Fluke 7381 - Thermal Conductivity; Thermal Expansion

7381
User’s Guide
16
Thermal Conductivity
Thermal conductivity measures how easily heat flows through the fluid. Thermal conductivity of
the fluid affects the control stability, temperature uniformity, and temperature settling time. Fluids
with higher conductivity distribute heat quicker, more evenly, and improves Bath performance.
Thermal Expansion
Thermal expansion describes how much the fluid volume changes with temperature. You must
consider fluid thermal expansion since the increase in fluid volume, as the Bath temperature
increases, can cause overflow into the fluid expansion reservoir, Rear Panel and the Fluid
Expansion Reservoir. The expansion can also cause loss of valuable bath fluid. Excessive
thermal expansion can be undesirable in applications where constant fluid level is important.
Thermal expansion coefficients of several fluids are shown in Tab le 2. Fluid manufacturers can
also provide this information. Thermal expansion coefficients are shown in units of cm/cm/°C.
However, the values are the same for any unit of length. Divide the value by 1.8 for °F
coefficients.
Use this equation to find the desired depth:
D
E
=D
S
[K(T
E
-T
S
)+1]
Or
D
S
=D
E
/ [K(T
E
-T
S
)+1] where D
E
≤ The Maximum Fill Depth
Where:
K=Expansion coefficient
T
E
=Ending temperature
T
S
=Starting temperature
D
E
=Ending depth
D
S
=Starting depth
The maximum-fill depth is typically 1.3 cm to 2.0 cm (0.5 in to 0.8 in) below the level of the gasket
at the top of the Bath tank top (not the top of the Bath lid). Use caution with different stirring
arrangements to prevent splashes on the Bath lid gasket.
Example:
The final depth of Dow Corning 710 silicone oil in the Bath tank is to be 23 cm (9.06 in) when
heated from 25 °C to 300 °C. What should the starting depth be?
Expansion coefficient for 710 oil on Table 2, K= 0.00077 cm/cm/°C
Ending temperature, T
E
= 300 °C
Starting temperature, T
S
= 25 °C
Ending depth, D
E
= 9.2 in (23 cm)