The most common fluids for manometers are water, red oil and mercury. Water is the best fluid to use if air is the fluid to be measured and the glass length is within reasonable limits. To minimize the effects of freezing and evaporation, kerosene or antifreeze may be used.
Mercury is used when the measured fluid mixes with the manometer luid or a very long tube length is necessary, while red oil is used as the indicating fluid when it is required to read a manometer from a distant place. It is easier to read and it does not combine chemically with some fluid in a dry system.