Thermal Conductivity; Thermal Expansion - Fluke 7341 User Manual

7341 Calibration Bath
User's Guide
8.1.4
8.1.5
28
proportional band may require some adjustment depending on the specific heat
of the fluid.

Thermal Conductivity

Thermal conductivity measures how easily heat flows through the fluid. Ther-
mal conductivity of the fluid affects the control stability, temperature unifor-
mity, and temperature settling time. Fluids with higher conductivity distribute
heat more quickly and evenly improving bath performance.

Thermal Expansion

Thermal expansion describes how much the volume of the fluid changes with
temperature. Thermal expansion of the fluid must be considered since the in-
crease in fluid volume as the bath temperature increases may cause overflow. It
may be dangerous to permit the fluid to overflow the tank. It may also cause
loss of valuable bath fluid. Excessive thermal expansion may also be undesir-
able in applications where constant liquid level is important.
Thermal expansion coefficients of several fluids are shown in Table 2 on page
32. Fluid manufacturers can also provide this information. The thermal expan-
sion 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. The fol-
lowing equation may be used to find the desired depth:
D = D [K(T
E S
Or
D = D / [K(T
S E
Where:
K=Expansion coefficient
T =Ending temperature
E
T =Starting temperature
S
D =Ending depth
E
D =Starting depth
S
The maximum fill depth is typically 0.5 to 0.8 inches below the level of the
gasket at the top of the bath tank (not the top of the bath lid). Judgement must
be made with different stirring arrangements to prevent splashing on the gasket
or lid of the bath.
Example:
The final depth of Dow Corning 710 silicone oil in the bath tank is to be 9.2
inches when heated from 25 to 300°C. What should the starting depth be?
Expansion coefficient for 710 oil on Table 2, K= 0.00077 inch/inch/°C
–T )+1]
E S
–T )+1] where D
E S E
≤ The Maximum Fill Depth