Resistor symbols have been added to the wires of the cable to show what is happening in a real
system. Bear in mind that these resistances can be minimized with heavy-gauge wire (at additional
expense) and/or their effects mitigated through the use of a high-resistance (null-balance?)
voltmeter for an indicator (at additional complexity).
Despite this inherent disadvantage, voltage signals are still used in many applications because of
their extreme design simplicity. One common signal standard is 0-10 volts, meaning that a signal
of 0 volts represents 0 percent of measurement, 10 volts represents 100 percent of measurement, 5
volts represents 50 percent of measurement, and so on. Instruments designed to output and/or
accept this standard signal range are available for purchase from major manufacturers. A more
common voltage range is 1-5 volts, which makes use of the "live zero" concept for circuit fault
indication.
• REVIEW:
• DC voltage can be used as an analog signal to relay information from one location to
another.
• A major disadvantage of voltage signaling is the possibility that the voltage at the indicator
(voltmeter) will be less than the voltage at the signal source, due to line resistance and
indicator current draw. This drop in voltage along the conductor length constitutes a
measurement error from transmitter to indicator.
Current signal systems
It is possible through the use of electronic amplifiers to design a circuit outputting a constant
amount of current rather than a constant amount of voltage. This collection of components is
collectively known as a current source, and its symbol looks like this:
A current source generates as much or as little voltage as needed across its leads to produce a
constant amount of current through it. This is just the opposite of a voltage source (an ideal
battery), which will output as much or as little current as demanded by the external circuit in
maintaining its output voltage constant. Following the "conventional flow" symbology typical of
electronic devices, the arrow points against the direction of electron motion. Apologies for this
confusing notation: another legacy of Benjamin Franklin's false assumption of electron flow!
Current sources can be built as variable devices, just like voltage sources, and they can be designed
to produce very precise amounts of current. If a transmitter device were to be constructed with a
variable current source instead of a variable voltage source, we could design an instrumentation
signal system based on current instead of voltage:
The internal workings of the transmitter's current source need not be a concern at this point, only
the fact that its output varies in response to changes in the float position, just like the potentiometer
setup in the voltage signal system varied voltage output according to float position.
Notice now how the indicator is an ammeter rather than a voltmeter (the scale calibrated in inches,
feet, or meters of water in the tank, as always). Because the circuit is a series configuration
(accounting for the cable resistances), current will be precisely equal through all components. With
or without cable resistance, the current at the indicator is exactly the same as the current at the
transmitter, and therefore there is no error incurred as there might be with a voltage signal system.
This assurance of zero signal degradation is a decided advantage of current signal systems over
voltage signal systems.
The most common current signal standard in modern use is the 4 to 20 milliamp (4-20 mA) loop,
with 4 milliamps representing 0 percent of measurement, 20 milliamps representing 100 percent,
12 milliamps representing 50 percent, and so on. A convenient feature of the 4-20 mA standard is
its ease of signal conversion to 1-5 volt indicating instruments. A simple 250 ohm precision
resistor connected in series with the circuit will produce 1 volt of drop at 4 milliamps, 5 volts of
drop at 20 milliamps, etc:
| Percent of | 4-20 mA | 1-5 V |
| measurement | signal | signal |
| 0 | 4.0 mA | 1.0 V |
| 10 | 5.6 mA | 1.4 V |
| 20 | 7.2 mA | 1.8 V |
| 25 | 8.0 mA | 2.0 V |
| 30 | 8.8 mA | 2.2 V |
| 40 | 10.4 mA | 2.6 V |
| 50 | 12.0 mA | 3.0 V |
| 60 | 13.6 mA | 3.4 V |
| 70 | 15.2 mA | 3.8 V |
| 75 | 16.0 mA | 4.0 V |
| 80 | 16.8 mA | 4.2 V |
| 90 | 18.4 mA | 4.6 V |
| 100 | 20.0 mA | 5.0 V |
The current loop scale of 4-20 milliamps has not always been the standard for current instruments:
for a while there was also a 10-50 milliamp standard, but that standard has since been obsoleted.
