MIL-HDBK-1005/7A
6.13.3.1 Use Limitations. Different types of measuring devices
are available for each application. See Table 15 for a listing
of primary devices and examples of their application. The
listed "capacity" of a device includes most sizes and types of
the device that are available. The "range" is the useful
turndown-ratio of a particular device.
6.13.3.2 Discrete vs. Analog Devices . Alarm functions and many
control functions require only the presence or absence of a
process variable input for their operation. For example, a sump
pump may start if the liquid level is above a certain point or a
tank heater may start if the temperature is below a selected
point. Control these functions by discrete devices such as flow
switches, temperature switches, level switches, and pressure
switches. If the actual status of the process variable is
required, rather than on/off, for indication or control, an
analog primary device should be used. Some alarm switches are
not included in the tables; for example, clarifier torque
switches, speed switches, and other equipment protection
switches that are normally supplied with the equipment.
6.13.4
Instrumentation. Instrumentation covers all secondary
instruments (such as gages, indicators, recorders, or
totalizers) needed for efficient operation of water supply
systems. Information sensed by a primary device is translated
by instruments into an operator usable form. Most analog
primary devices require secondary instruments, although a few
(such as displacement meters) contain built-in counters.
Instrumentation should be used only where operating convenience
and cost savings outweigh added maintenance needs. Data logging
devices should be considered where cost can be offset by reduced
operating manpower needs. See Table 14 for recommended
instrumentation usage. Refer to Water Treatment Plant Design,
Chapter 20, "Process Instrumentation and Controls," AWWA, third
edition.
6.13.4.1 Use Limitations. Instruments may be obtained in any
combination to total, indicate, or record the information
developed by primary devices. Other more sophisticated forms of
instruments (such as summation and multiplication of variable)
are possible, but are not normally needed.
71
Previous Page
