From the Research
The easiest way to determine the amount of nitrous oxide (N2O) in an E cylinder is to weigh the cylinder and subtract the tare weight (empty cylinder weight) which is typically stamped on the cylinder. This method is reliable because nitrous oxide exists as both liquid and gas in the cylinder, making pressure readings unreliable indicators of remaining content 1. Some key points to consider when using this method include:
- An E cylinder contains approximately 1,590 liters of N2O when full, weighing about 5.5 pounds (2.5 kg) of gas.
- The tare weight is usually stamped on the shoulder of the cylinder along with other markings.
- To perform this measurement, you'll need an accurate scale capable of weighing the cylinder. Unlike oxygen cylinders where pressure directly correlates with remaining gas, nitrous oxide maintains constant pressure (approximately 750 psi at room temperature) until almost all liquid has vaporized 2. Only when the liquid phase is depleted will the pressure begin to drop, at which point the cylinder is nearly empty. This weight-based approach provides the most accurate assessment of remaining nitrous oxide content, and is supported by recent studies on the measurement of nitrous oxide leakage from hospital manifold-pipeline networks 1.
Some other methods for measuring nitrous oxide content have been described, including the use of headspace gas chromatography coupled to mass spectrometry (HS-GC-MS) 3, but these methods are not as practical or widely available as the weight-based method. Additionally, studies have investigated the ideal oxygen/nitrous oxide fresh gas flow sequence with the Anesthesia Delivery Unit machine 4, but these studies do not provide information on the measurement of nitrous oxide content in E cylinders. Overall, the weight-based method is the most reliable and practical way to determine the amount of nitrous oxide in an E cylinder.