Fluke Bath Fluids User Manual

Choosing a bath fluid

by Steve Iman

Hart Scientific

Application Note

Fluke Hart Scientific manufactures a large number of constant tempera ture baths that cover a temperature range from –100 to 550°C. How ever, the usable range of each bath highly depends on the fluid chosen. Hart’s definition of “usable range”, is the temperature range over which the fluid will provide the best performance. The ideal bath fluid would have a low viscosity, high heat capacity, very low vapor pressure, and a high flash point. It would also need to cover a very wide temperature range. Unfortu nately, no single fluid has all of these attributes, so care must be taken when choosing a bath fluid. From both an operational and safety standpoint, some considerations are: safety precautions, flash points, viscosity, heat capacity, thermal conductivity, fluid expansion, specific gravity, vapor pressure, gel time, usable life and storage.

1.0 Fluid Safety

Always obtain data sheets and/or material safety data sheets for the fluids that will be used. It is impor tant to read and understand all safety requirements. When it comes to safety, Hart strongly recommends the following:

1. Have the appropriate fire extinguishing equipment nearby in case of a fire.

2. Never mix fluids or put any chemicals into the fluid. Doing so may cause contamination or an adverse chemical reaction.

3. Always exercise caution when working around extremely cold or hot bath fluids. Wear protective clothing to prevent accidental injury.

4. Use adequate ventilation for fluids at elevated temperatures. (See Figure 1).

5. Never operate a bath on or around combustible materials.

6. Provide safety training for all personnel who will either use the baths or be around them.

7. Abide by federal and state laws regarding storage and disposal of any hazardous or flammable liquid.

2.0 Terms and Definitions

Now that you have the data sheets, what do all those terms mean?

2.1 Flash point

This is the temperature at which an adequate mixture of fluid vapor and air will ignite if in the presence of an open flame or spark. It is important to note that if the fluid is nonflammable it’s only the vapor that will burn and not the fluid. There are two units of measure for flash point.

1. Open Cup (oc). As the term implies, the air and fluid vapor are not enclosed. In an open cup, there is a higher ratio of air to fluid vapor.

2. Closed Cup (cc). The mixture of air and fluid vapor are

contained in an enclosure. In this instance, the ratio of fluid vapor to air is higher.

When specifying the flash point, fluid manufacturers only have a list of options to pick from. They do not have open text fields that they can fill in. So when it says >101.1°C, this is just stating that the flash point is greater than 101.1°C. This is just used to classify the material (ie. flammable or combustible, etc. for the hazard profile, not as an ac tual value). It’s like saying how old are you? Then giving you choices of >15, >25, >35, etc. So when it co mes to the flash point definitely go with the product data sheet.

2.2 Viscosity

Viscosity is the unit of measure for the thickness of a fluid at 25°C. Generally it is a constant consis tency under fixed pressure and temperature. Ideal fluids offer no resistance to shear and have zero consistency. Viscosity dimensions

Figure 1 Ventilation system for removing oil vapors.

are force per area x time. The unit of viscosity is the poise (P) = 1g/(cm) (sec) and is a measure of mass flow of a liquid. One poise is equal to 0.1 Pa.s in SI units. Where Pa.s is the Pascal-second, the SI unit for viscosity, equaling 1kg(m.s) or 10 poise.

A common unit of measure with

bath fluids is kinematic viscosity.

This differs from viscosity in that it is the measure of volume flow of a liquid, defined as a stoke (st). A

stoke equals 1 cm

–4m2

/Sec. A centistoke, cst = 0.01 St = 1mm can be converted to viscosity (poise) by multiplying by the density of the fluid.

used at more than one temperature, the viscosity will change when it is heated or cooled. Because the vis cosity changes, each fluid will have a “viscosity temperature coeffi

Since bath fluids generally are

/ Sec or

/sec. Kinematic viscosity

Hart Scientific

cient.” Example: Over a range of 0°C to 100°C, VTC = 1–(viscos

ity@100°C/viscosity@0°C). Thus the lower the VTC, the less change there will be in viscosity over the range.

