Intelligent pressure transmitters

Gauge pressure sensor Absolute pressure sensor Differential pressure sensor 

Sealed vacuum reference (evacuated to full vacuum) 

Because of the wide range of the sensors, only four different sensor units are needed to cover the entire range of dp spans from 100 kN/m to 20 MN/m (4 in water to 3000 Ib/in ) An internal temperature sensor monitors the temperature of the pressure sensor and is used to compensate the sensor output for the effects of temperature changes. The sensor temperature may also be transmitted digitally for monitoring, alarming, and for other appropriate applications. 

Biplanar design and process cover support maintains standard process connection spacing without need for special flanges 

Small size means only 2 bolts are required to achieve precise alignment and tight seal between process sensor and covers. 

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Intelligent transmitters have two major components (1) a sensor module which comprises the process connections and sensor assembly, and (2) a two-compartment electronics housing with a terminal block and an electronics module that contains signal conditioning circuits and a microprocessor. Figure 6.9 illustrates how the primary output signal is compensated for errors caused in pressure-sensor temperature. An internal sensor measures the temperature of the pressure sensor. This measurement is fed into the microprocessor where the primary measurement signal is appropriately corrected. This temperature measurement is also transmitted to receivers over the communications network. 

Figure 6.11. Intelligent differential-pressure cell with transmitter...

The size of orifice-type flow sensors is limited only by the pipe size they are installed in, and their pressure and temperature ratings depend only on the limitations of the d/p detector used. Their measurement error is the combined orifice and d/p readout error, which in a standard installation over a 3 1 range is about 2% FS, and with an intelligent and multirange transmitter it can be reduced to 1% AF over a 10 1 range. 

Most pressure sensors detect the difference between the measured value and a reference. In the case of absolute pressure sensors, the reference chamber cannot be evacuated to absolute zero, because it can only be approached within a few thousands of a millimeter of mercury (torr). In the case of positive pressure detectors, when the barometric pressure is the reference, atmospheric pressure variations can cause errors up to 25 mmHg. Intelligent transmitters can also operate with multiple references and switch them as required. 

Differential pressure-type detectors have already been discussed in connection with both flow (Section 3.9.9) and level (Section 3.11.2) measurements. Therefore, only their ranges and accuracies will be briefly mentioned here. The basic error of d/p transmitters ranges from 0.1 to 0.5% of the actual span. Added to this are the errors caused by the temperature and pressure effects on the span and zero of the instrument. For intelligent transmitters, the pressure and temperature corrections are automatic, and the overall error is 0.1 to 0.2% of span. 

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