Selecting a Desiccator

Contents

1. What is a Desiccator
2. Non-Vacuum Desiccators
3. Non-Vacuum Desiccators with Desiccant
4. Non-Vacuum Desiccators with Purge
5. Flow Meter Guidelines
6. Vacuum Desiccators
7. Vacuum Desiccators with Desiccant
8. Vacuum Desiccators with Vacuum
9. Vacuum Guidelines

What is a Desiccator

Desiccators are sealed containers designed to store samples shielded from dust and contamination and/or in a low humidity and in some cases a low oxygen environment. Ted Pella, Inc. offers desiccators in two main classifications, non-vacuum and vacuum. Both types will shield from dust and contamination when simply used as a storage container. They allow for reduction of humidity/oxygen within the chamber as discussed below. Desiccators will not control temperature, humidity or oxygen to a setpoint.

Non-Vacuum Desiccators

In the non-vacuum desiccators, humidity can be reduced in two ways.

Non-Vacuum Desiccators with Desiccant

The first way to reduce humidity is by placing a desiccant which absorbs moisture into the desiccator along with the samples. The humidity level achieved is dependent on the desiccant, moisture content of the samples, moisture content of the air, time, etc. See the Desiccant Technical Note for guidance on desiccant usage and regeneration. The non-vacuum desiccators are equipped with a hygrometer to allow monitoring of the relative humidity within the desiccator chamber. This can be used in conjunction with the indicating desiccant to monitor the moisture level within these desiccators.

Non-Vacuum Desiccators with Purge

The second way to reduce humidity is to purge the desiccator with a low humidity atmosphere, such as CDA (compressed dry air) or dry nitrogen. Purging with dry nitrogen will also reduce the oxygen level in the desiccator. The humidity/oxygen level of the chamber is going to be dependent on the humidity/oxygen level and flow rate of the gas you flow into the chamber and the moisture content of your samples, along with the humidity of the ambient air. When you open the desiccator doors, outside air enters that has to be purged by the gas flow. A rule of thumb is to set the flow at three turnovers per hour. Calculate the cubic feet of your desiccator, multiply by three and set your Flow Meter for this flow rate in standard cubic feet per hour. On multi-chamber desiccators, the flow is input through one flow meter into a plenum which distributes it to all chambers. On non-plenum desiccators, a flow meter is required for each chamber. A Pressure Relief Valve is required on each chamber to allow flow into each chamber and out the relief valve. The faster the flow rate, the faster it replaces the ambient air with the lower humidity gas. There is a hygrometer on each chamber that reads relative humidity. This can be used to determine if the desired humidity is achieved in a time acceptable to you and maintain it at an acceptable level. If not, increase the flow rate to the minimum amount needed. Some users manually increase the flow rate after opening the chamber and reduce it to a lower flow rate (e.g. 1 turnover/hr.) to conserve purge gas. Once again, the hygrometer can be used to determine the required flow setting to maintain a low humidity. This high/low purge function is performed automatically with the Controlled Purge System. It is not recommended to use both a desiccant and a purge gas to maintain low humidity.

Flow Meter Guidelines

Rule of thumb: One, 2-20 SCFH flow meter is required per door (chamber) for non-plenum desiccators. For plenum desiccators with three and four doors (chambers), one 2-20 SCFH flow meter is required. One, 10-100 SCFH flow meter is required per plenum desiccator with five to ten doors (chambers). One pressure relief valve is required on each chamber on both non-plenum and plenum desiccators.

Vacuum Desiccators

In the vacuum desiccators humidity can also be reduced in two ways.

Vacuum Desiccators with Desiccant

The first way to reduce humidity is to place a desiccant which absorbs moisture into the desiccator along with the samples. The humidity level achieved is dependent on the desiccant, moisture content of the samples, moisture content of the air, time, etc. See the Desiccant Technical Note for guidance on desiccant usage and regeneration. Since the vacuum desiccators have good seals, the samples are considered protected from humidity as long as the indicating desiccant remains blue.

Vacuum Desiccators with Vacuum

The second way to reduce humidity is to evacuate the air from the chamber using a vacuum pump. This reduces both humidity and oxygen content in the chamber. Typically a low grade vacuum of 25" Hg or better is sufficient for this purpose. All vacuum desiccators can hold a full vacuum of 30" Hg without collapsing. It is a misconception that increased vacuum can cause collapse since the maximum pressure exerted on the outside of a chamber is determined by the 14.7 psi atmospheric pressure and not by the internal vacuum. This pressure is almost reached by the time the internal vacuum reaches 100 Torr or 27" Hg.

Vacuum Guidelines

The length of time a vacuum desiccator holds vacuum is a function of the leak rate of the chamber seal and valve as well as the outgassing of the stored contents. Only Product Number 2251 has a vacuum gauge to indicate the internal vacuum level. On all others, the way to determine if it is under vacuum is if the lid cannot be removed without opening the vent valve. Vacuum grease must be used on ground glass seals and valves, found on the glass desiccators, to achieve low leak rate.

Typically, it is not recommended to use a desiccant in conjunction with vacuum as it can outgas and degrade the vacuum. Some use a desiccant in conjunction with vacuum to protect the samples should the vacuum release.

The only vacuum desiccators offered by Ted Pella, Inc. that are configured for pump then backfill purge with another gas are the 2253 series PELCO® N2 Purge Vacuum Desiccators.

It is good practice to periodically re-evacuate a vacuum desiccator to maintain the vacuum.