Vapor pressure is the pressure that a vapor exerts when it is in equilibrium with its own liquid. At a constant temperature, the vapor pressure is just as unique a characteristic of a liquid as is its boiling point (Table 8.20a). Therefore, by measuring the vapor pressure of a fluid, one can determine the composition of that fluid. If the temperature of the process varies, temperature compensation is needed. With microprocessor-based, smart instruments, such compensation is easily provided, if the relationship between vapor pressure and temperature is entered into the compensation software (Figure 8.20b).The differential vapor pressure transmitter is a d/p cell with one of its sides connected to a temperature bulb filled with a reference fluid (Figure 8.20c), while the other side is connected to the process. This way the sensor can continuously compare the vapor pressure of the process material with the vapor pressure of the sealed-in reference fluid. If the reference bulb is inserted into the process, the need for temperature compensation can be eliminated. This way the temperature of the reference fluid is always the same as that of the process. Consequently, if the two compositions are the same, their vapor pressures will also be identical.On the other hand, a differential vapor pressure analyzer will correctly measure the binary composition under variable column pressure conditions, because as the column pressure drops, it also lowers the boiling point on the tray. This in turn will lower the reference bulb temperature and with it the vapor pressure inside the reference bulb. Therefore, as long as the tray composition is constant, the pressure differential will also be constant, even if the column pressure varies.