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T E M P E R AT U R E M E A S U R E M E N T
Considerations for Choosing
Temperature
Measurement
Devices
T
emperature is the physical variable most often
measured in industrial processes, says Bill Tanner,
president, Sensoray Company, Inc. There are a variety
of reasons we need to know the temperature of an
object or a process – to prevent product damage, ensure
sterilization, ensure mixture blending, control chemical
reactions (including food cooking), or to ensure drying,
curing, and out-gassing, to name just a few. Temperature
measurement can also be a regulatory requirement: for
example, the US Food and Drug Administration requires
temperature monitoring of food and drug products.
Selecting the sensor and measurement device to match
a specific process is extremely important and knowing the
various options is the first step to optimizing temperature
measurement. Temperature measuring sensors play an
extremely important role in heat-treatment of metals: for
example structural steel used in buildings and metals used in
aircraft; or for some automobile parts, that need to be tough
enough to withstand wear and tear, certain surfaces must be
heat-treated in accordance with a specific ASTM schedule.
In these cases, a manufacturer must be able to guarantee
that the metal was heat-treated in a particular way to
ensure it has the desired properties: it is simply not possible
to physically look at something (or even conduct X-ray
fluorescence) to tell if it has been properly treated.
Sensor
Thermocouple
RTD (resistive temperature
detector)
Thermistor
Diode reverse current
Diode forward voltage
drop
Optical infrared
Physical property sensitive to temperature
Generates voltage
Increases resistance
Decreases resistance
Current increases with temperature
Forward voltage decreases with temperature
IR emissions at multiple wavelengths
measured
Table 1 Temperature sensor properties
Temperature 1700ºC
Sensor types
Sensors used in temperature measurement have an
electrical property that is sensitive to temperature changes
– see Table 1.
The temperature sensor chosen for a particular process
depends on its cost, range of operation, sensitivity, response
time, repeatability, and ability to survive its environment.
There is usually some measurement range overlap, and more
than one sensor type may be suitable for an application: for
example, biological systems can often be monitored with
non-contact infrared (IR) detectors, thermistors, or silicon
diodes. Occasionally, the only practical way to monitor an
internal temperature is to embed a sensor into a product
during the manufacturing procevss: for example an electric
motor with an embedded resistive temperature detector
(RTD), or a thermistor on one of the copper windings.
A thermistor is a type of resistor, generally made
of ceramic or polymer, whose resistance varies with
temperature. Unfortunately, thermistors require more
complicated software to account for their very non-linear
temperature response. RTDs use such metals as platinum or
nickel and are useful over larger temperature ranges, while
thermistors typically achieve a higher precision within a
limited temperature range, typically −90ºC to130°C.
Some industrial processes operate at such high
temperatures that only a few sensor types can survive and
give repeatable results after temperature cycling. Steel
making requires measuring up to 1700ºC, requiring the use
of either thermocouples, which can be immersed in molten
metals for internal measurements, or IR thermometry. IR
optical sensors are sometimes used for high temperature
measurements but they can only measure hot surface
temperatures. Non-contact IR thermometry may be the
only choice if the product is moving or if penetrations are
not allowed in the product: for example, ceramic firing in
a kiln. Non-contact methods can only measure surface
temperatures and not internal temperatures.
Table 2 provides an overview of temperature sensor
applications, including the process, approximate sensor
temperature ranges, and appropriate sensor types.
Temperature measurement application
examples
Manufacturing plastic products often requires thermocouples
due to the high operating temperatures, where the sensors
are very close to the electric heaters in the machines that
extrude and form the finished products.
Excessive drying temperatures can cause damage
and waste energy, which makes it desirable to control how
products, objects, and even people, are dried. For example,
streams of hot air drying human hair and hands must be
controlled and limited to prevent injury. In the case of food
preparation, insufficient drying can cause product damage or
allow harmful bacteria to grow. Thermistors often monitor
400-150ºC
200-90ºC
150-32ºC
100-35ºC
120- (-30)ºC
(-113)- (-272)ºC
-273.15ºC
Petro chemical
Plastic melting
Drying
Food
processing
Life sciences
HVAC
Refrigeration
Research
Thermocouple
RTD
Diode Voltage
Process
Metal melting
Sensor type
Thermocouple Thermocouple
Non-contact IR RTD
Thermocouple RTD
Thermistor
Thermistor
RTD
Thermistor Noncontact IR Non-contact IR
Diode current
Table 2 Temperature sensor applications
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T E M P E R AT U R E M E A S U R E M E N T
built into a cable that has a heat-resistant
outer sheath. At one end of the cable,
the two conductors are mechanically and
electrically connected together by crimping,
or welding. This end of the thermocouple
(known as the hot junction) is placed at
the location that is to be monitored: the
other end of the thermocouple (the cold
junction, also known as the reference
junction) is connected to a thermocouple
measurement system. The term ‘hot’
junction is somewhat of a misnomer,
since this junction will be subjected to a
temperature below that of the reference
junction if relatively cold temperatures are
being measured – see Figure 2.
