How Accurate Are Glove Box Moisture Sensors

What are the critical measurements needed in inert glove boxes and how do you make them?

Glove boxes are used in thousands of applications in laboratories and factories across the world. Their main purpose is to allow materials to be handled safely and often have an inert atmosphere. The relatively small space inside the glove box makes it easier to maintain the correct conditions as well as keeping operators safe.

This post discusses the variables that need to be controlled – pressure, temperature, humidity as well as gas purity and composition – and recommends best practice to install measurement equipment correctly.

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What are common inert gases used in glove boxes?

Unsurprisingly, the most common inert gases used in glove boxes are nitrogen (N2) and argon (Ar) as they are easily acquired and cost-effective. They protect reactive chemicals from oxidisation and will also ensure that volatile substances will not catch fire or explode. Helium (He) is also sometimes used to create an inert atmosphere. Other gases sometimes used may include hydrogen (H2) and carbon dioxide (CO2) to create specialist atmospheres – for example for experiments with plants. Hydrogen in specific quantities may be necessary for chemical reactions to take place. In these instances, control is needed to ensure that the quantity of hydrogen with the inert gas is exactly right.

What are the key parameters to monitor?

Nitrogen

Monitoring trace nitrogen (N2) in an inert glove box is the most effective and reliable method to detect leaks in argon (Ar) or helium (H2) – air is 78% nitrogen so the presence of N2 is a very good indication of a leak.

Using an online trace impurities analyzer installed as part of an extractive sampling system will provide a reliable indication of leaks.

Oxygen

Monitoring trace oxygen in an inert glove box is the most effective method to detect leaks in inert atmospheres of argon and helium and can also be used to detect leaks if the glove box is filled with nitrogen as an inert gas.

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There are two measurement techniques to ensure the purity of an inert gas used in a glove box:

• 1. Spot checking the purity of supply before the gas is fed into the glove box. This can be carried out using a portable oxygen analyzer.
• 2. Monitoring the inside of the glove box for trace oxygen which is a good indicator of leaks. Oxygen transmitters are small and easily installed in situ (directly inside the glove box) for these applications.

Humidity

In glove boxes where ultra-dry conditions are maintained, trace moisture transmitters are often installed to detect potential leaks. If moisture levels rise, it’s a good indicator that ambient air is entering the system.

Often humidity levels are controlled at a specific level to ensure the right conditions for the process or the experiment. In the case of more humid conditions, a polymer humidity will provide fast and accurate measurements.

All of the above parameters – N2, O2 and H2O – can be measured using a single online impurities analyzer for multiple gases.

Temperature and pressure

These are also two important parameters which impact on humidity or dew-point measurements. You can learn more about this in an earlier post: Humidity Academy Theory 5: Effect of temperature and pressure on %rh.

Why might the temperature and pressure of the glove box be different to ambient conditions? Depending on the process or experiment being carried out in the glove box, keeping a consistent temperature could be essential. It may need to be generally higher or lower than typical ambient conditions for example.

Pressure in a glove box is often to do with safety. When handling potentially hazardous materials – such as bacteria or viruses – the pressure of the glove box is set to be lower than ambient. In case of a leak, air will be sucked into the glove box and protect the operators from the contents. Higher pressures are used in a glove box when the contents need to be protected from ambient air: in this case the atmosphere from the glove box is expelled if a leak occurs. It provides added protection since there will be a slight delay between the leak occurring, an alarm being set off and the system being shut down.

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In some instances, processes require a vacuum in a specialist vacuum glove box, and in this case any instruments selected will need to be capable of operating in a vacuum.

Creating and maintaining an inert atmosphere in a glove box

There are four stages to consider: Purging. At this stage, a dry inert gas is flooded through the glove box to remove any ambient air or excess humidity. Purging takes place before first set up and also after cleaning or maintenance. Nitrogen is often used for this since it is cost-effective. Establishing the correct atmosphere and conditions. The glove box is then filled with the inert gas for the process – this might be N2, Ar, He or a specific mixture of gases. Humidity and temperature levels are also stabilised. Monitoring conditions. Oxygen, humidity, temperature and pressure are all monitored continuously to ensure that the glove box is stable and sealed. Regeneration. The inert gases in the glove box need constant regeneration. Although purging (as described above) is common in larger facilities gas management systems are also used. These systems cycle gases through the glove boxes and use purifying columns to clean and regenerate the inert gases.

Installing moisture and oxygen sensors for best accuracy

Correctly installing oxygen and moisture transmitters is essential for accurate and reliable measurements. Carefully read your manufacturer’s installation guidelines which should give detailed instructions for installations. The most important factors to take into account are:

• Avoiding dead volumes. Dead volumes are areas that are not in the direct flow path and can trap moisture. Installing a dew-point transmitter here will lead to wetter than expected readings.
• The effect of flow. Changes in flow rates can lead to unexpected response speeds and accuracy from both moisture and oxygen transmitters – always follow manufacturers’ guidelines regarding optimal flow rates.
• The effect of pressure. Either negative or positive pressure can have an effect on the performance of sensors: check manufacturers’ recommendations before installing. It’s especially important in vacuum glove boxes to ensure that any instruments selected are appropriate for use in vacuums.
• Exposure to contaminants. Sensors installed in glove boxes may be exposed to a range of contaminants such as powders, solvents, chemical and bleach that may be used for wash downs. Always make sure appropriate filters are used with all sensors and if the atmosphere inside the glove box will be especially aggressive using extractive sampling may give better results.
• Using the correct fittings that are compatible with both the glove box and the transmitters being installed. KF40 and KF25 flanges are standard.

For more details on correctly installing trace moisture transmitters, read our related post: Measuring dew point reliably: How to install a dew point transmitter for best measurement accuracy

PST recommends

We recognise that specific glove box applications have very different requirements. Our ranges of trace moisture transmitters and oxygen transmitters allow for use in a range of conditions and situations. The sensor exchange program for moisture and oxygen transmitters ensures that maintenance is fast, easy and results in minimal disruption and downtime.

For more complex glove box applications, where multiple trace impurities are analysed, our integrated online trace impurities analyzers, gas stream selection and gas purifiers provide complete turnkey systems which are both affordable and reliable.

Contact us to discuss your requirements.

Source: https://t-tees.com
Category: HOW