1. Does expired mean the warranty is expired or that I can't use the gas anymore?
Expired means toss it out. Some people will tell you to use it as bump gas in the future, but I'm pretty firmly against that. Gases react with the inside of the cylinder wall over time. Each year that goes by, the probability of your mix staying stable gets lower. The reason we don't suggest still using it as bump gas is that someone INEVITABLY calibrates with it because they don't know the difference. People get handed gas detectors all the time with little to no training and they get told to calibrate or "make them work" and they often don't know the difference between a bump test and a calibration. For this reason, we say that when it expires you should vent it out and either toss the cylinder or contact the manufacturer to return it to them. Some companies even give you a bonus if you return their cylinders, so look into that end if you have a bunch to return.
2. Can I use the same cylinder for different types of monitor?
You can, but we don't really recommend it unless you really know what you're doing, and even then it's a good idea to document it in case you change positions or someone else takes your gas detection program over. Essentially, you'll need to change the calibration gas settings on your monitor to make sure they match. You'll want to keep in mind that some monitors prefer a different gas for the explosive sensor (ISC likes Pentane, MSA likes Pentane Simulant, BW Technologies like Methane, etc). So yes... you can, but again we'd really recommend you go with the gas blend recommended by your gas detection manufacturer of choice.
3. What is a Simulant gas?
4. When do I need a stainless steel regulator instead of a nickel plated brass?
5. Can I use a fixed flow regulator to calibrate a pumped instrument?
You shouldn't, but you can in a pinch. We'd recommend using a demand flow type regulator, which inflates a small internal chamber that lets it flow gas at the exact rate the gas detector needs it. If you use a fixed flow regulator, you're either giving it too much gas (weakenes the pump seals) or you're giving it too little (which runs the pump hard and shortens life of the pump). Demand flows deliver the correct amount of gas at all times. If you need to use a fixed flow in a pinch, you can set a T fitting in the tubing between the gas detector and the regulator. It'll vent the excess pressure so your detector calibrates properly.
6. Can I use a 0.5lpm regulator when my gas detector wants a 0.25lpm regulator? What's the difference?
Always use the flow rate your gas detector asks for. You can occasionally use a different regulator for a bump test in a pinch, but for calibration you need the right flow rate. The calibration adapters are all designed with that flow rate in mind, and if you use a different rate it can overpressurize or not provide enough gas to the cells. Always use the correct flow rate regulator for your calibrations.