With growing sophisticationCONTROL ENGINEERING China版权所有, instrumentation devices can often provide more than one variable. These measurements are free in that they don’t require any additional sensors or process penetrations. All they require is a way for you to extract the information.
        Multivariable approaches fall into three categories depending largely on the needs of the primary variable:
        Corrective measurements—Most electronic sensors are influenced to some extent by more than one variable. For example, pressure sensors that use capacitive or strain gage technologies are affected by temperature. ConsequentlyCONTROL ENGINEERING China版权所有, the transmitter for such a device takes its own temperature measurement and uses that data to correct the primary reading. Since that measurement is in the transmitter, it is usually a simple matter to provide it to the Control system.
        The caution of using corrective measurement data is making sure you understand where it comes from. The temperature in this example will be taken where it is needed to correct the primary variable and may not reflect the process at all; it may only reflect the ambient temperature around the transmitter or electronic devices. Make sure you understand what it is before you use such data.
        Multiple measurements—One of the most common flow measurement methods is using an orifice plate and differential pressure gage. There are many implementation variationsCONTROL ENGINEERING China版权所有, but the basic concept calculates flow based on pressure readings on both sides of a known obstruction. While the flow measurement only needs the differential pressure valueCONTROL ENGINEERING China版权所有, it isn’t difficult to extract line pressure measurements as well.
        Calculated measurements—With the growing sophistication of transmitter electronics,