One reason for the eventual supremacy of the 4-20 milliamp loop was safety: with lower circuit
voltages and lower current levels than in 10-50 mA system designs, there was less chance for
personal shock injury and/or the generation of sparks capable of igniting flammable atmospheres
in certain industrial environments.
• REVIEW:
• A current source is a device (usually constructed of several electronic components) that
outputs a constant amount of current through a circuit, much like a voltage source (ideal
battery) outputting a constant amount of voltage to a circuit.
• A current "loop" instrumentation circuit relies on the series circuit principle of current
being equal through all components to insure no signal error due to wiring resistance.
• The most common analog current signal standard in modern use is the "4 to 20 milliamp
current loop."
Tachogenerators
An electromechanical generator is a device capable of producing electrical power from mechanical
energy, usually the turning of a shaft. When not connected to a load resistance, generators will
generate voltage roughly proportional to shaft speed. With precise construction and design,
generators can be built to produce very precise voltages for certain ranges of shaft speeds, thus
making them well-suited as measurement devices for shaft speed in mechanical equipment. A
generator specially designed and constructed for this use is called a tachometer or tachogenerator.
Often, the word "tach" (pronounced "tack") is used rather than the whole word.
By measuring the voltage produced by a tachogenerator, you can easily determine the rotational
speed of whatever its mechanically attached to. One of the more common voltage signal ranges
used with tachogenerators is 0 to 10 volts. Obviously, since a tachogenerator cannot produce
voltage when its not turning, the zero cannot be "live" in this signal standard. Tachogenerators can
be purchased with different "full-scale" (10 volt) speeds for different applications. Although a
voltage divider could theoretically be used with a tachogenerator to extend the measurable speed
range in the 0-10 volt scale, it is not advisable to significantly overspeed a precision instrument
like this, or its life will be shortened.
Tachogenerators can also indicate the direction of rotation by the polarity of the output voltage.
When a permanent-magnet style DC generator's rotational direction is reversed, the polarity of its
output voltage will switch. In measurement and control systems where directional indication is
needed, tachogenerators provide an easy way to determine that.
Tachogenerators are frequently used to measure the speeds of electric motors, engines, and the
equipment they power: conveyor belts, machine tools, mixers, fans, etc.
Thermocouples
An interesting phenomenon applied in the field of instrumentation is the Seebeck effect, which is
the production of a small voltage across the length of a wire due to a difference in temperature
along that wire. This effect is most easily observed and applied with a junction of two dissimilar
metals in contact, each metal producing a different Seebeck voltage along its length, which
translates to a voltage between the two (unjoined) wire ends. Most any pair of dissimilar metals
will produce a measurable voltage when their junction is heated, some combinations of metals
producing more voltage per degree of temperature than others:
The Seebeck effect is fairly linear; that is, the voltage produced by a heated junction of two wires
is directly proportional to the temperature. This means that the temperature of the metal wire
junction can be determined by measuring the voltage produced. Thus, the Seebeck effect provides
for us an electric method of temperature measurement.
When a pair of dissimilar metals are joined together for the purpose of measuring temperature, the
device formed is called a thermocouple. Thermocouples made for instrumentation use metals of
high purity for an accurate temperature/voltage relationship (as linear and as predictable as
possible).
Seebeck voltages are quite small, in the tens of millivolts for most temperature ranges. This makes
them somewhat difficult to measure accurately. Also, the fact that any junction between dissimilar
metals will produce temperature-dependent voltage creates a problem when we try to connect the
thermocouple to a voltmeter, completing a circuit:
The second iron/copper junction formed by the connection between the thermocouple and the
meter on the top wire will produce a temperature-dependent voltage opposed in polarity to the
voltage produced at the measurement junction. This means that the voltage between the voltmeter's
copper leads will be a function of the difference in temperature between the two junctions, and not
the temperature at the measurement junction alone. Even for thermocouple types where copper is
not one of the dissimilar metals, the combination of the two metals joining the copper leads of the
measuring instrument forms a junction equivalent to the measurement junction:
This second junction is called the reference or cold junction, to distinguish it from the junction at
the measuring end, and there is no way to avoid having one in a thermocouple circuit. In some
applications, a differential temperature measurement between two points is required, and this
inherent property of thermocouples can be exploited to make a very simple measurement system.
