BE/PLS 479/579
Applied Instrumentation for Controlled Environment Agriculture
Module 3. Temperature Measurement
Dr. Murat Kacira
©KACIRA2018
©KACIRA2023
Department of Biosystems Engineering
Controlled Environment Agriculture Center
CEA Building, Room 101, 1951 E. Roger Road, Ph: 520 626-4254
mkacira@arizona.edu
Learning Objectives
• Operating principles and applications for various temperature sensors
• Sensors and their applicability
• Sources of errors
• Measurements and data logging with temperature sensors
Main principles of temperature sensors
Temperature-dependent properties
• Expansion of liquid
• Different degrees of expansion of two metals
• Electric potential difference exists due to temperature difference at junctions of
two metals
• Electrical resistance of metal increases with temperature
• Semiconductor resistance decreases as temperature increases
Expansion of liquid (Liquid-in-glass thermometer)
Colored alcohol used
• Increase in temperature and increase in volume
Common liquids used:
• Toluene, Ethyl alcohol, Pentane
• Has a limited application in CEA study due to its relatively
large size and no electric readings to record.
• Certified (standard) liquid-in-glass thermometer: more accurate and most reliable for
calibrating other thermometers/sensors
• Certified liquid-in-glass sensors are as accurate as 0.1 oC, while non certified, massproduced sensor are often 2-3 oC off.
Note: mercury thermometers are not recommended due to the risks of personal and
environmental exposure when broken.
Bimetallic Strip
• Operating principle: Two different metals with different
Example: Thermostat
coefficients of thermal expansion (CTE)
• Bonded together
• Bend proportional to temperature
• Calibrated to obtain true temperature
www.hk-phy.org
Electrical Methods
•
•
•
•
Good for continuous measurement and data logging
Can measure and store values without anyone present
Preserve Tmax, Tmin, Tave., hourly, etc. values
Come in many shapes and sizes
• Important to know where and when each is appropriate
Basic Types
Thermocouples
Thermistors
Resistance Temperature Detectors (RTD)
Basic Electronics Review
Units
•
•
•
•
Voltage (V) - Volts = potential difference that drives electron flow
Current (I) - Amps = flow of electrons through a circuit
Resistance (R) - Ohms = resistance to current flow
Power (P) - Watts = energy dissipated by current flow through a circuit
Ohm’s Law:
P = VI
Georg Simon Ohm
Thermocouple
• Thermocouples consist of a pair of
different metals joined to form two
junctions in a circuit.
• The accuracy of thermocouples is
generally 0.1oC if reasonable precautions
are taken.
• Type T thermocouple consisting of copper
(+)/constantan copper-Nickel (-) wires
which is the most common thermocouple
type used in CEA application. Type K
thermocouple consists chromel-alumel
wires.
Seebeck effect
T1
T2
A voltage is generated according to the difference in
temperature between two junctions of dissimilar
metals used, called seebeck effect.
Knowing this relationship and measuring voltage,
one can tell the temperature difference created
between the two junctions. If one junction is held at
a known (reference) temperature, then the
temperature of the unknown (measurement) junction
can be calculated.
Thomas Seebeck
T1 > T2
T1
T2
Voltmeter
Voltage proportional to ΔT
V =  (Th − Tc)
The Electromotive Force Coefficient (EMF coefficient)
Voltage response to temperature can be logged
MUST KNOW TEMPERATURE AT ONE JUNCTION (Tref)
Th ?
Tref
Tref
Cu
Ref Temp (Tc) = 22.5 C
Measured Th voltage = 0.98 mV
Cu-Constantan TC calibration = 40 μV/C = 
𝑉 = 𝛼(𝑇ℎ − 𝑇𝑐)
𝑇ℎ = (𝑉/𝛼) + 𝑇𝑐
𝑇ℎ = (980/40) + 22.5
𝑇ℎ = 47𝐶
Cu
Thermocouple Types and Color Codes
Thermocouple types
Thermistors
“Thermally Sensitive Resistor”
 Change resistance with temperature
 R vs. T relationship follows form
ln RT = 

T
where α and β are calibration constants
for individual thermistors
Thermistors differ from resistance temperature
detectors (RTD) due to materials being used for
manufacturing
Thermistor is generally a ceramic or polymer,
while RTDs use pure metals.
The temperature response is also different; RTDs
are useful over larger temperature ranges, while
thermistors typically achieve a higher precision
within a limited temperature range.
Resistance Temperature Detector
Thermistor response
Temperature sensor specifications
Many exists but essential specifications:
•
•
•
•
•
Measuring range
Stability
Accuracy
Sensitivity
Response time
Thermocouples
Advantages
• Wide variety of measuring ranges,
• Many physical sizes and configurations;
• Fast response times;
• Tiny measuring point;
• Moderate price; and
• Very simple configuration (You can even make your own!)
Limitations
• Medium accuracy and sensitivity;
• Linearity is only fair;
• Specific types have to have matching cable (e.g., type K thermocouple
has to have type K cable.); and
• Signal strength is very low
Thermistors
Advantages
• Accuracy and response time comparable to thermocouples;
• Highest sensitivity;
• Least expensive; and
• Robust signal
Limitations
• Narrowest measuring range, by far;
• Lowest stability and linearity;
RTDs
Advantages
• More stable;
• More accurate;
• Greater repeatability;
• Better sensitivity and linearity; and
• More robust signal
Limitations
• Narrower measuring range,
• More expensive;
• Require an external power source;
• Slower response time; and
• At some temperatures, the reference voltage can
actually heat the sensor and throw it off.
