Molecular Weight Measurement

Does anyone know if instrumentation exists for measuring the molecular weight of a gas stream. The application involves combining a flow of natural gas (CH4/methane) with a flow of a gas with variable composition. The composition of the gas is mainly hydrogen and carbon monoxide (over 90%) but with a few other impurities such as carbon dioxide, nitrogen, methane and hydrogen sulphide. Ideally the device would have 4-20mA outputs and be capable of surviving in the harsh environment of an industrial plant but I am flexible! The mixture of gases is used for combustion for gas heating furnaces.
Thanks in advance for any help.

 

[samlukeben]

Gas molecular weight is an indirect measure of gas density or gas gravity. Gas gravity is the relationship - ( density of the subject gas / density of air at std Conds.) or similarly the (molecular weight of the subject gas / Molecular weight of air at st.conds). Therefore if you measure the gas density the molecular weight of the gas is simply - (Gas density / density of air) * Molecular weight of air.

Molecular weight of air at st.conds = 28.94 g/mol
density of air = 1.225 kg/m3

Possible instument for measuring the density of a pressurised gas stream :

http://www.solartronmobrey.com/density/density_main.html