Measurement of small pressure differences. The two$-$fluid U$-$tube manometer shown in Fig. Q$4$ is being used to measure the difference in pressure at $2$ points $(a$&$b)$ in a line

containing air at $1$ atm abs pressure. When $Pa=Pb,$ the reading of R in the manometer is $0.$ The lighter fluid is hydrocarbon with a density of $812k\frac{g}{m^3}$ and the heavier fluid is water with ${}_{\text{w}\text{a}\text{t}\text{e}\text{r}}=998k\frac{g}{m^3}.$ The inside diameters of the $U$ tube and reservoir are $3\mathrm{.}2$ mm

and $54\mathrm{.}2$ mm $,$ respectively. Calculate the pressure difference in mm Hg and Pascal.

Just wondering how to properly take into account having different diameters?

Thanks