Measurement Technology - The Electronic Nose Artificial Olfaction Technology - page 19

parameters are important in discriminating normal from abnormal function. Dis-
placement sensors can be used in both direct and indirect systems of measurement.
Direct measurements of displacement are used to determine the change in diameter
of blood vessels and the changes in volume and shape of cardiac chambers.
Indirect measurements of displacement are used to quantify movements of liq-
uids through heart valves. An example is the movement of a microphone diaphragm
that detects the movement of the heart indirectly and the resulting heart murmurs.
Displacement measurement can be of two types: contact and noncontact types.
Besides the measurement principles can be classified into two categories: electrical
sensing and optical sensing. In electrical sensing, passive electrical sensors are
used and variation of either inductance or capacitance with displacement is
measured. On the other hand, the optical method mainly works on the principle of
intensity variation of light with distance.
Potentiometer, LVDT, piezo electric transducer, inductive and capacitive type
measurement, tachogenerator, gagnetic pickup, and proximity are mostly used
sensors for displacement and speed measurement.
1.9.5 Level Measurement
A level measurement is an integral part of process control, and may be used in a
variety of industries [ 6 ].
• Level measurement may be divided into two categories:
- Point level measurement
- Continuous level measurement
• Point level sensors are used to mark a single discrete liquid height, a preset level
condition (as a high alarm or a low alarm condition).
• Continuous level sensors provide an analog output that directly correlates to the
level within the containing vessel.
This analog signal may be directly linked to a visual indicator or to a process
control loop, forming a level management system.
There are several instances where we need to monitor the liquid level in vessels.
In some cases the problem is simple, we need to monitor the water level of a tank;
a simple float type mechanism will suffice. But in some cases, the vessel may be
sealed and the liquid a combustible one; as a result, the monitoring process
becomes more complex. Depending upon the complexity of the situation, there are
different methods for measuring the liquid level, as can be summarized as follows:
1. Float type
2. Hydrostatic differential pressure gage type
3. Capacitance type
4. Ultrasonic type
5. Radiation technique