However, in most applications the intent is to measure temperature at a single point only, and in
these cases the second junction becomes a liability to function.
Compensation for the voltage generated by the reference junction is typically performed by a
special circuit designed to measure temperature there and produce a corresponding voltage to
counter the reference junction's effects. At this point you may wonder, "If we have to resort to
some other form of temperature measurement just to overcome an idiosyncrasy with
thermocouples, then why bother using thermocouples to measure temperature at all? Why not just
use this other form of temperature measurement, whatever it may be, to do the job?" The answer is
this: because the other forms of temperature measurement used for reference junction
compensation are not as robust or versatile as a thermocouple junction, but do the job of measuring
room temperature at the reference junction site quite well. For example, the thermocouple
measurement junction may be inserted into the 1800 degree (F) flue of a foundry holding furnace,
while the reference junction sits a hundred feet away in a metal cabinet at ambient temperature,
having its temperature measured by a device that could never survive the heat or corrosive
atmosphere of the furnace.
The voltage produced by thermocouple junctions is strictly dependent upon temperature. Any
current in a thermocouple circuit is a function of circuit resistance in opposition to this voltage
(I=E/R). In other words, the relationship between temperature and Seebeck voltage is fixed, while
the relationship between temperature and current is variable, depending on the total resistance of
the circuit. With heavy enough thermocouple conductors, currents upwards of hundreds of amps
can be generated from a single pair of thermocouple junctions! (I've actually seen this in a
laboratory experiment, using heavy bars of copper and copper/nickel alloy to form the junctions
and the circuit conductors.)
For measurement purposes, the voltmeter used in a thermocouple circuit is designed to have a very
high resistance so as to avoid any error-inducing voltage drops along the thermocouple wire. The
problem of voltage drop along the conductor length is even more severe here than with the DC
voltage signals discussed earlier, because here we only have a few millivolts of voltage produced
by the junction. We simply cannot afford to have even a single millivolt of drop along the
conductor lengths without incurring serious temperature measurement errors.
Ideally, then, current in a thermocouple circuit is zero. Early thermocouple indicating instruments
made use of null-balance potentiometric voltage measurement circuitry to measure the junction
voltage. The early Leeds & Northrup "Speedomax" line of temperature indicator/recorders were a
good example of this technology. More modern instruments use semiconductor amplifier circuits
to allow the thermocouple's voltage signal to drive an indication device with little or no current
drawn in the circuit.
Thermocouples, however, can be built from heavy-gauge wire for low resistance, and connected in
such a way so as to generate very high currents for purposes other than temperature measurement.
One such purpose is electric power generation. By connecting many thermocouples in series,
alternating hot/cold temperatures with each junction, a device called a thermopile can be
constructed to produce substantial amounts of voltage and current:
With the left and right sets of junctions at the same temperature, the voltage at each junction will
be equal and the opposing polarities would cancel to a final voltage of zero. However, if the left set
of junctions were heated and the right set cooled, the voltage at each left junction would be greater
than each right junction, resulting in a total output voltage equal to the sum of all junction pair
differentials. In a thermopile, this is exactly how things are set up. A source of heat (combustion,
strong radioactive substance, solar heat, etc.) is applied to one set of junctions, while the other set
is bonded to a heat sink of some sort (air- or water-cooled). Interestingly enough, as electrons flow
through an external load circuit connected to the thermopile, heat energy is transferred from the hot
junctions to the cold junctions, demonstrating another thermo-electric phenomenon: the so-called
Peltier Effect (electric current transferring heat energy).
