Author: admin
-
14.6 Proper care and feeding of pneumatic instruments
Perhaps the most important rule to obey when using pneumatic instruments is to maintain clean and dry instrument air. Compressed air containing dirt, rust, oil, water, or other contaminants will cause operational problems for pneumatic instruments. First and foremost is the concern that tiny orifices and nozzles inside the pneumatic mechanisms will clog over time. Clogged orifices tend…
-
14.5 Analysis of practical pneumatic instruments
To better understand the design and operation of self-balancing pneumatic mechanisms, it is helpful to examine the workings of some actual instruments. In this section, we will explore three different pneumatic instruments: the Foxboro model 13A differential pressure transmitter, the Foxboro model E69 I/P (electro-pneumatic) transducer, the Fisher model 546 I/P (electro-pneumatic) transducer, and the…
-
14.4 Comparison of Op-Amp Circuits With Analogous Pneumatic Mechanisms
Self-balancing pneumatic instrument mechanisms are very similar to negative-feedback operational amplifier circuits, in that negative feedback is used to generate an output signal in precise proportion to an input signal. This section compares simple operational amplifier (“opamp”) circuits with analogous pneumatic mechanisms for the purpose of illustrating how negative feedback works, and learning how to…
-
14.3 Pilot valves and pneumatic amplifying relays
Self-balancing mechanisms consisting solely of a baffle/nozzle detector coupled to a feedback bellows, while functional, are not always practical as field instruments. Nozzles and orifices must be made rather small in diameter in order to minimize compressed air usage4 , but this means the mechanism will require significant time to alter its output pressure (i.e. to…
-
14.2 Self-balancing pneumatic instrument principles
A great many precision instruments use the principle of balance to measure some quantity. Perhaps the simplest example of a balance-based instrument is the common balance-beam scale used to measure mass in a laboratory: A specimen of unknown mass is placed in one pan of the scale, and precise weights are placed in the other pan until…
-
14.1 Pneumatic sensing elements
Most pneumatic instruments use a simple but highly sensitive mechanism for converting mechanical motion into variable air pressure: the baffle-and-nozzle assembly (sometimes referred to as a flapper-and-nozzle assembly). A baffle is nothing more than a flat object obstructing the flow of air out of a small nozzle by close proximity: The physical distance between the baffle and the nozzle…
-
Chapter 14 Pneumatic instrumentation
While electricity is commonly used as a medium for transferring energy across long distances, it is also used in instrumentation to transfer information. A simple 4-20 mA current “loop” uses direct current to represent a process measurement in percentage of span, such as in this example: The transmitter senses an applied fluid pressure from the process…
-
13.7 Troubleshooting current loops
A fundamental principle in instrumentation system troubleshooting is that every instrument has at least one input and at least one output, and that the output(s) should accurately correspond to the input(s). If an instrument’s output is not properly corresponding to its input according to the instrument’s design function, there must be something wrong with that…
-
13.6 4-wire “passive” versus “active” output transmitters
Some self-powered (4-wire) analog electronic transmitters are designed to behave as electrical loads rather than as electrical sources. Such transmitters are commonly referred to as having passive or sinking 4-20 mA outputs, as opposed to the active or sourcing 4-wire transmitters previously described: At first this seems needlessly confusing. Why build a self-powered transmitter requiring a second power supply in the circuit to…
-
13.5 2-wire (“loop-powered”) transmitter current loops
It is possible to convey electrical power and communicate analog information over the same two wires using 4 to 20 milliamps DC, if we design the transmitter to be loop-powered. A loop-powered transmitter connects to a process controller with only two wires, which is why loop-powered transmitters are synonymously known as 2-wire transmitters: Here, the transmitter is not really…
-
13.4 4-wire (“self-powered”) transmitter current loops
DC electric current signals may also be used to communicate process measurement information from transmitters to controllers, indicators, recorders, alarms, and other input devices. Recall that the purpose of a transmitter is to sense some physical variable (e.g. pressure, temperature, flow) and then report that quantity in the form of a signal, in this case a 4…
-
13.3 Controller output current loops
The simplest form of 4-20 mA current loop is the type used to represent the output of a process controller, sending a command signal to a final control element. Here, the controller supplies both the electrical power and signal information to the final control element, which acts as an electrical load. To illustrate, consider the…
-
13.2 Relating 4 to 20 mA signals to instrument variables
A 4 to 20 mA current signal represents some signal along a 0 to 100 percent scale. Usually, this scale is linear as shown by this graph: Being a linear function, we may use the standard slope-intercept linear equation to relate signal percentage to current values: Where, y = Output from instrument x = Input to instrument…
-
Chapter 13 Analog electronic instrumentation
An “analog” electronic signal is a voltage or current proportionate to the value of some physical measurement or control quantity. An instrument is often classified as being “analog” simply by virtue of using an analog signal standard to communicate information, even if the internal construction and design of the instrument may be mostly digital in…
-
12.10 How to teach yourself PLC programming
First and foremost, you need to get your very own PLC to work with. Computer programming of any kind is not a spectator sport, and can only be learned by significant investment of time and effort at the keyboard. In many ways, learning to program is like learning a new spoken or written language: there…