Transmitter range, zero, span

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. 

Comment pressure transmitters deliver signals but require power. You can set the zero, span, measurement scale and range, and more. They are pricey but very... 

Figure TAb shows a temperature transmitter which accepts thermocouple input signals and is set up so that its current output goes from 4 to 20 m A as the process temperature varies from 0 to 250 F. The range of the temperature transmitter is SO to 2S0"F, its span is 200 F, and its zero is 50 F. The gain of the temperature transmitter is...

The basic rangeability of this meter is the same as that of an orifice plate (3 1), but if two (a high span and a low span) transmitters are used, and the flow element is accurately calibrated over the complete flow range, it can be increased to 10 1. This performance can be obtained from all properly calibrated d/p flow elements, not just from the V-cone design. The V-cone flowmeter should be installed horizontally so that the two pressure taps are at the same elevation. This guarantees that the d/p cell will detect a zero pressure differential when there is no flow. 

We now introduce several terms that are commonly associated with the accuracy of process instruments. The measurement error (or error) is the difference between the true value and the measured error. Instrument vendors often express accuracy as a percentage of full scale (% FS) where the term full scale refers to the span of the instrument. Suppose that the % FS error of a temperature transmitter is reported as 1% and the zero and span are adjusted so that the instrument operates over the range of 10-70 °C. Since the span is 70-10 = 60 °C, the measurement error is 1% of 60 °C, or 0.6 °C. Consequently, the relative error (obtained by dividing the error by the value of the measurement) at 10 °C is 0.6/10 = 6%. Thus, when instrument accuracy is expressed as % FS, the relative error can be quite large for small values of the measured variable. 

Composition measurement The zero and span of each composition transmitter are 0 and 0,50 (mass fraction), respectively. The output range is 4-20 mA. A one-minute time delay is associated with each measurement. 

---