About Sink-Float Standards
Sink-Float Standards are weighted, hermetically sealed, amber glass capsules for visually measuring and monitoring the density of liquids. Each Sink-Float Standard is precisely calibrated at 23°C (73.4°F) to measure a specific density of a liquid within a 0.7 gm/cc to 7.50 gm/cc range. The Standards are in 1/4″ (6.4 mm) in diameter and usually 3/4″ (19 mm) in length; for higher densities, the lengths may be longer. Each Sink-Float Standard is also permanently marked for identification.
Density is determined by simply observing the position of a Sink-Float Standard in a liquid. When a Sink-Float is swirled in a liquid and thoroughly wet, it will either float in a more dense medium, sink in a less dense medium or stay suspended when the densities match. Continuous monitoring of liquid densities is performed by keeping a Sink-Float in the liquid. Since a Sink-Float will have a normal position (up, down or suspended), any change of position will signal a change in density.
Methods of Use
Beaker Method – The most common technique for using Sink-Floats is the “beaker” method. A transparent container such as a lab beaker, cylinder, or battery jar is employed to hold the density liquid, or filled with product for quality control testing. In general, where a series of separations are to be made by adjusting the liquid, it is better to start with a low density. The surface can then be skimmed again to remove new floaters. A series of Sink-Floats makes adjusting the liquid, by bringing the next heavier Sink-Float into suspension, a simple procedure.
Semi-Continuous Monitoring – This method is primarily for use with tank and piping systems. When frequent testing is required, it may justify setting up a vertical sight gauge to contain the Sink-Floats. If flow cannot be stopped long enough for the Sink-Floats to indicate the density, a by-pass can be arranged.
Tanks or pipes equipped with bypasses must provide continuous flow through the by-pass so the sample in the Sink-Float chamber is the same as in the tank or pipe line.
To make a density determination, close valves or shut off mixers, or other agitating systems, to permit the test chamber to come to equilibrium and the Sink-Floats to respond to the density of the liquid.
The sight gauge system must be equipped with screens to prevent the Sink-Float from being washed through. For accurate testing, the temperature of the batch, or at least the test section must be controlled. Broader temperatures are allowed if three or more floats are used as described.
Using One or Multiple Sink-Floats
- Gives the most accurate control when kept suspended in the liquid.
- Indicates minimum density when liquid is higher. A drop in liquid density causes Sink-Float to sink.
- Indicates maximum density when liquid is lower. A rise in density causes the Sink-Float to float.
- Sink-Floats indicate when a liquid changes beyond the minimum or maximum when a density range is permissible. For a liquid within limits, one Sink-Float sinks, the other floats. If both sink or float, liquid is beyond limits.
Three Or More Sink-Floats
- The density of a liquid will vary with temperature. Rather than take time to bring temperature or a liquid or liquid product to the standard temperature, select Sink-Floats for the test at the liquid temperature. The table below shows the Sink-Floats to be selected.
EXAMPLE: Liquid is 75°F, Sink-Floats 2.215 and 2.220 would be used as minimum and maximum limits for acceptance.
NOTE: If 4 Sink-Floats are used as listed in table, all can be put in the test liquid. For 75°F the density would be satisfactory if two sink and two float.
- A series of Sink-Floats can also be used to adjust a liquid through steps for a series of skim separation. If all Sink-Floats are put in the container and the lowest density is used first, as the density is raised to the next point, one Sink-Float will become suspended.
- Temperature changes cause density changes in liquids. The operator must know the degree to which a temperature change will affect his result. Density varies inversely with temperature.
- The simplest method of temperature control is to standardize tests, when possible, at average room temperature. Any adjustments, if they must be made, are then minor.
- Changing the temperature of the liquid can be avoided by preparing a table and using several Sink-Floats. Once the temperature of the liquid is taken, the appropriate Sink-Floats can be selected. A liquid product can then be tested, or a test liquid adjusted by addition of end members. See typical table.
- A liquid can be protected from sharp temperature changes caused by air circulation or cold drafts by putting the beaker inside a larger beaker. Greater temperature stability is achieved by filing the space between the beakers with water or clean oils. This technique also assists when tests are conducted at elevated or lowered temperatures.
- Faster response to the density of the liquid can be accomplished by stirring or swirling the liquid and watching the Sink-Floats come to equilibrium.