Summary for thermometers
Type
Principle
Advantage
Disadvantage
Liquid-in-glass
expansion or contraction
of liquid (mercury or
ethanol)
Ease in handling
High accuracy with
certification
Fragile
Reading errors
No recordable output
Large size of sensor
Thermocouple
Thermoelectric emf
(Seebeck effect)
Small size (sensing point)
Recordable
Long distance between
measurement and
recording
Complex
Difficulty in handling
Sensor accuracy depends
on the whole
measurement/recording
setup.
Thermistor
Electric resistance of semi Ease in handling
conductor
Recordable
Large size of sensor
Infrared thermometer
Longwave radiation
Low accuracy due to view
angle and emissivity
Remote sensing (without
disturbance)
Recordable
Calibration
• All thermometers need calibration. A common calibration method
for thermocouples is by comparing values measured by standard
thermometer (certified liquid-in-glass thermometer).
• Thermistors and Infrared (IR) thermometers are recommended to
have a manufacturer’s calibration.
• IR thermometers also need an on-site calibration to adjust the
emissivity.
Important factors to consider for air temperature
measurement under radiation
• Air current speed
• Radiation flux and type of shield
• Sensor size
Energy balance of the sensor
• Longwave radiation exchange
determined by:
•
Temperatures of sensor itself and of
the surrounding objects, and the
emissivity of the sensor;
• Sensible heat exchange determined by:
•
Tair
Tsens
Air-sensor temperature difference.
Heat exchange rate determined by
air current speed and size of the
sensor.
• Shortwave radiation exchange
determined by:
•
Incident shortwave radiation and
absorptance of the sensor surface.
• When surrounding temperatures are higher than
sensor temperature, the positive net longwave
radiation (effective radiation) contributes to
increasing sensor temperature.
• When sensor temperature is higher than air
temperature, heat dissipates as sensible heat from
the sensor. This rate will be enhanced at higher air
current speed and at smaller sensor size.
• Under sensor exposure to sun radiation conditions,
Effects of shortwave radiation and wind providing a radiation shield is important to
speed on temperature reading errors (Erell et minimize the error due to the potential increase of
al., 2003). Sensor diameter was 1 mm.
sensor temperature by absorbing radiation, unless
the sensor size is very small.
Effects of sensor size (diameter) and wind speed on error (Erell et al., 2003). The sensors are
exposed under 300 W m-2 shortwave radiation.
NOTE: Super fine thermocouples are with 0.1 mm diameter.
Potential errors generated
Air temperatures measured using aspirated and non-aspirated thermocouples.
Shielded
Aspirated+Shielded
Other sensors
Commercially available temperature sensors with internal micro data logger
In recent years, many sensors with data recording function have been available at a
reasonable cost. However, the same issues apply for accurate air temperature
measurement and special cautions will be needed for measurements under radiation.
When using such sensors in greenhouse, comparison should be made with aspirated
certified thermometer readings or with a very thin thermocouple readings calibrated with
certified thermometer.
What is the best shield?
• Color
•
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White
Metallic silver
• Material
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Wood
Plastics
Aluminum
• Ventilation
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•
Aspiration
Natural convection
When you measure leaf temperature using thermocouples, you need
to make sure:
• there is good contact of thermocouple measurement joint to the leaf
• there is negligible amount of conductive heat contributed to the
thermocouples (especially when bare wires are used)
• there is minimal effect from thermocouple placement on the leaf
temperature.
A 0.1 mm type T thermocouple is used with a
thicker thermocouple (type T) as extension.
Thermocouple joint can be inserted under
the thin layer of epidermis if the
measurement is for a short period of time.
Use of a very small amount of super glue is
reportedly effective for longer period of time
http://www.avogadro-lab-supply.com/
Assmann Psychrometer
 Aspirated type portable hygrometer
 In the Assmann psychrometer, the wet-bulb
thermometer is installed in a duct where the air
is flowing at reasonable velocity (2.5 m/s wind
into 2 lower pipes)
Sling Psychrometer
www.tech-faq.com
Richard Assmann
Infrared Transducer
• By measuring radiation emitted from the target, infrared thermometer can compute the
surface temperature of the target body.
• Measure intensity of IR radiation
• Intensity proportional to T4
• Useful for measuring temp of surfaces (remote)
 All surfaces emit radiation proportional to their absolute temperature raised to
the fourth power:
R = ε σ [T + 273]4
R: radiation emitted from the surface (W m-2);
ε: emissivity constant (< 1.0);
σ: Stefan-Boltzmann constant (5.673 x 10-8 W m-2 K-4);
T: surface temperature in oC.
Radiation at ε=1.0 is called “perfect black body radiation”.
Thermal Cameras and Thermal Imaging
Lettuce in NFT system
Before water stress
Tomato crop canopy temperature
at 2:00pm on a sunny day.
Top canopy temperature are 2-3 oC
higher than bottom canopy
temperature.
(Story and Kacira, 2015)
After stressed induced
Stress recovered
Crop water stress index =
(Kacira, 2000)
Empirical CWSI Approach
Tc – Ta (oC)
C
(Tc – Ta)UpperLimit
B
(Tc – Ta)LowerLimit A
CWSI =
(Tc −Ta ) − (Tc −Ta )LL
(Tc −Ta )UL − (Tc −Ta )LL
=
BA
CA
Vapor pressure deficit (VPD) (kPa)
(Kacira, 2000)
CWSI and ET rates
1500
0.8
1350
0.6
1200
0.4
1050
0.2
900
0.0
750
-0.2
600
-0.4
450
-0.6
300
-0.8
150
-1.0
0
Measured ET (g m-2 hr-1)
Crop water stress index
1.0
Days
CWSI(S)
CWSI(C-ave)
Etm(S)
ET(C-ave)
(Kacira, 2000)
(Chaerle et al., 2007)
Growing Media Temperature
Phoenix Mars Lander
Decagon probe
http://www.delta-t.co.uk/
Decagon devices
http://www.stevenswater.com/