Thermal probes and thermocouples A.C. Power Supply Constant Power - - PowerPoint PPT Presentation

Thermal probes and thermocouples

Temperature indicator



0000

 Set Off on

Switch

300 600 900 1200 1500

 

big

S

000.0 000.0 000.0

Constant Power Supply Unit



A.C. Power Supply

Field Thermal Probe

000.0

Timer

Temperatures Fine tuning

Coarse tuning

small

Current

Laboratory thermal probe

THERMODET DDTHERM (software)

Field thermal probe

Various Devices used for Thermal Property Determination

1

2 2

          r r r t        

Transient Method Governing Equation for Line Heat Source in an Infinite Medium Initial and boundary conditions:  = 0 , for t = 0, r = 

Q r θ r k. . π 2 lim

r

   



Solution of the Differential Equation:

   

           



 1 n n n

n.n! u 1 γ lnu k π 4 Q ) θ θ (

t α 4 r u

2



 is the Euler’s constant and is equal to 0.5772.

r

1 2 1 2

t t ln k π 4 Q ) θ θ (  

1 T

4π Q s. R



      

For r0 and t, the higher order terms of u can be neglected

5 10 15 20 25 30 35 40 20 40 60 80 100

(b)

t (min)

0.1 1 10 100 20 40 60 80 100

(a) s

 (

0C)

70 mm Thermocouple leads Power leads 25 mm thick Perspex disk 20 mm thick Styrofoam Thermal probe Top cap Bottom cap Thermocouple Rubber washer Compacted soil 140 mm Cap of the probe Rubber washer 220 mm long SS tube 25 mm thick Styrofoam 5 mm thick Perspex disk

Details of the thermal property detector (THERMODET)

10 20 30 40 50 60 70 80 90 100 20 30 40 50 60 70 (b)

 (

0C)

t (min)

1 10 100 20 30 40 50 60 70

(a)

Variation of temperature with time for THERMODET

 =

50 2

t T D

where  is the thermal diffusivity D is the diameter of the soil sample T is the time factor corresponding to 50% change in temperature t50 is the time corresponding to 50% change in temperature

120 100 80 60 40 20 0.01 0.1 1

H=2D H= T

(%)

Percentage change in temperature versus time factor curves