Transcript Temp-12 - CREATE.stfx.ca

Temperature Measurements
• thermocouples, thermistors and
resistance thermometers
• exposure and shielding of
thermometers
• soil temperature measurements
• response times and sampling rates
Thermocouple measures temperature difference
(T1 – T2) between two junctions
T1
Copper
Constantan
T2
Voltage output
Copper
- Easy to construct. Just twist together Copper and
Constantan wires, and solder.
- Beautifully suited to measuring temperature
differences directly.
- Requires knowledge of temperature at T2
(“reference” temp) to get actual temperature at T1.
Can construct “thermopile” of several thermocouples
connected in series.
- Increases signal strength. Calibration factor increases
according to the number of junction pairs in the “pile”.
- Allows for spatial averaging, if desired.
- For example, measuring soil temperature gradients…
Voltage to Data Logger
Soil surface
Upper level
Lower level
= Cu,
= Con
Thermocouple measures temperature difference
(T1 – T2) between two junctions
T1
Copper
Constantan
T2
Voltage output
Copper
Copper
Data logger
Constantan
1. Where is the second junction, when using logger?
2. What temperature difference is being measured?
The output
signal from a
thermocouple
is not quite
linear.
Voltage change = (a + b T) (Temperature change)
Microvolts per degree= 38.58 + 0.0428T for copper/constantan
Thermocouples
1. Estimate the maximum signal (in microvolts) you should
expect from a thermocouple that is measuring an air
temperature of 40 C.
2. Suppose you ignore the non-linearity of a thermocouple
and always use the calibration factor for 0 C. What would
the temperature error be at 40 C?
Thermocouples
1. Estimate the maximum signal (in microvolts) you should
expect from a thermocouple that is measuring an air
temperature of 40 C.
Correct calibration factor is:
38.58 + 0.0428 (40) = 40.29 microvolts/degree C
So signal is (40.29 microvolts/degree C) x 40 C
= 1,611 microvolts or 1.611 millivolts
2. Suppose you ignore the non-linearity of a thermocouple
and always use the calibration factor for 0 C. What would
the temperature error be at 40 C?
Signal to be translated to temperature is 1,611 microvolts.
Using calibration factor for 0C… 38.58 microvolts/C:
1,611 microvolts /38.58 microvolts/C = 41.76 C.
Answer is 1.76 C too high.
Electronic Temperature (and RH) probe which uses
a thermistor, a semi-conductor material whose
resistance changes with temperature.
Advantages are… stronger signal change with temperature
than thermocouple, and no reference needed.
Thermistor resistance decreases very non-linearly with
increasing temperature.
Suppose we take
two resistance
readings and
average them.
250 Kohms
50 Kohms
Avg = 150 Kohms
Average resistance
gives correct
average
temperature?
No!! Must convert to temperature before averaging.
Thermistor-based probes can contain electronics to give
a linear voltage output with temperature.
Platinum resistance-temperature detector
- Resistance of wire changes with temperature
- Platinum wire is typically used, wound inside a protective
casing.
- stable and almost linear resistance change with temp
- non-linearity can be accounted for in logger program,
yielding very accurate temperature measurements
that may be used to calibrate other temperature
sensors.
Exposure of thermometers (Str – P. 41)
Unshielded sensor will warm up during the day
until heat loss to the air by convection matches
the gain from radiation.
(Tsensor – Tair) is the radiation error.
Radiation error is reduced by:
- Small sensor size
- More air flow
Radiation gain
- Blocking the radiation
Convection
loss
Temperature probe will heat above air temperature if
exposed to solar radiation.
So….. what are features of a good thermometer shield?
• shades the sensor
• allows wind (or artificial ventilation)
• doesn’t warm incoming air (high solar reflectivity)
• avoids long wave gain from inside surface of shield to sensor
(low emission efficiency of shield, poor conductor so inner
temperature of shield not higher than air temperature)
• avoids heat conduction down signal wires
Cut-away view
Here’s a shield that is often
used at weather stations
(the Gill shield).
- a stack of upside-down
plastic saucers .
Good features?
Possible improvements?
Temp/RH probe, or thermocouple, can be used in
“stacked saucers” shield.