Figure 1 – Thermistor application to measure humidity
a product’s temperature directly or the atmosphere around
the product.
The relative humidity of the atmosphere is an
important variable related to drying. A thermistor in the
humidity sensor of a chilled mirror hygrometer (CMH) helps
measure the dew point, which is related to the moisture
content of the nearby atmosphere. Figure 1 shows a
thermistor monitoring a temperature-controlled chilled
mirror to regulate the mirror temperature.
Monitoring outside and inside temperatures
in heating, ventilating and air conditioning (HVAC)
equipment often calls for the use of thermistors, RTDs, or
silicone diodes, all of which are low cost and sensitive to
temperature changes. Among these, thermistors have the
highest sensitivity and are often found in home thermostats
because of their high sensitivity and low cost.
Refrigeration systems often use thermocouples due to
their ruggedness and low cost. The T-type thermocouple is
commonly used because it resists corrosion from moisture
condensation on refrigerated surfaces better than other
thermocouple types, including types K and J. It also has the
lowest temperature calibration data of any thermocouple,
with data down to -185ºC.
The lowest end of the temperature scale is used
by researchers to study material properties close to
absolute zero, -273.15ºC. Sensors in these devices use the
dependence of the forward voltage drop in a p-n junction
diode biased at a constant current, typically 10µA. Some
germanium and silicon diodes can get to within fractions of
zero degrees Kelvin.
Measuring with thermocouples
A thermocouple consists of two electrical conductors made
from different metal alloys. Typically, the two conductors are
Figure 2
20 | Sept/Oct 2015 | ME
Thermocouples generate an open-circuit voltage
proportional to the temperature difference between the
hot and reference junctions. It is important to have an
accurate cold junction measurement, even when several
thermocouples are connected to the same board. If the cold
junction sensor is too far away from the thermocouple, there
may be a significant measurement error – more than 1ºC.
To avoid such errors, some measurement systems (including
Sensoray’s models 2608 and 2418) use eight cold junction
sensors to ensure each thermocouple is less than ½-inch
from a cold junction sensor – see Figure 3.
Figure 3 Multiple cold junctions reduce temperature errors
It is not unusual to attach the hot end of a
thermocouple sensor next to an electrical heater or to the
machine frame used by high voltage motors. These two
conditions often lead to high voltages finding their way
onto thermocouple sensors. Thermal cycling of electrical
heaters often weakens their electrical insulation, resulting
in a high common mode voltage being applied to the
millivolt-level thermocouple signal. This condition obscures
the temperature signal and in the worst case destroys the
measurement electronics. One solution is to isolate the
electronics and let them float up to the high common mode
voltage of the thermocouple.
Analog/digital (A/D) products that measure resistance
must have a wide measurement range to work with a
wide range of resistance values that sensors produce. All
supported resistive sensors, including RTDs and thermistors,
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T E M P E R AT U R E M E A S U R E M E N T/ M O T O R S
 Continued from page 20
are acquired by applying a constant voltage across the
sensor or by forcing a constant current through the target
sensor – see Figure 4.
Figure 4 Measuring with resistive sensors (Note The A/D converter
uses 4-wire measurement on all 16 channels.)
Precision temperature measurements for
industry require special considerations
Sensors must be chosen to survive their environment and
produce repeatable results after repeated exposure to
temperature extremes. Thermocouples and RTDs can survive
high temperatures but sacrifice resolution; they need more
complex circuitry, for example, cold junction measurement
(for thermocouples) and a four-wire measurement (for RTDs).
The millivolt-level signal from thermocouples limits high
resolution measurements in noisy industrial environments.
Malfunctioning machinery can add high common mode
voltages that obscure the tiny sensor signals and they can
destroy measurement circuitry. Isolated measurement circuitry
(such as Sensoray’s model 2418) minimizes the effects of high
common mode voltages while resisting high voltage damage.
Some applications require resolutions of 0.001ºC.
Thermistors provide this resolution at the expense of a limited
temperature range; however, unlike thermocouples and
RTDs, they cannot survive high temperature environments.
Thermistors have the fastest time response to temperature
changes: they have the added advantage of being inexpensive
and not requiring an additional cold junction measurement.
The reversed biased silicon diode is the least expensive
sensor. It has a well-defined voltage versus temperature
curve, but does not have the thermistors’ high sensitivity –
nor can it survive the high temperature environments where
thermocouples excel. It is sometimes built into integrated
circuits designed for temperature measurement, where the
added circuitry makes the diode’s non-linearity appear linear.
t: +1 503 684 8073
sensoray.com
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How Efficient are
your Motors Really?
A
round the world, legislation relating to electric motor
efficiency is tightening in an attempt to reduce carbon
emissions and damage to the environment. However, far less
attention is paid to some of the practicalities. Here, Lucie
Hodkova of Exico Electric Motors looks at how best to determine the
efficiency of an installed motor.
