Gassing up your LI-6800: How to connect your own gas supply

Sometimes, the needs of an experiment dictate that the LI-6800 utilize air from a tank, rather than the room or outside air. In some cases, users may want to supply pure CO₂ from a larger tank rather than use the 8-gram cartridges. Other users may want to use specialty gas mixtures with their LI-6800; for example, running a low O₂ experiment will require utilizing a 2% (or less) O₂ gas cylinder. Fortunately, there are various ways to connect external gas tanks to the LI-6800. Which one you use will depend upon your specific needs. Here we present three different options.

Option 1: Using a tank of pure compressed CO₂ in place of the 8-gram cartridges

A tank of pure, compressed CO₂ can be used in place of the 8-gram cartridges. This option requires using the CO₂ tank adapter kit (LI-COR part number 9968-109). The tank is connected to the console via the CO₂ injection system. The needed equipment is:

• Tank of pure, compressed CO₂ with a two-stage regulator capable of delivering pressure between 500 to 2000 kPa (73 to 290 psi). This is the range of pressure that the adapter kit can tolerate. Note that the type of regulator will need to be appropriate to the type of pressurized gas tank being used; different countries have different sizes and standards for gases (CGA sizes in the United States, BS sizes in the UK, DIN sizes in many European countries, etc.)
• CO₂ tank adapter kit (part number 9968-109; available as an optional accessory for the LI-6800).
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• 1/8-inch or 4-mm diameter metal tubing to connect the tank air to the console. Note that this is a high pressure application and bev-a-line or other plastic tubing should not be used. The tank adapter kit contains fittings to connect either diameter of metal tubing to the adapter kit.
• Appropriate reducing fittings to connect the outlet of your tank regulator to the metal tubing.
• Phillips screwdriver for removal of the cartridge injector assembly and installation of the adapter kit. Further details can be found in the LI-6800 instruction manual (see Tank adapter kit for details).

The next two options are for users who want to use compressed gas mixtures with the LI-6800 rather than pure CO₂. Thus, they are plumbed into the system differently using one of two possible ways: through the auxiliary air inlet or through the console pump air inlet (Figure 1‑1).

Option 2: Using a compressed gas cylinder (e.g., low O₂) and bypassing the pump

If you want to bypass the console pump entirely, you can utilize the console auxiliary air inlet (Figure 1‑1, left). This air inlet is labeled auxiliary air inlet on the back of the LI-6800 console. The auxiliary air inlet plumbs into a place in the air flow path that is downstream of the console pump and soda lime chemical scrub tube, but is upstream of the Stuttgarter Masse, Drierite® and CO₂ injector. This makes it possible to use the CO₂ and moisture control systems of the LI-6800 to condition the tank air, however, the lowest achievable CO₂ concentration will be dependent on the CO₂ concentration in the supplied tank air. Note that in this setup, the maximum allowable pressure from the tank is 28 kPa (4.06 psi) and the largest allowable flow rate is 2.5 liters per minute (LPM). The general arrangement is shown in Figure 1‑2.

The needed equipment will be:

• A two-stage regulator capable of accurate pressure delivery around 28 kPa (4.06 psi) is needed. Many two-stage regulators may not be capable of accurate pressure delivery in this low pressure range; thus it is possible that an additional low pressure line regulator may be needed. Again, the type of regulator will need to be appropriate to the type of pressurized gas tank being used; different countries have different sizes and standards for gases (CGA sizes in the United States, BS sizes in the UK, DIN sizes in many European countries, etc.)
• Flow meter capable of measuring flows 2.5 LPM or lower. There are many options available that range in price from tens to hundreds of dollars. In this setup, a relatively inexpensive flow meter will suffice. The Dwyer Series MM or RM flow meters work well. Choose one with a metering valve that has a maximum flow rate of 2.5 or 5.0 LPM (2500 or 5000 cc/min) air. Having a valve on the flow meter is useful as it allows for fine-tuning of the gas flow rate; otherwise, a separate in-line flow valve will be required.
• Tubing to connect the gas tank to the console. For this low-pressure application, the Bev-a-line (ethylene vinyl acetate lined with polyethylene, LI-COR part number 222-01824) tubing supplied in the LI-6800 spares kit can be used.
• Fitting to connect to the auxiliary inlet. If using the 222-01824 Bev-a-Line tubing, you can use the brass hose barb fitting that comes with the LI-6800 spares kit (LI-COR part number 300-02547, 1/8 inch × 10-32). Another, possibly more convenient, fitting that works with the same Bev-a-Line tubing is a brass quick connect (LI-COR part number 300-08251, 1/4 inch × 10-32). This quick-connect is not included with the LI-6800 spares kit, however, and will have to be ordered from LI-COR or another supplier. Alternatively, you can use a different type of tubing if desired, but you will need to use a different fitting. The threaded end of the fitting will need to conform to the M5 or UNF 10-32 standard.

Option 3: Connecting via the main console air inlet

If you prefer to use the pump to control the bulk flow rate, the setup will be slightly different. The main difference is that the tank will be plumbed directly to the main console air inlet (Figure 1‑1, right). Also, a T-fitting with about 15 cm of vent tubing should be used; the tank must supply slightly more air than the pump requires, thus venting a little bit of air out of the tubing. This venting can be verified by placing the vent tube near your face or lips; you should be able to feel a small flow of air leaving the tube. This connection method allows for the pump to drive bulk flow and also for the full range of console air treatment options if desired (CO₂ scrubbing, humidification, and CO₂ addition). The general arrangement is shown in Figure 1‑3.

The needed equipment is:

• A two-stage regulator capable of accurate pressure delivery around 28 kPa (4.06 psi) is needed. Many two-stage regulators may not be capable of accurate pressure delivery in this low-pressure range; thus it is possible that an additional low pressure line regulator may be needed. Note (once again) that the type of regulator will need to be appropriate to the type of pressurized gas tank being used; different countries have different sizes and standards for gases (CGA sizes in the United States, BS sizes in the UK, DIN sizes in many European countries, etc.)
• Flow meter capable of measuring flows 2.5 LPM or lower. There are many options available that range in price from tens to hundreds of dollars. In this setup, a relatively inexpensive flow meter will suffice. The Dwyer Series MM or RM flow meters work well. Choose one with a metering valve that has a maximum flow rate of 2.5 or 5.0 LPM (2500 or 5000 cc/min) air. Having a valve on the flow meter is useful as it allows for fine-tuning of the gas flow rate; otherwise, a separate in-line flow valve will be required.
• Tubing to connect the gas tank and T-fitting to the console. For this low-pressure application, the Bev-a-line (ethylene vinyl acetate lined with polyethylene, LI-COR part number 222-01824) tubing supplied in the LI-6800 spares kit can be used.
• Quick-connect T-fitting. If using the 222-01824 Bev-a-line tubing, the 300-15712 T-fitting from LI-COR can be used.
• Fitting to connect to the auxiliary inlet. If using the 222-01824 Bev-a-Line tubing, you can use the brass hose barb fitting that comes with the LI-6800 spares kit (LI-COR part number 300-02547, 1/8 inch × 10-32). Another, possibly more convenient, fitting that works with the same Bev-a-Line tubing is a brass quick connect (LI-COR part number 300-08251, 1/4 inch × 10-32). This quick-connect is not included with the LI-6800 spares kit, however, and will have to be ordered from LI-COR or another supplier. Alternatively, you can use a different size or type of tubing if desired, but you may need to use different fittings. The threaded end of the fitting will need to conform to the M5 or UNF 10-32 standard.