Reasonably priced sensors are now available in a variety of packages, making them attractive for a wide variety of embedded system precision sensing applications, including liquid level measurements. This article describes differences among modern pressure sensors and emphasizes the benefits of recent advances in microelectro mechanical (MEMS) temperature-compensated, silicon pressure sensors.
The article then describes a cost-effective, low-power, liquid level-measurement data acquisition systems (DAS) using a compensated silicon pressure sensor and a high-precision delta-sigma ADC. The article will explain how to select the compensated silicon pressure sensor. It will suggest system algorithms, analyze noise, and offer calibration ideas for improving system performance while reducing complexity and cost.
Measuring Pressure-a Look Back
It could be argued that modern pressure measurement was started by the Italian physicist Evangelista Torricelli1 through his inventions in 1643, the mercury barometer. Torricelli filled a glass tube one meter long with mercury, hermetically closed the tube at one end, and set it vertically with the open end in a vessel filled with mercury. The column of mercury fell to about 760mm, leaving an empty space above its level. The pressure unit, Torr, was named in honor of this inventor and has a ratio of 1 to 760 standard atmospheres. Blood pressure is measured in Torr (millimeters of mercury) in most of the world.
Contemporary pressure units include the Pa (Pascal) defined by System International (SI) as the main pressure unit (Pa = N/m). In the U.S. a popular pressure-measurement unit is the “bar” which measures pounds per square inch (PSI). Conversion to a standard unit measure among the various pressure units is quite a cumbersome task because of historical and technical reasons. Nonetheless, widely available free conversion tables or free online unit converters can make the task easier for engineers.
There are two main categories of pressure sensors classified by type of the measurement:
1. Absolute pressure sensor, which measures the pressure relative to perfect vacuum pressure. An example of an absolute pressure sensor is the mercury (Hg) barometer shown in Figure 1 below.
Figure1. Mercury (Hg) in the barometer tube adjusts until the weight of its column balances the atmospheric force. Patm is exerted on the open reservoir.
2. Differential pressure sensor, which measures the difference between two or more pressures introduced as inputs to the sensing unit. An application example of such a sensor is the differential pressure flow meter (Figure 2 below) where change in the velocity of the fluid produces a change in the pressure and creates a pressure difference, ΔP = P1 – P2.
Figure 2. For this type of flow meter, the volume flow rate, Q, is related to ΔP by a simple formula that measures the rate of the flow and, consequently, determines consumption.
A gauge pressure sensor is another type of differential sensor that is constructed to measure the relative pressure to an atmospheric pressure. An example of this sensor is the popular tire-pressure gauge. When the tire-pressure gauge reads zero, it is really reading atmospheric pressure at a given location.