- Small changes in density can be made by changing the temperature of the liquid. This avoids constant additions to adjust the liquid. Sink-Floats will indicate when the new density has been reached.
- For a given low viscosity liquid an operator can develop a feel for the density difference indicated by the rate a Sink-Float may rise or drop. This technique can be used to keep a liquid within a narrow range with one Sink-Float, or to keep the temperature of the liquid within a narrow range.
Making and Adjusting Density Liquids – Density liquids can be made by mixing appropriately selected liquids until the proper Sink-Float remains suspended in the liquid. The same Standard can be left in the liquid as a monitor, and adjustments can be made by adding the appropriate end member or varying the temperature if the density if the liquid should change.
Separation – Example: Separating gems. Since the possibilities are known, a density liquid or series of liquids can be selected that lie between the densities of the gems being separated. Note: A single Sink-Float will control the liquid, but a pair will require less operator attention. The pair should be selected to give the widest range for variation of the liquid consistent with clean separation between types of stones.
Identification – Example: Diamonds. If in the extraction process a number of crystalline materials are found and the diamond is wanted to the exclusion of all other materials, two liquids would be required. If the diamonds range from 3.50 to 3.54, a liquid of 3.50 would float any crystals of lower density and the diamonds would sink. In liquid of 3.54 the diamonds would float and heavier materials would sink.
Concentration Control – Example: Solution preparation. When a solution is to be prepared from a concentrate, and the diluent has a different density than the concentrate, the concentration may be controlled by density, using one or a pair of Sink-Floats.
Contamination Control – Materials being separated by a density liquid may carry minute amounts of liquids into the density liquid. Monitoring with Sink-Floats will indicate a change in density due to addition of the contaminants.
Product Control – Example – Liquid product to be separated from solvent. Where a solvent process is used in manufacture and the solvent is distilled off, a Sink-Float can be used to indicate when a minimum density or maximum density is reached. The accuracy depends on the density difference between the solvent and the product. Distillation would continue until a Sink-Float just sinks, just floats, stays suspended, or, using a pair to establish an acceptance density range, one sinks and one floats.
Purity Control – When a liquid product’s purity can be related to maximum or minimum density, an appropriately selected Sink-Float will indicate whether the desired purity has been reached, or has changed by contamination or deterioration.
Quality Control – Example: Dielectric properties of ceramic parts. Porosity (air inclusion) of pressed parts affect dielectric constants. By use of density liquids, parts with excessive air inclusions can be separated. One Sink-Float would give critical control; a pair of Sink-Floats would allow a small range of control of the liquid that would reduce operator time in adjusting the density liquid, and yet avoid the possibility of extreme change of density liquid without operator knowledge.
Reaction Concentration Control – When a reaction process consumes a material in solution and the drop in concentration changes density, a test with a properly selected Sink-Float will show when concentration has dropped below minimum level.
Solder Fluxes – The solvents and diluents in fluxes used with automatic soldering equipment slowly evaporate during use and must be replaced to maintain uniform fluidity. Also, the flux solids are depleted during application and must be replaced. Density is commonly used to control fluidity since there are measurable differences between the stable (denser) components and the lighter solvent. Sink-Floats are used to monitor densities and to immediately alert the user to a change in density and that fluidity may be approaching a “problem level.”
Most commonly used Sink-Floats are in the range .700 to 3.00 gm/cc. These are also the most readily available and the least expensive.
A tolerance of ±0.0005 is the most widely used, having proved highly satisfactory for quality control testing, identification and separation work.
The most precise Sink-Floats, having a tolerance of ±0.0005, are generally used in research, and precision analytical work, or where concentration, contamination and purity control is dependent on very minor density changes.
Sink-Floats are stocked by intervals dependent on the tolerance as follows:
EXAMPLE OF SERIES
4.00, 4.01, 4.02
2.000, 2.005, 2.010
2.0000, 2.0005, 2.0010
- For control of a liquid with one Sink-Float, order one matching the density of the liquid.
- For control of a liquid with a pair of Sink-Floats, order one above and one below the liquid density, representing minimum and maximum limits.
- For series separations, or product density quality control at the temperature of the product, the proper series of Sink-Floats must be selected according to the application.
- Sink-Floats are available with densities from 0.70 to 7.5 gm/cc. Available tolerances vary with the density of the Sink-Float. Tolerance is indicated by an extra mark above or below density number, as follows:
0.700 to 4.00
0.700 to 4.00
4.000 to 7.50