Pressure is actually quite simple; it refers to the magnitude of the force acting on a given area. This applies to gases, liquids, and any force acting on an area. Pressure is one of four unique measurement parameters within a plant, the other three being flow rate, temperature, and level. What makes pressure unique is that it can actually be used to infer the other three parameters. Flow rate can be inferred by installing pressure transmitters at both ends of a venturi, pitot tube, or orifice plate. Level can be measured using the principle of hydrostatic pressure to determine the amount of liquid in a tank. Finally, pressure can also be used to measure temperature, although this method is less common. For example, in a closed space, a given amount of gas will experience pressure changes within the chamber as the temperature changes; by observing the pressure change, the temperature change can be determined.
What are the important fundamental characteristics of pressure in industrial applications?
There are three important characteristics. First, the pressure is the same regardless of your location within the chamber. That is, whether you are at the top, bottom, left, or right, the pressure remains constant. Second, if the amount of gas in the chamber remains constant, changing the size of the chamber will change the pressure within it. For example, if the gas chamber shrinks, the pressure will increase. Third, temperature also affects pressure. If the gas chamber size is fixed and it is filled with gas, increasing the temperature will increase the pressure, and decreasing the temperature will decrease the pressure. Understanding these three properties is very important.
What is Pressure Reference?
When measuring pressure, you are actually comparing two pressure values, and there are many ways to do this. One is gauge pressure measurement, which compares the measured pressure to atmospheric pressure. This is very similar to measuring tire pressure on a car, where you compare the pressure inside the tire to the pressure outside. However, a drawback of this method is that if local pressure changes, the measurement results will appear inaccurate. For example, weather conditions such as thunderstorms and cold fronts can cause pressure changes because local atmospheric pressure fluctuates. Another method is absolute pressure measurement. Absolute pressure measurement compares the pressure to be measured to absolute zero pressure. Since absolute zero pressure doesn't exist anywhere, it's difficult to manufacture a transmitter with absolute zero as a reference. You need to create a vacuum in the chamber for the comparison. The advantage of absolute pressure measurement is that it doesn't have the errors common in gauge pressure measurement. However, the disadvantage is that absolute pressure measurement is very expensive. Some customers accept using gauge pressure as a reference because it's cheaper or more accurate than absolute pressure. There is also differential pressure reference, which directly compares two different pressure values.
How is pressure measured in industrial applications?
Pressure transmitters are commonly used. A pressure transmitter contains a sensor that measures pressure. It works by having a physical property that changes with pressure. Therefore, you infer the change in pressure by measuring the change in this physical property. There are many different ways to achieve this; commercially available sensors use technologies such as frequency, capacitance, and inductance. With a wide variety of sensors available, you can find transmitters using various technologies.
Is the function of a pressure transmitter limited to this?
Not at all. The transmitter takes information from the sensor and transmits it or communicates it with other devices that need that information. These devices can be controllers, distributed control systems (DCS), or even cloud-based data acquisition programs. The key is that the transmitter can complete the task quickly, accurately, and reliably. That's the most important thing.
How do transmitters communicate?
Most transmitters connect via hardwired connections to devices that require information, but some wireless solutions allow information to be broadcast to the desired location.
What communication language do transmitters use?
There are two communication methods: analog communication, digital communication, or a combination of both. Analog communication uses a 1-5 volt DC or 4-20 mA current signal, corresponding to pressure changes. Digital communication uses protocols through which the controller and transmitter communicate to obtain information. Some commonly used protocols include HART, FieldBus, and MODBUS. Some companies even have proprietary communication languages. While there are many options, ensure that all devices use the same protocol.
What characteristics should a pressure transmitter possess?
As we discussed earlier, you need a transmitter that can quickly, accurately, and reliably transmit signals to the desired location. Unfortunately, many pressure transmitter specifications on the market are difficult to understand. When you talk about speed, you're referring to response time. You need a fast response. Accuracy is also easy to understand, usually referring to reference accuracy. Reliability is a bit more difficult to understand; it's related to the stability of the product. Therefore, you need to examine the response time, accuracy, and stability of the transmitter you're interested in.
