Fisher Viscosity Cups

All prices are F.O.B. Pompano Beach and are subject to change without notice.

Cup Temperature

Most materials change in viscosity as a function of temperature. Those normally measured by viscosity cups change in the range of 3% to 8% per degree Celsius change in temperature. Usually, the higher the viscosity the greater the change. For acceptable accuracy, it is necessary to measure temperature at the same time that viscosity cup readings are taken. When many determinations are to be made on similar products in the same viscosity range, it may be helpful to produce a graph for converting measured temperature and viscosity cup efflux time in seconds to seconds at a specified temperature, normally 25° Celsius.

There are three variables to consider: viscosity, efflux time seconds and temperature. All three can be shown on a graph with a family of curves as shown in the following example. Viscosity level is indicated by the diagonal lines, increasing from the lower left to the upper right. Such a graph can be prepared for a given material by taking readings with the Gardco/Fisher cup over a limited temperature range as shown in the example. Within this limited range the plots of the obtained data will normally result in a straight line such as the heavy diagonal line. Draw parallel lines to the line of experimental data, as shown, representing different viscosity levels. Enter on the graph the material represented and the Gardco/Fisher cup number designation.

Use the prepared graph by plotting on it measured temperature and efflux time in seconds. At "A" in the example these values are 26.0°C and 39.5 seconds. Read parallel to the diagonal lines to the intersection with the heavy vertical line which is 25.0°C, the target temperature. Then, reading horizontally to the left, it is found that the correct efflux time in seconds at 25°C is 41.5. Similarly, at "B" in the example, it is found that a reading taken at 23.5°C, when corrected to 25°C, changes from 40.5 to 37.5 seconds.

Compensating for temperature measured close to 25˚C must be done with caution. Even within the limited range of ±2.0°C the variation of viscosity with temperature may not be linear and any thinning materials used to adjust viscosity may also change the rate of this variation.

Cup Care

The Gardco/Fisher viscosity cups are precision instruments. They are ruggedly constructed and with reasonable care will give many years of satisfactory service, requiring only thorough cleaning following each use. Particular care should be used in cleaning the orifice to avoid leaving deposits or scratches on internal surfaces.

Never strike the orifice directly when removing it from the cup. Place a wood dowel rod (VI-WD) against the orifice and strike the dowel with a heavy object such as a paper weight and catch the orifice Prior to inserting an orifice into the cup, insure that the exterior of the orifice and the receiving cone of the cup are clean.

It is good practice to retain one or more standard oils which can be used to periodically insure that the cup retains its initial calibration. Such oils are available from the Paul N. Gardner Company. Refer to the Specification Table (Fig. 1) for the oil recommended for use with each cup/orifice combination.

Guide for removing the calibrated oil from Gardco viscosity cups: Any remaining material in the cup must be removed by flushing with a suitable solvent. Light naphtha, heptane, octane, highly aromatic solvents, and or any other petroleum-derived hydrocarbon solvent can be used. Varsol® is a commercial solvent that works very well for this purpose. Varsol is a registered trademark of the Exxon Company

Completely dry the viscosity cup with a lint free cloth. Use a highly volatile solvent for a second cleaning as since any remaining hydrocarbon solvents from the first process will evaporate quickly after the sample has been flushed from the cup. Hypersolve, MEK and Alcohol can be used in aluminum cups and Hypersolve and Alcohol for the stainless steel cups. Acetone is commonly used as the second solvent because of its high volatility and its ability to dissolve traces of petroleum solvents and water.

In the third process a low velocity stream of clean air will be sufficient to evaporate remaining traces of any volatile solvent. Be aware, avoid rapid evaporation of these solvents as this can cool the surface to such an extent that humid air may be brought below the dew point, causing a film of water to form on the cup.

Warning: Silicone fluids should not be used to calibrate viscosity cups. These materials change the interface between the cup surface and the test material and therefore change the cup calibration. The following is taken from ASTM D445: Viscometers used for silicone fluids should be reserved for the exclusive use of such fluids. Solvent washings from these viscometers should not be used for cleaning other viscometers.

Viscosity Oils

Gardco produced viscosity cups are calibrated with standard "G" Series oils. Centistoke viscosity value of these oils is traceable to the National Institute of Standards and Technology and are available from the Paul N. Gardner Company.

Shown in the graph is the viscosity cup designation and the standard oil used for its calibration. Normally, cup calibration is at 25 degrees Celsius, shown on the graph by bold lines intersecting with the curve in the circle. This graph is included with each Gardco/Fisher Standard cup sold by Gardco.

