Transcript Thermometry - Texas A&M University
Consider following problem
• Want to measure temperature range of: -15 o C < T 1 < 35 o C.
• Reference junction, T 2 , = 0 o C.
• Output must be in range of -5Volts < V 3 +5Volts. V 3 cannot be greater than +5 V.
< • Using a copper - constantan thermocouple of: g a = -0.09 m = 0.041 m / o C V, b = 38.7 m V/ o C,
V
V
1
V
2
V
V
a b
T
1
T
2 g
T
1
T
2 2 0.09
m
V
571.365
m
V
38.7
m
V o C
15
o C
0.041
mV o C
2 15
o C
2
V
V
V
a b
T
1 0.09
m
V
T
2 38.7
g
T
1 m
V
T
2 2
o C
35
o C
0.041
m
V
1404.635
m
V o C
2 35
o C
2 • To get V 3 = 5 Volts when T=35 must amplify the signal.
o C, then
G
V
3
V
5
V
1404.6
m
V
5
V
.0014046
V
3559.7
• Then, voltage output for -15 o C would be:
V
3
3559.7
571.4
mV
V
3
3559.7
571.4
x
10
6
V
V
3
2.03
V
• Such large gain requirements are potentially expensive and susceptible to introducing noise into the output signal.
• Advantages of Thermocouples – Accurate - Can be made very small – Highly reliable – Rugged - Wide temperature range - Low cost – Fast time response - Simple & Easily installed – Compatible with most modern measuring systems.
• Disadvantages of thermocouples – Small output signal. Needs amplification.
– Poor sensitivity – Need a known reference – Metal corrosion – Metal fatigue – Interference from nearby strong EMF sources: (radio stations, cell telephone transmitters)
• Applications – Micrometeorology • Fast response & small size • Turbulence – Agriculture • Environment near plants • Temperature of leaves, etc.
– Soil moisture • Rate at which heat is dissipated into soil is a function of the soil moisture.
• Probe composed of thermocouple and small heater is inserted into soil. Rate of heat dissipation measured.
• Sensor – Thermistor – Platinum PRT – Thermocouple (Type T)
Comparison
Range smallest largest moderate Accuracy best moderate least
Radiation Thermometers
• Infrared Thermometer • • • Brightness Thermometer
E
e es
T
4 = emissivity 150000 100000 s = Stefan 50000 Boltzman 0 1 constant 2 3 4 Wav elength ( m m) 5 6 1300K 1200K 1100K 1000K 800K 7
Infrared Thermometer
• Target radiation is focused on a sensor plate or plates which is (are) heated by the radiation. • Typically, incoming radiation is filtered to a selected wavelength band. (as small as 1 m m). (For most meteorological temperature ranges, we are interested in the 8 - 14 m m band).
• May be classified as: – Spot measuring IR thermometers – Line measuring IR thermometers – Area measuring IR thermometers
• Spot Measuring: – Sensor is usually a thermocouple thermopile or a thermister • Near touching: – Thermocouple sensor – Used for measuring bearing temperatures, computer chip production, etc.
• Line Measuring – These sensors measure a linear region over a defined angular range. The device produces a linear trace along the line seen by the sensor.
• Area measuring IR thermometers – Two-dimensional temperature map.
Brightness Thermometer
• Temperatures are determined by adjusting a rheostat on the instrument that changes the lamp’s intensity. Using the inherent ability of the human eye of the operator and the unknown intensity of light radiated from a hot target, a color blend is made between the apex of the pyrometer's calibrated lamp and the target. The current value to the lamp then is output via analog or digital signal to a temperature display. Temperature ranges can be measured between approximately 1,300 to 5,800°F (700 to 3,200°C).
ASOS Temperature Sensor
• Platinum wire Resistance sensor.
• Air drawn past sensor by a small fan.
• Sensor shielded to protect from radiation.
• Samples once every 30 seconds.
• Determines a 1-minute mean.
• The last 5-minutes of 1-minute mean values are averaged for a 5-minute average.
• Maximum and minimum temperatures are determined from the 5-minute averages and reported at proper synoptic time.