Another application for thermocouples is in the measurement of average temperature between
several locations. The easiest way to do this is to connect several thermocouples in parallel with
each other. The millivolt signal produced by each thermocouple will average out at the parallel
junction point. The voltage differences between the junctions drop along the resistances of the
thermocouple wires:
Unfortunately, though, the accurate averaging of these Seebeck voltage potentials relies on each
thermocouple's wire resistances being equal. If the thermocouples are located at different places
and their wires join in parallel at a single location, equal wire length will be unlikely. The
thermocouple having the greatest wire length from point of measurement to parallel connection
point will tend to have the greatest resistance, and will therefore have the least effect on the
average voltage produced.
To help compensate for this, additional resistance can be added to each of the parallel
thermocouple circuit branches to make their respective resistances more equal. Without custom-
sizing resistors for each branch (to make resistances precisely equal between all the
thermocouples), it is acceptable to simply install resistors with equal values, significantly higher
than the thermocouple wires' resistances so that those wire resistances will have a much smaller
impact on the total branch resistance. These resistors are called swamping resistors, because their
relatively high values overshadow or "swamp" the resistances of the thermocouple wires
themselves:
Because thermocouple junctions produce such low voltages, it is imperative that wire connections
be very clean and tight for accurate and reliable operation. Also, the location of the reference
junction (the place where the dissimilar-metal thermocouple wires join to standard copper) must be
kept close to the measuring instrument, to ensure that the instrument can accurately compensate for
reference junction temperature. Despite these seemingly restrictive requirements, thermocouples
remain one of the most robust and popular methods of industrial temperature measurement in
modern use.
• REVIEW:
• The Seebeck Effect is the production of a voltage between two dissimilar, joined metals that
is proportional to the temperature of that junction.
• In any thermocouple circuit, there are two equivalent junctions formed between dissimilar
metals. The junction placed at the site of intended measurement is called the measurement
junction, while the other (single or equivalent) junction is called the reference junction.
• Two thermocouple junctions can be connected in opposition to each other to generate a
voltage signal proportional to differential temperature between the two junctions. A
collection of junctions so connected for the purpose of generating electricity is called a
thermopile.
• When electrons flow through the junctions of a thermopile, heat energy is transferred from
one set of junctions to the other. This is known as the Peltier Effect.
• Multiple thermocouple junctions can be connected in parallel with each other to generate a
voltage signal representing the average temperature between the junctions. "Swamping"
resistors may be connected in series with each thermocouple to help maintain equality
between the junctions, so the resultant voltage will be more representative of a true average
temperature.
• It is imperative that current in a thermocouple circuit be kept as low as possible for good
measurement accuracy. Also, all related wire connections should be clean and tight. Mere
millivolts of drop at any place in the circuit will cause substantial measurement errors.
pH measurement
A very important measurement in many liquid chemical processes (industrial, pharmaceutical,
manufacturing, food production, etc.) is that of pH: the measurement of hydrogen ion
concentration in a liquid solution. A solution with a low pH value is called an "acid," while one
with a high pH is called a "caustic." The common pH scale extends from 0 (strong acid) to 14
(strong caustic), with 7 in the middle representing pure water (neutral):
pH is defined as follows: the lower-case letter "p" in pH stands for the negative common (base ten)
logarithm, while the upper-case letter "H" stands for the element hydrogen. Thus, pH is a
logarithmic measurement of the number of moles of hydrogen ions (H
+
) per liter of solution.
Incidentally, the "p" prefix is also used with other types of chemical measurements where a
logarithmic scale is desired, pCO2 (Carbon Dioxide) and pO2 (Oxygen) being two such examples.
The logarithmic pH scale works like this: a solution with 10
-12
moles of H
+
ions per liter has a pH
of 12; a solution with 10
-3
moles of H
+
ions per liter has a pH of 3. While very uncommon, there is
such a thing as an acid with a pH measurement below 0 and a caustic with a pH above 14. Such
solutions, understandably, are quite concentrated and extremely reactive.
While pH can be measured by color changes in certain chemical powders (the "litmus strip" being
a familiar example from high school chemistry classes), continuous process monitoring and control
of pH requires a more sophisticated approach. The most common approach is the use of a
specially-prepared electrode designed to allow hydrogen ions in the solution to migrate through a
selective barrier, producing a measurable potential (voltage) difference proportional to the
solution's pH:
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