A pneumatic transmitter is a device that senses some process variable and translates the measured value into an air pressure which is transmitted to various receiver devices for indication, recording, alarm, and control. The signal range of 3-15 psig is the accepted industry standard; however, other ranges may be encountered. This signal is proportional to the range of measurement of the process variable. For example, 3-15 psig can represent 0-100 psi, 500-1000 gpm, –50 to +50°F, etc. The prime function of a transmitter is to reproduce the low energy measurement signal with sufficient energy that it may be transmitted over an appreciable distance or used as a power source to a control device. The low-energy measurement signal is the position or movement associated with the action of the process variable on the sensing element (bellows, diaphragm, Bourdon tube, etc.). Pneumatic transmitters operate in a manner similar to proportional controllers.
Read also: Temperature sensor
Electronic transmitters perform the same function as pneumatic transmitters: a low energy process-related signal is converted into a higher energy signal suitable to connect to other instruments in the system. The output signal of most electronic transmitters is a 4-20 mA, 10-50 mA, or 1-5 Vdc signal. Other ranges often encountered are: 0-10 Vdc, 2-10 Vdc, and 0.25-1.25 Vdc. Electronic transmitters are also classified as force balance or motion balance types.
Of great concern to the instrument engineer is the method by which electronic transmitters are connected in the instrumentation system. The “two-wire,” “three-wire,” and “four-wire” classifications are often used to
describe the method of connection.
Two-wire transmitters (Fig. 4-17a)
These are the simplest and most economical and should be used wherever load conditions will permit. In a two-wire system the only source of power to the transmitter is from the signal loop. Referring to Fig. 4-18a, the 4 mA “zero-end” current is sufficient to drive the internal circuitry of the transmitter and the current from 4 to 20 mAs represents the range of the measured process variable. The power supply and the instruments are usually mounted in the control room.
Three-wire transmitters (Fig. 4-17b)
Some transmitters require more power than the signal loop (4- 20 ma, etc.) can supply to support their internal circuitry. A DC common wire is run from the instrument to the transmitter. This permits the transmitter to draw whatever power it needs from the power supply and produce the desired signal current at the transmitter output.
Four-wire transmitters (Fig. 4-17c)
Some transmitters have their own internal power supply and require no connection to the DC power supply. A 120 vac source is connected directly to the transmitter and its output signal loop is connected only to the receiving instrument. These are often used where an instrument is “added on” to an existing instrumentation installation to avoid adding to the load of the DC supplies. The disadvantage is the need for AC power at the instrument site.
Signal converters are used either to achieve compatibility between different types of instruments or for isolation purposes. Some common forms of signal converters are:
Pneumatic-to-electronic (P/I) — These are electronic pressure transmitters designed for 3-15 psig input range and the desired output range (4-20 ma, etc.). Electronic-to-pneumatic (I/P) — I/P converters are pneumatic transmitters with an electro-magnetic device connected to a nozzle-baffle arrangement which generates a pneumatic output signal which is proportional to the input signal.
Isolators — These are usually electronic current-to-current or voltage-to-voltage converters which provide electrical isolation to eliminate unwanted ground loop currents or common mode voltages.
Electric signal converters — These fit the same category as I/Ps and P/Is in that they change the signal from one range to another. Examples are 4-20 mA to 0-10 vdc, 1-5 Vdc to 10-50 mA, etc.
Frequency converters — Frequency to DC converters typically receive pulse inputs from turbines or positive displacement flowmeters and provide a proportional 4-20 ma, 10- 50 mA or voltage output. Voltage output converters are often referred to as F/V (frequency-to-voltage) converters or transmitters. V/F (voltage-to-frequency) converters are often used to interface standard “current-loop” type instrumentation to control devices requiring frequency or pulse-train setpoint inputs. These are commonly used in speed indicators for high speed centrifugal equipment