Motors are the greatest consumers of electricity in the world,
with about 70% of all power generated being used by them. Since the
Rio Earth Summit a generation ago, there has been a continuous effort
to improve motor efficiency, with much effort being put into drafting
efficiency standards and new testing methods for newly manufactured
motors, which get gradually tighter over the years.
However, this approach misses the point that there is a huge
number of motors already
in use around the world and
their efficiency does not
come under anything like so
much scrutiny. So, a review
of the motors in your place
of work may well lead to the
identification of significant
improvements to your
energy efficiency, carbon
footprint and – perhaps
most importantly – saving
on your utilities bill.
If you use machine
tools, automated
manufacturing systems,
conveyors, or other ‘technical’ equipment, each of these is likely to
use a number of motors. If you have air-conditioned offices, there will
be motors driving pumps and fans. Medical equipment, displays, test
stands, lifts, escalators, extractor fans in kitchens and laboratories are
all motor-driven and, typically, their efficiency may never have been
checked!
All motors
should have their
efficiency checked
and it may be
best to start with
those motors that
are used a lot;
if a motor runs
constantly or
for long periods,
any inefficiency
will have a
compounding effect over time. Other motors that would be worth
investigating early include those that are more than 7–10 years old,
that have been repaired or rewound, that were originally installed for
one purpose but have since been switched to a different duty, or those
that appear to be oversized for their role.
In essence, a motor’s efficiency is the ratio between the
electrical power input to the motor and the mechanical power it
outputs. Both types of power are measured in kilowatts or horsepower,
Continued on page 25 
ME | Sept/Oct 2015 | 23
ELECTRIC MOTORS
 Continued from page 23
so are directly comparable. The Energy-related
Products (ErP) Directive however changed the
testing method of how the efficiency has been
calculated. This means that the IE efficiencies for a
particular motor may be declared lower than it was
under EFF efficiency rating scheme.
All motors have a rated load, but most are
designed so that they run over a range of loads.
Typically, the load will be 50% to 110% rated load
and generally, motors are at their most efficient at
70% to 80% load. The drop-off in efficiency can
be very significant outside of this small optimum
window, particularly in smaller motors.
Fortunately, there are several quick and easy
ways to check that a motor is optimally loaded.
The simplest is probably to feel its temperature (be
careful, it could be very hot). If it is unusually warm
it is probably overloaded.
The next step is to check the nameplate values on the
motor and compare these with the calculated load. This will
require a little technical knowledge and some interpretation.
It is important to note that the stated values for the motor
are only true at the rated motor voltage, so the supply voltage
must be checked too. Further, the total load on the motor may
be the sum of several separate loads including the driven load
and mechanical losses in the transmission system.
At this point power factor may need to be considered.
Power factor is the ratio between the theoretical or calculated
power being supplied and the actual power delivered to the
motor. Again using a multimeter, the power in the mains
near the motor can be measured. If a power factor correction
unit is present, the theoretical and actual values will be very
similar giving a small power factor ratio. Larger power factor
ratios must be incorporated into relevant calculations.
The next step is to calculate the power output. A simple
practical method to do this is to compare the rated and actual
output speeds of the motor. The rated speed is calculated
from the theoretical speed of a motor and is related to the
frequency of the supply and the number of poles in the motor.
The actual speed can be measured using a tachometer, strobe
light, calculated from production rates etc.
There are other methods for calculating motor
efficiency; these may include special devices, computer
models or complex analytical processes. In practical terms,
the best answer may be to bring in an expert, who will
probably be able to check ten motors in the time it would
take you to do one!
There is no absolute answer to the question ‘What
is acceptable efficiency for an installed motor?” In each
case you have to compare the current efficiency to the
potential improved efficiency and work out the energy/cost
implications over the duty cycle and remaining life of the
installed motor.
Once you have determined the efficiency of each
motor you have to decide whether to accept it, rebuild
it, or replace it. As a general rule, smaller motors are
better replaced than repaired; it is a cheaper option and
automatically ensures that the latest efficiency standards are
being met. With larger motors, the repair/replace decision is
harder and unique to each situation. However, we can define
a rule-of-thumb strategy along the lines of:
 Motors that are significantly oversized or inefficient should
be urgently replaced with properly sized ones that meet
current efficiency standards.
 Motors that are moderately oversized or somewhat
inefficient should be replaced when appropri ate.
 Motors that are properly sized but older should be
replaced with modern high efficiency units when
appropriate.
 Sometimes, fitting a variable-speed drive to an existing
motor will pay significant efficiency dividends.
 Check the duty cycle of each motor: if there is a lot of
idling time, consider PLC control to switch it off instead.
 Some motors are deliberately oversized so that they
have a bit of extra power in reserve to overcome stiction,
blockages, icing, etc. A properly sized motor and variablespeed drive will provide the same effect far more
efficiently.
 And finally, developing a motor management strategy
may be best done with the help of an expert.
[email protected] exico.co.uk
Wish to Comment?
Find these articles at maintenanceonline.co.uk and at the
end of the editorial there is the option to add a comment
– eg Was it relevant?, Can you add to it?
ME | Sept/Oct 2015 | 25