Electrical Resistivity; Fluid Lifetime; Safety - Fluke 7341 User Manual

7341
Users Guide
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 undesirable in applications where constant
liquid level is important.
Thermal expansion coefficients of several fluids are shown in Table 8-1. Fluid
manufacturers can also provide this information. The 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. The following equation may be used to find
the desired depth:
D
E
Or
D
S
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). Judgment 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
Ending temperature, T
Starting temperature, T
Ending depth, D
D
S

Electrical Resistivity

Electrical resistivity describes how well the fluid insulates against the flow of electric
current. In some applications, such as measuring the resistance of bare temperature
sensors, it may be important that little or no electrical leakage occur through the fluid.
In such conditions choose a fluid with very high electrical resistivity.

Fluid Lifetime

Many fluids degrade over time because of vaporization, water absorption, gelling, or
chemical break-down. Often the degradation becomes significant near the upper
temperature limit of the fluid, substantially reducing the fluid's lifetime.

Safety

When choosing a fluid always consider the safety issues associated. Obviously where
there are extreme temperatures there can be danger to personnel and equipment. Fluids
may also be hazardous for other reasons. Some fluids may be considered toxic. Contact
with eyes, skin, or inhalation of vapors may cause injury. A proper fume hood or
8-4
= DS [K(T –T )+1]
E S
= D / [K(T –T )+1] where D
E E S
= 300°C
E
= 25°C
S
= 9.2 inches
E
= 9.2/[0.00077 (300-25) + 1] = 7.59 inches
≤ The Maximum Fill Depth
E