density measurement of natural gas

This is just gonna be a shot in the dark wild guess, but how about a gas density meter?

If it is a process plant, at the battery limits they will usually have a process Gas Chromatograph, the software can calculate BTU and Density.

You can measure gas density by measuring and correlating some of the gas's physical properties (e.g. thermal conductivity, electrical breakdown, acoustic attenuation, etc.) with pressure. Or you can procure commercial equipment such as a Yokogawa gas density analyzer.

How and where do I reply to the original queation?

The answer you need: Gas Density and Specific volume meters. Common gases such as methane have values obtainable from the utility supplier or manufacturer of the gas.

Your answer is gas chromatograph.

But if you are not interested in spending money, or your need is only occasional, you can even do it manually. You got to know the gas analysis and P & T conditions to calculate this. See GPSA or Perry handbook.