Viscosity of most liquids, including the standard oils, are dependent on temperature. Efflux time in seconds for the indicated cup-oil combination from twenty (20) to twenty seven (27) degrees Celsius is shown in the graph. The cup may be checked with this graph and the indicated oil with reasonable accuracy within these limits. For best accuracy, the temperature of the standard oil should be 25 degrees Celsius. The standard oil value in centistokes is printed on the standard oil bottle label. Conversion from viscosity in centistokes to efflux time in seconds is by formula or table that defines the cup characteristics. The cup applicable formula and table are furnished with each cup sold as an additional customer service by the Paul N. Gardner Company or by licensed distributors.

Poise, Centistokes, Efflux Time

The POISE is the fundamental unit of viscosity. It is a defined mechanical measurement of the resistance of a liquid to flow where gravity is not a factor. 100 CENTIPOISE = 1 POISE. However, gravity is the driving force causing liquid in a viscosity cup to flow through the orifice. A high density material will flow from a cup in a shorter time than a low density material of the same viscosity. The STOKE is defined as the POISE divided by density (or weight per gallon LBS. times 0.120). 100 CENTISTOKES = 1 STOKE. The CENTISTOKE is the unit of reference in all viscosity cup measurements.

The graphs relate viscosity in CENTISTOKES to cup efflux time in SECONDS for each of the four cups of the Gardco/Fisher series. The graphs may be used for determining the rough relationship between these factors but usually reference will be made to the table that is furnished with each cup which gives the relationship to the nearest tenth of a second. If there is a necessity to determine the relationship beyond the range of the table, the mathematical formulas shown on the graphs may be used.

The lower formula shown in the graph is used when cup efflux time in SECONDS is known. As an example, assume 42.5 SECONDS in the #2 CUP. Multiply 42.5 by 2.32 and the result is 98.6. Divide 190 by 42.5, which is 4.5 and subtract this value from 98.6. The result is 94.1, the CENTISTOKE value of 42.5 SECONDS efflux time from this cup.

The top formula shown in the graph is used when the CENTI-STOKE value is known. As an example assume, 425 CENTISTOKES in the #4 cup. Square 425 which is 180625 and add 15876 for a total of 196501. Take the square root of this value, which is 443 and add 425 for a total of 868. Divide 868 by 37.8 and the result is 23.0 SECONDS, the efflux time value of 425 CENTISTOKES from this cup.

Flow characteristics of the Gardco/Fisher viscosity cups are accurately defined by mathematical formula relating them to the viscosity of standard oils which are traceable to the National Institute of Standards and Technology (NBS). The formula for each cup in the series is shown on the cup graphs. For convenience, the formula for each cup has been solved for each tenth of a second within the normal cup range. Results are available in table form as shown at the left and are furnished with each cup. They are also available in sets of four for the four cup series.

Use the table as follows: Assume an efflux time of 44.5 seconds. Read down the left column to the 44.0 second line and then to the right on this line to the 0.5 column. The value at the intersection is 33.9 centistokes. The tables may be read in reverse to find efflux time in seconds from a known centistoke value. Note that conversion values are accurate only for those materials that do not deviate greatly from true liquids.

Flow characteristics of all Gardco produced viscosity cups are accurately defined by mathematical formula relating their Efflux time to the viscosity of standard oils which are traceable to the National Institute of Standards and Technology. The formula for the Gardco/DIN 4mm cup is shown in the conversion graph. For convenience, the formula has been solved for each tenth second within the normal cup range. The resulting table is furnished with each cup and is shown here in reduced form.

As an example for the use of the table, assume a cup Efflux time of 66.7 SECONDS. Read down the left column to the 66 SECOND line and then to the right on this line to the 0.7 column. The value at the intersection is 298.0 CENTISTOKES. The table may be read in reverse to find Efflux time SECONDS from a known CENTISTOKE value.

The information contained herein, or that supplied by us or on our behalf in any other manner is based on data obtained by our own research and is considered accurate. However, NO WARRANTY IS EXPRESSED OR IMPLIED REGARDING THE ACCURACY OF THESE DATA, THE RESULTS TO BE OBTAINED FROM THE USE THEREOF, OR THAT ANY SUCH USE WILL NOT INFRINGE ANY PATENT. This information is furnished upon the condition that the person receiving it shall make his own tests to determine the suitability thereof for his particular purpose.