The Fluke Calibration 5010 Silicone Oil, Type 200.05, 1 liter (0.26 gal) has a usable range of -40 °C to 130 °C 133 °C and a flash point of 133 °C.

How to Select a Calibration Bath Fluid

Usable Range

Fluke Calibration defines the "usable range" of a bath fluid as the range of temperatures over which a fluid can safely provide a good environment in which to compare thermometers. The ranges we define for each fluid may be different than what the manufacturers of those fluids specify. That's simply because we're taking the application (thermometer testing in baths) into account.

Range can be limited by viscosity, flash points, freeze points, boiling points, evaporation rates, propensity to gel (or polymerize), etc. Safety-related issues should never be discounted.

Unfortunately, no magic fluid exists to cover extremely wide temperature ranges. We wish one did! Most fluids cover smaller ranges than we'd like. Ideally, you have a separate bath for every common temperature point you use. Doing so increases calibration throughput by eliminating the need to change fluid and then waiting for the bath to reach temperature and stabilize.

Viscosity

Viscosity is a measure of a fluid's resistance to flow-we often think of it simply as "thickness." Kinematic viscosity is the ratio of absolute viscosity to density and is measured in "stokes" (at a specific temperature), which are commonly divided by 100 to give us more helpful "centistokes." The higher the number of centistokes, the more viscous (or thick) a fluid is. Viscosity is always stated at a specific temperature (often at 25 °C) and increases as the fluid's temperature decreases (and vice versa).

Bath fluids which are too viscous create strain on stirring and pumping mechanisms and don't adequately transfer heat uniformly from temperature sources to thermometers.

Fluke Calibration recommends using fluids with less than 50 centistokes viscosity, which is reflected in the usable ranges we state for each fluid. Less than 10 centistokes viscosity, however, is ideal. Low-uncertainty calibrations require a homogeneous temperature within the "calibration zone" of a bath. High-viscosity fluids promote unwanted temperature gradients.

Flash Points

This is the temperature at which an adequate mixture of fluid vapor and air will ignite if in the presence of a spark or flame. (The vapor may even stop burning if the flame is removed.)

There are two ways to measure flash points. With the "open cup" method, neither the fluid nor the air around it is enclosed, so there is a higher ratio of air to fluid vapor. With the "closed cup" method, the fluid, fluid vapor, and air are enclosed. Closed cup flash points are typically lower than open cup flash points.

Also, fluid manufacturers list flash points in various places. On material safety data sheets (MSDS), the flash point is often given non-specifically to fit into a classification scheme used for hazard control. Actual product specification sheets usually give more specific information. For example, the flash point of one silicone oil is listed on its MSDS as "> 101.1 °C," whereas a more specific "211 °C cc" is listed on its specification sheet.

For Fluke Calibration fluids that have flash points, we list the closed cup method and limit the upper end of the fluid's range to slightly below the flash point.

Heat Capacity

Specific heat is the amount of heat required to raise the temperature of a unit of a substance by 1 °C. The higher the heat capacity, the more difficult it is to raise a fluid's temperature, therefore it is both slower and more stable.

Thermal Conductivity

Thermal conductivity is a fluid's ability to transfer heat from one molecule to another. The better the heat transfer, the quicker the fluid will heat or cool. Better thermal conduction improves bath uniformity.

Expansion

All fluids have a coefficient of thermal expansion. This coefficient tells how much a fluid's volume will change (expand or contract) with changes in temperature. Fluid expansion has important ramifications for safety, cleanliness, and care of equipment. If baths are filled too high with a fluid at a low temperature and then heated without regard to volume increase, they can obviously spill. Also, if the fluid in a bath is allowed to run too low, it can leave bath heaters exposed, which can damage them.

Specific Gravity

The specific gravity is the ratio of a fluid's density to that of water. The higher the specific gravity, the more dense (and heavy) a fluid is. If the fluid is too heavy, it may not work well in a bath equipped with a pump mechanism or circulator.

Vapor Pressure

Vapor pressure is (at least for our purposes here) the temperature at which the rate of evaporation of that fluid equals the rate at which the fluid's vapor is condensing back into the fluid-i.e. the two are at equilibrium. Raising the temperature increases a fluid's vapor pressure over ambient pressure, thereby driving vapor into the air.

Fluids that have high vapor pressures (such as alcohols and water) evaporate quickly and require frequent replenishment. Furthermore, rapid evaporation at the fluid surface has a cooling effect on the fluid, making temperature control more difficult, especially with an uncovered bath. Such fluids generally are only suitable for low temperature use. In some cases, vapors in the air can provide a health hazard and should be carefully vented.

Gelling (Polymerization)

Here's an area that can get people into trouble! Given enough time, temperature, and catalysts, silicone oils will eventually polymerize. That is, they'll suddenly turn into a molasses-like "goop," doubling in volume and making an unpleasant mess.

Oxidation is the root cause. While silicone oils may be used safely to near their flash points, susceptibility to polymerization increases with use above their oxidation points, which we list for each silicone oil.

To delay polymerization, limit a bath's time above a fluid's oxidation point, have it idle below its vapor point when not being used, keep contaminants out of the oil (including salts, other oils, and oxidizers), and change your oil if it becomes too dark, too viscous, or too unstable in temperature.

Water

There are a few things to understand about water in non-water baths. First, never introduce water into a salt or hot oil bath as this can be extremely dangerous.

Second, water may condense in an oil bath being used at low temperatures, particularly where there is high ambient humidity. The water can freeze to cooling surfaces and cause bad stirring conditions. Occasionally the water needs to be boiled off.

Lastly, alcohols absorb water. This isn't all bad. In fact 5 % water in methanol will allow methanol to be used at -100 °C. Also, water that is absorbed will not freeze onto cooling surfaces. However, when too much water is absorbed, the alcohol becomes saturated and an ice slurry forms, negatively affecting stability and uniformity. At that point, the fluid needs to be changed.

Ventilation

Always using good ventilation with baths will prevent bath users from breathing fumes from bath fluids. Suction devices that open near the bath's access opening and exit out of doors are best. Oil vapor can settle on the surfaces of the eyes which causes some discomfort. Silicone oils can create benzine and formaldehyde as they break down at high temperatures - i.e. at about the flash point or above. Keep baths sealed up as much as possible to prevent fumes from coming into the work space. This will help with safety but will also increase the lifetime of the oil and improve performance of the bath.