Assertion : An open-tube manometer is a useful instrument for measuring pressure differences.

Reason : $1 bar = {10}^5 Pa$ .

The pressure difference of the gas in the container is indicated by the height of mercury level on the tube of the open tube manometer. Hence, the open tube manometer is a useful instrument to measure pressure differences.

 $1 bar = {10}^5 Pa , \mathrm{is} \mathrm{a} \mathrm{conversion} \mathrm{relation} \mathrm{for} \mathrm{pressure} \mathrm{in} \mathrm{different} \mathrm{units}.$

Hence, Both are correct but reason does not explain about assertion.