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The days of loading up nitrous bottles and lugging them somewhere to be refilled are over. Do it yourself with an NOS Nitrous Refill Pump Station that doesn't just spare you that hassle; it saves you money, too. Now every bit of nitrous you pay for will make it into your bottles and not be wasted in a "mother" bottle.
Here's how to do it, step by step: First, place the nitrous cylinder on an accurate scale (NOS offers kits both with and without a scale) to determine how much nitrous oxide is left in the bottle. If it's more than one-third full, you can top it off; if there’s only a little left, open the valve to release the pressure. If you’re not sure how much nitrous is in the bottle you’ll need to first weigh it and then subtract the bottle weight from the total weight shown on the scale. (The empty bottle weight and the maximum weight when full should both be labeled on the bottle.)
Once you’ve determined how much nitrous you can add, connect the line with the nitrous-control valve to the NOS cylinder you want to refill, placing a PTFE washer between the -6AN bottle nut and the cylinder. If the cylinder has a -4AN fitting, use a -4AN/-6AN adapter fitting with the one-foot -4AN hose; if it has an old-style valve, just use the standard valve adapter. Weigh the bottle again. You should notice a slight weight increase – that's the control-valve assembly. If your scale has a zeroing feature, push it now to zero it. If not, the additional "tare" weight must be added to the total filled weight of the cylinder mentioned earlier.
Next, determine whether your mother bottle has a siphon tube. Just look near the base of the valve – it'll have an "S" if there's a siphon or a "DP" to designate a dip tube. If your bottle has a siphon tube, it can be left with the valve in the upright position. If it doesn't have one, just invert the cylinder with a bottle-fill stand so that the valve is at the bottom. Close the shut-off valve on the nitrous-control valve assembly, then fully open the valve on the mother bottle and the one on the NOS cylinder you're refilling. Open the shut-off valve on the nitrous control-valve assembly and wait for the pressure in both cylinders to equalize. (You’ll hear a district sound that slowly fades away.) Next, connect the transfer pump to a clean air supply with a water trap, making sure that there's no oiler in the air line so that the nitrous oxide won't be contaminated. Slowly open the control valve on the compressed air-valve assembly. Watch the scale reading and close the air pressure-control valve when the nitrous cylinder reaches its full weight.
If the bottle reaches 1100 PSI before the full weight of the bottle is obtained, stop the pump by turning off the compressed air valve. Flip the bottle upside down and then upright several times until the temperature of the bottle drops. By now, the pressure on the gauge should have dropped too. If so, turn the pump back on and continue filling until you reach the bottle's maximum weight. Once the bottle is at the correct weight, close the valve on the nitrous bottle you just filled, then close the nitrous-control valve on the line running back to the mother bottle. That's all there is to it. Just make sure you never fill a nitrous bottle above the "FULL" weight stated on the cylinder label.