8.1.3
Specific Heat
Specific heat is the measure of the heat storage ability of the fluid. Specific
heat, to a small degree, affects the control stability. It also affects the heating
and cooling rates. Generally, a lower specific heat means quicker heating and
cooling. The proportional band may require some adjustment depending on the
specific heat of the fluid.
8.1.4
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.
8.1.5
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
into the fluid expansion reservoir (see Section 7.3.1, Fluid Expansion Reser-
voir, for how to use this). It may also cause loss of valuable bath fluid. Exces-
sive thermal expansion may also be undesirable in applications where constant
fluid level is important.
Thermal expansion coefficients of several fluids are shown in Table 2 Table of
Bath Fluids. Fluid manufacturers can also provide this information. The ther-
mal expansion coefficients are shown in units of cm/cm/°C, however, the values
are the same for any units of length. Divide the value by 1.8 for °F coefficients.
The following equation may be used 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 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.
6331 Calibration Bath
User’s Guide
28
