15-12-2012, 05:55 PM
Process Instrumentation
1Process Instrumentation.ppt (Size: 352 KB / Downloads: 99)
Differential Pressure Transmitter
Transmitters are available that bolt flush to the bottom of the tank and thus
eliminate that tubing connection; transmitters are also available that have a
hydraulic filled tube between the dP cell diaphragm and a remote diaphragm
Differential Pressure
While a variety of pressure measurement methods are available, such as
manometers, bourdon tubes and bellows, most pressure transmitters today, both
single pressure and differential, measure pressure by sensing the deflection of a
diaphragm. The sensing device for that deflection is a strain gauge or other
technique and is often on a secondary diaphragm for temperature and shock
protection. shows the internals of a differential pressure transmitter
and the secondary diaphragm that is coupled by oil filled channels. The output
of the sensor is then amplified for transmission. The sensor is analog whether
the signal conditioning and transmission is digital or analog
Measurement Concepts
Accuracy is the ratio of the error to the full-scale output, generally expressed as
a percentage of span.
Repeatability is how well an instrument gives the same output for the same
input when the input is applied in the same way over a short time period. It is
also often expressed as the error as a percent of span.
Linearity only applies to measurements that are supposed to be linear; then, it
also is a percent of span of the deviation of the measurement versus actual value
from a straight line.
Response speed is defined as the length of time required for the measured value
to rise to within a certain percentage of its final value as a result of a step
change in the actual value. A 98% response time, for example, while indicative
of the time to get a good measurement, is much longer than a first order time
constant. The first order time constant of the measurement is of most interest in
the performance of a control loop.