Hart baths generally perform the best with a viscosity that does not exceed 10 cst. But in the real world, the viscosity will change with temperature. So viscosities that do not exceed 50 cst will work satisfactorily. Any higher than this, and the stability and uniformity may be very poor. In addition, if the viscosity is too high, it will put too much of a load on the stirring mo

tor. This may cause it to overheat or to stop altogether.

2.3 Heat Capacity

The specific heat capacity of a solid or liquid is defined as the heat re

quired to raise a unit of mass or substance by one degree of temperature.

ΔΔQmcT=

Where:

ΔQ = heat applied to fluid. m = fluid mass. c = specific heat capacity. ΔT = rise in temperature.

2.4 Thermal Conductivity

Thermal conductivity is a fluid’s ability to transfer heat from one molecule to another. This can be determined by:

λ() () () ()TaTcTpT

=× ×

Where:

λ = thermal conductivity T = temperature a = diffusivity of material cp = specific heat p = density

The better the heat transfer, the quicker the fluid will heat or cool. Better thermal conduction will help with bath uniformity.

2.5 Coefficient of Volume Expansion

All fluids have a thermal expansion coefficient. This unit of measure tells how much the fluid will either expand or contract with changes in temperature. Unless the bath is equipped with an overflow device, it must be considered, otherwise the bath may overflow.

2.6 Specific Gravity

The specific gravity is a specifica

tion of the density or weight of a fluid as compared to that of water. The specific grav ity of water is 1. A

The specific gravity of water is

cubic foot of water weighs 62.4 pounds.

The higher the specific gravity, the more the fluid will weigh. If the fluid is too heavy,

To calculate an unknown specific gravity:

Sp gr

Weight of an equal volume of water

Sp gr

Density of a fluid

Density of water

it may not work well in a bath equipped with a

Figure 2 Calculation of an unknown specific gravity.

pump mechanism or circulator.

2.7 Vapor Pressure/Volatility

The temperature at which a liquid is on the verge of vaporization is called vapor pressure. At this tem

perature, the vapor pressure of the liquid is equal to that of ambient pressure. Another way of saying this is that the vapor and ambient pressures are at equilibrium. If the temperature is below this point, the vapor will condense into liquid. Conversely, if the temperature is above this point, the liquid will vaporize. A fluid that has a low vapor pressure such as alcohol will evaporate quickly and require frequent replenishment. Furthermore, rapid evaporation at the fluid surface will have a cooling effect, making tem

perature control more difficult. These fluids generally are only suitable for low temperature use.

With some liquids, the processes of condensation and evaporation can be delayed, which is referred to as supersaturation and super

heating, respectively. A good exam ple is adding ethylene glycol to water. This raises the boiling point of the water as well as the vapor pressure.

2.8 Gel Time

Gel time is usually associated with silicone oils when used at elevated temperatures. This is the time that it takes silicone oil to gel or poly

merize. Oxidation of the oil is the root cause. When this occurs, it’s a molecular chain reaction that hap

pens instantly and can cause the fluid to nearly double in volume. Polymerization is a metrologist’s worst nightmare; the oil will either

turn to a jelly-like substance or even worse, a “molasses in winter” goop! It can be very difficult to re

62 4

Weight of the fluid

move from the bath and its parts. Fortunately, Dow Corning makes a solvent that can be used to remove polymerized oil. The solvent is called OS-2 and can be purchased from an authorized distributor of Dow Corning fluids. It will require approximately 2 gallons of OS-2 for every 7 gallons of polymerized oil.

Polymerization of silicone oil in an open system may not be avoidable. However, there are steps that can be taken to prolong the oil’s life.

1. Keep the time that the bath is at high temperatures to a minimum.

2. If the bath isn’t being used, either turn it off or set the idling temperature below its vapor point.

3. Avoid cross-contamination of oils.

4. Keep oxidizers such as bath salts out of the oil.

5. Change the oil if it becomes

too dark in color, to viscous, or there is a notable difference in bath stability.

3.0 Fluid Life and Storage

The life of a fluid depends on how it is used, at what temperature it is used, and the length of time at that temperature. Generally, most bath fluids will have a long life as long as their limitations are not ex ceeded. Unused liquids should be left in their original unopened con tainer. If storage life is a concern, please check with the fluid manu facturer for specifics about shelf life and storage requirements.

2 Fluke Hart Scientific Choosing a bath fluid

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