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:  =  V,  = 38.7  V/ o C,  =  / o C

To get V 3 = 5 Volts when T=35 o C, then must amplify the signal.

Then, voltage output for -15 o C would be: 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- Wide temperature range –Rugged- 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.

Comparison SensorRangeAccuracy –Thermistorsmallestbest –Platinum PRTlargestmoderate –Thermocouplemoderateleast (Type T)

Radiation Thermometers Infrared Thermometer Brightness Thermometer  = emissivity  = Stefan - Boltzman constant

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). (For most meteorological temperature ranges, we are interested in the  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.

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