest way to test to see where a leak is coming from is to perform a simple bubble test using a leak detecting fluid. To do this, simply put soapy water at suspected leaky points such as the join between the glove and the main chamber while the glove box is trying to achieve an over pressure. If the soap begins to create large bubbles, there is gas escaping from the glove box.

There are a number of places where leaks can occur in a glove box, including:

• 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
• Feed-throughs for power
• Window seals/wall seals

The seals on the doors between the main chamber, the antechamber and the outside environment are all common sources of leaks.

The doors between the antechamber and the main chamber are the most heavily used and may be exposed to solvents, so are likely to be the first seals to deteriorate. It is 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 may struggle to reach vacuum when cycling for vacuum-based antechamber glove boxes. In addition, the pressure within the glove box may begin changing whenever the antechamber is being evacuated if there is a compromise in the door seals. 

To avoid this, check the seal between these doors for damage or dirt that may compromise it. Clean them and coat with vacuum grease if necessary. Every seal will begin to degrade over time, so it is important that seals are regularly checked, and replaced when needed.

If neither the seals or the glove box gloves are the cause of the leak, check every join in the glove box. Ensure the sealant material is not damaged, clean the seals and ensure that all bolts are tight (the vibration of the vacuum pump may cause them to come loose over time). Additionally, check the window is securely fixed to the glove box front, and a tight seal is formed around it.

Leaks in the Glove Box Gloves

The most common source of leaks in a glove box is through small (or large) holes in the gloves. The gloves are the thinnest, most rubbery element of glove boxes, and are therefore the most vulnerable to punctures. They are also the most heavily used part of the glove box so the most likely to undergo significant stress. Small holes can be fixed with electrical tape but spare gloves should always be kept handy in case a complete replacement is needed.

With care, you can replace the gloves on your glove box without compromising the internal atmosphere.

 

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