  We consider a state estimation problem for gas flows in pipeline networks
where hydrogen is blended into the natural gas. The flow is modeled by the
quasi-linear isothermal Euler equations coupled to an advection equation on a
graph. The flow through the vertices where the pipes are connected is governed
by algebraic node conditions. The state is approximated by an observer system
that uses nodal measurements. We prove that the state of the observer system
converges to the original system state exponentially fast in the $L^2$-norm if
the measurements are exact. If measurement errors are present we show that the
observer state approximates the original system state up to an error that is
proportional to the maximal measurement error. The proof of the synchronization
result uses Lyapunov functions with exponential weights.
