Page 386 - Boiler_Operators_Handbook,_Second_Edition

Controls 371

noticed that the force required to push the float down in- rapidly, commonly from a negative voltage to a positive
creased with depth. If you didn’t notice, go back and do voltage so there’s no way you can put a meter on the
it again. The additional force is equal to the difference signal terminals and measure it. The value of a digital
between the weight of air in the float and the weight signal is a function of the number of changes in value
of water it displaces, the buoyancy principle. Displace- and the time between each change, so complex that it
ment transmitters balance the force on the float with a requires a computer to read it. Why are they better? Be-
force produced by a feedback bellows. cause the actual value is not important. Any significant
Pressure transmitters use the same principles of resistance in the signal wiring for a voltage signal, like a
force balance to produce an output by using another bel- loose terminal, can alter the signal to produce an error.
lows or a diaphragm sensing the pressure in the process Digital signals represent zeros and ones where a zero is
and balancing that with an output feedback. Different considered anything between plus five and plus fifteen
pressures are accommodated by changing the size of volts and a one is considered anything between a minus
the process bellows or diaphragm. Pressure transmitters five and minus fifteen volts. That considerable range of
would be very expensive if a special bellows had to be voltage minimizes errors and the additional features of
made for each pressure range so they are made adjust- digital signal transmission provide more accuracy and
able within standard ranges by allowing adjustment of a reliability than analog signal transmission. All that and
pivot between two beams connected to the bellows and the lower cost, both hardware and installation, of digital
feedback. controllers have made them the controls of choice, re-
Temperature transmitters work the same, they placing all other types of control.
just need a way to get motion or force proportional to My understanding of control operation is based on
the temperature then convert it to a signal. Bimetallic pneumatic controls so I’m going to continue using them
sensors use the movement or force produced by the dif- as examples in describing concepts. You may never see
ference in thermal expansion of two metals, fluid filled a pneumatic control system but the concepts work with
transmitters use the thermal expansion of the liquid to any type of controller and a pneumatic understanding
produce movement and gas filled transmitters use the will help you comprehend them. I’ll even use a control-
increase in pressure proportional to temperature. ler that’s no longer available (like most pneumatics), a
Electronic pressure and differential transmitters Hagan Ratio Totalizer as shown in Figure 11-5.
sense process values using the same techniques as de- The totalizer has four diaphragm chambers but
scribed for pneumatic transmitters converting a force they could also be fitted with bellows. The totalizer
or movement to a voltage or current and generating a was designed to provide universal use by adapting it.
feedback force using an electromagnet. Temperature The output chamber and A input chambers are secured
transmitters use a resistance to electric current where to the base of the transmitter. Sliding in the middle are
the resistor’s resistance varies with temperature. An- clamps that connect to the base and the beam. The beam
other means of measuring temperature that has been floats in the middle of the assembly, attached to the
around for years is a thermocouple. Two wires of dif- diaphragms and the beam clamp. A very thin piece of
ferent materials connected at their ends will produce spring steel connects the two clamps to form the pivot
an electric voltage when the two ends are subjected to of the controller. The two clamps can be loosened and
different temperatures. Note that the reference tempera- slide along the base and beam to positions right and left
ture (one end of the two wires) has to be stable to get a of center. The valve floats in the output chamber and
reliable signal proportional to temperature at the other
end. Digital transmitters use similar methods then con-
vert the analog signal to a digital one.
Gee, we got this far in the discussion of transmit-
ters without mentioning the word “analog.” That wasn’t
hard because, for all practical purposes, all pneumatic,
voltage and current signals are analog signals. The sig-
nal represents (is analogous to) a process value, you can
take a measurement of the signal and can determine the
process value from the value of the signal. That’s all an
analog signal is, a value that represents another one.
What makes digital signals different? They change Figure 11-5. Hagan Ratio Totalizer