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

Thermal probes and thermocouples

A.C. Power Supply  Constant Power Supply Unit Set Off on 0 300 600 900 1200 1500 Switch     0000 000.0 big small S Current 0 Timer Temperature indicator 000.0 000.0 000.0 Temperatures Field Thermal Probe Fine tuning Coarse tuning

Various Devices used for Thermal Property Determination Laboratory thermal probe Field thermal probe THERMODET DDTHERM (software)

Transient Method r Governing Equation for Line Heat Source in an Infinite Medium       2 1           2   Initial and boundary conditions: t r r r  =  0 , for t = 0, r =   θ   π lim 2 . k. r Q   r r 0 Solution of the Differential Equation:         n n Q 1 u       2 θ θ γ   r ( ) lnu  0 π u 4 k n.n!    1 n α 4 t  is the Euler’s constant and is equal to 0.5772.

For r  0 and t  , the higher order terms of u can be neglected 100 (a) Q t 80   θ θ 2 ( ) ln 2 1 π 4 k t 60 1 s 40  1   Q  R s.   0 C) 20 0.1 1 10 100 T  4 π   ( 100 (b) 80 60 40 20 0 5 10 15 20 25 30 35 40 t (min)

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

Variation of temperature with time for THERMODET (a) 70 60 50 40 30 20 0 C) 1 10 100  ( 70 (b) 60 50 40 30 20 0 10 20 30 40 50 60 70 80 90 100 t (min)

Percentage change in temperature versus time factor curves 0 20 40  (%)  = D 60 2 T 80 H=  t 50 100 H=2D 120 0.01 0.1 1 T where  is the thermal diffusivity D is the diameter of the soil sample T is the time factor corresponding to 50% change in temperature t 50 is the time corresponding to 50% change in temperature