The “Stevenson Screen” thermometer
shield
World-wide way of housing manuallyread thermometers, and automated T
and RH sensors
Max and min
thermometers
T & RH probe
Good and poor features of the
“Stevenson Screen” thermometer shield?
-louvered for air flow
- white, & double roof, for solar
protection
- wood for poor heat conduction
- world-wide “standard”
- needs regular repainting
- bulky
- too warm on calm, sunny days if
no supplementary ventilation
Soil temperatures on an ideal sunny day.
- temperature range decreases with depth,
and max/min temperatures lag with depth.
•Sensors must
be waterproof
•Sensor in a
metal tube can
give some
spatial
averaging
•Place
horizontally to
spatially
average at one
depth
•Place at an
angle to
average over a
layer
Signals, sampling and sensors
Imagine we take
a sample every
second with our
data logger, for
10 seconds.
For which of the 5
signals will our
sampling yield a
good 10-second
average?
Sampling rate must be at least twice as fast as the period
of the signal you wish to average.
How quickly does a sensor respond?
A step change
is applied to
the sensor at
time zero.
63% level
Tc = 2 sec
Tc = 6 sec
Time constant
is time required
for sensor to
reach 63% of
the step
change.
Guidelines for good sampling over time.
1. Sampling rate must be at least 2x as fast as the
period of the signal you wish to average.
2. Time constant of sensor should be 4x faster
than period of the signal you wish to detect.
Sensor’s response speed controls the signal
fluctuations it “sees”.
Signal period 2x slower
Sampling rate
Sensor 2x faster
1. How could you modify the time constant of a
temperature sensor?
2. A very small thermocouple could be used without
radiation shielding. Any disadvantages of a very small
sensor?
3. You decide to ask your data system to sample once
each minute. What time constant should your sensor
have, and what is the period of the fastest signal you
can resolve?
4. Suppose you need to measure temperature fluctuations
as fast as 10 cycles per second. What sensor time
constant is required, and how often would you sample?
Infra-red Thermometer (IRT)
IRT
Looks at I-R
radiation
from object.
Sensor assumes object obeys the Stefan-Boltzmann law
which links radiation emitted to object temperature:
Radiation in W/m2 = s TIRT 4
where s = 5.67 X 10-8 and T is in 0K
(0K = 0C + 273.2)
Senses radiation  solves S-B equation  TIRT signal
Infra-red Thermometer (IRT)
But real objects are not “perfect” emitters so the
S-B equation needs a reduction factor called
the emissivity (e) , which ranges from 0  1
Radiation in W/m2 = e s Tobject 4
If an object is not a perfect emitter (that is, e < 1),
then it is also not a perfect absorber, so it will
reflect some incoming radiation from the
surroundings. The fraction reflected is 1-e .
Therefore a real object will send out an emitted
I-R stream and a reflected I-R stream.
Emitted
I-R
Reflected
I-R
I-R from
surroundings
Infra-red Thermometer (IRT)
• IRT sees IR emissions from two sources when
pointed at an object…
Emission = e s Tobject 4
Reflection = (1-e) (I-R from surroundings)
Total IR seen = e s Tobject 4 + (1-e) (I-R surroundings)
But the IRT changes IR radiation seen into a
temperature using the “perfect” S-B law, so…
s TIRT 4 = e s Tobject 4 + (1- e) (I-R surroundings)
This means TIRT does not equal Tobject unless e = 1.
Errors are usually small, since e > 0.95 for most
objects. Shiny metals are a notable exception. Their
typical e < 0.5, so T-measurement with an IRT can be
seriously degraded by reflected I-R from surroundings.
Practice with the IRT equation.
1. Suppose an IRT pointed at the sand on a beach shows
the surface temperature is 41.2 C. The sand has an
emissivity of 0.97. The sky is emitting 412 W/m2.
What is the error between the true sand temperature
and the value from the IRT? (~ 0.6 C error)
2. A piece of aluminum foil (e = 0.15) was careless left on
the grass near the beach. The foil temperature is
34.2 C. What is the error between the foil temperature
and the value from the IRT? (~ 13 C error)
Maximum and minimum thermometers – design tricks
Using thermocouples for spatial sampling.
2. Link in parallel
- same signal size as 1 couple
(resistors 20x longest t/c).