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Temperature Displays, DAS,
I/O Modules, Transmitters and Controllers Most
temperature sensors used in monitoring and control do not have built in temperature
displays, with the exception of non-electrical devices like glass thermometers,
bimets, phase change and filled systems. Whether in manufacturing or R&D, wherever
there is an electrical output temperature sensor or some sort of control action
will be taken based on temperature values, there is a need for add-on devices
to interpret the output signals. The
simplest of these for those sensors having an analog electrical output (thermocouples,
thermistors, RTDs and radiation thermometers) is the meter. Meters come in a variety
of styles, shapes and sizes, but the major division of types is between analog
and digital. Analog meters are familiar enough, especially to anyone recalling
the dashboard on a car with a meter that has a pointer that physically points
to "H" or 200° or some other mark painted on the meter scale. Digital meters are
simply displays of numerical temperature values in familar digits that change
as the temperature reported by the sensor varies. Still
other temperature sensors find use connected to data acquisition systems or computer
controllers. They need their signal carried, sometimes over extended distances,
from the measuring point to the computer or system performing the actual control.
On and on it goes. There are many variations on the same theme. The net result
is a large variety of temperature sensor add-ons particularly in the monitoring
and control of large power plants, process and various manufacturing plants. Of
course there are suppliers of each type of device and the intent of these pages,
after breaking down the different type into categories, is to provide names and
links to the web sites of such companies.
Thanks for visiting.
Meters
for Temperature Display
The
meter category actually covers a very wide ground but suffice it to say the main
intent of these devices is to display temperature. There are two principal types
of meter displays: Analog Meters and Digital Meters
Recorders
for tracking Temperature Signals
The
old time paper chart recorder has all but disapeared from the market, but those
desperate enough to want one will have to pay a premium in terms of maintenance
and in the marketplace. These days, liquid crystal displays (LCD) have been developed
that are light and rugged and almost low cost. The cost trends are expected to
continue downwards enough to become an acceptable replacement, in conjunction
with digitally recorded data, for paper charts altogether. Data
Acquisition Systems (DAS) for digitally recording temperature signals and
I/O Modules for converting analog temperature signals to digital
The PC revolution has spawned
a raft of PC plug-ins for use as digital data acquisition system building blocks.
Then, too, there is an almost equally large supply of DAS software to run the
PC-based DAS.
I/O modules are another variation of the same theme, but usually
tied into a control or recording system. With many data busses available there
seems to be an endless variety of such devices for temperature and other analog
signals.
Current
Transmitters
Thermocouples and RTDs are used widely in large process (and other) plants where
the temperature signals must be brought to one or more distant data acquisition
devices or computer interfaces. Sometimes these distances can be a mile (~1.5
KM) or more. One of the analog methods for transmitting the signals through wire
pairs is to use current transmitters. These devices convert the analog sensor
voltage output signal into a 0-20ma or 4-20 ma current value. The current is unattenuated
by a long wire length as long as there is a sufficiently large voltage source
to provide the power to drive the transmitter and its current output. These days,
however, digital busses are rapidly taking over the role of the current transmitter
and many provide seperate local interfaces for analog sensor signals.
Temperature Controllers
Many simple ovens,
furnaces and similar heating devices are essentially small systems operated by
a relatively simple temperature controller. The controller input(s) often comes
from a temperature sensor mounted in the oven, furnace etc. There are literally
thousands of such devices in uses that range from baking cookies and breads to
hot-melt glueing of disposable diapers, to heat treating plastic parts prior to
vacuum forming, to epoxy coating of re-bar, drying paper, etc, etc. etc.
Meter, Recorder, Data Acquisition and Controller
Suppliers | | |
