The BYK-Gardner Weight Per Gallon Cup is a simplified form of pycnometer specially designed to measure the density of a fluid at a standard temperature of 77°F (25°C). The net weight of the air-free sample is determined to the nearest 0.1 gram, and converted by simple factors to either density (pounds per gallon) or specific gravity (density relative to water). These corrosion resistant stainless steel cups are available in 3 sizes: Regular, Midget and Imperial.
- Mix the material thoroughly before pouring a representative sample into the cup. Fill the cup until the liquid level is just below the rim at 25°C or other agreed upon temperature. Avoid trapping air bubbles in the sample.
- Place the perforated lid on top of the cup and seal it with a careful twisting motion. An excess of the liquid should ooze (not squirt) put of the small hole on top of the snugly fitting lid. Wipe off this excess material, using a soft cloth or tissue paper.
- Weigh the properly filled cup, (usually to the nearest 0.1 gram), on an appropriate laboratory balance. A tare weight is available with the 3 sizes of cups. It can be used as a counterpoise to determine rapidly the net weight of the sample when an evenarm or trip balance is used. The weight of the tare equals the exact weight of the empty cup and its lid.
- Using the net weight, compute the pounds-pergallon density or the specific gravity of the sample by the following methods
The Regular Cup contains 83.2 grams of water at 25°C. The net weight in grams of the fluid sample is divided by 10 to obtain the weight in pounds per U. S. Standard Gallon. This weight per gallon is multiplied by 0.12 to obtain a specific gravity or the net weight of the cup can be divided by 83.2 for a direct conversion to specific gravity.
The Midget Cup has a capacity of 8.32 grams of water at 25°C. The net weight of the fluid sample in grams equals the sample‘s density in pounds per U. S. gallon. To obtain specific gravity, multiply pounds per gallon by 0.12 or divide the net weight of the cup by 8.32.
The larger Imperial Cup contains 100.0 grams of water at 25°C; the net weight in grams of the fluid sample is divided by 10 to obtain the weight in pounds per British Imperial Gallon. This weight in pounds is multiplied by 0.10 to obtain the specific gravity or the net weight of the cup is divided by 100 to obtain this figure directly.
Importance of Temperature Control:
All weight per gallon cups are calibrated with distilled water at the factory before shipment so that the correct net weight of water is retained in the cup at the prescribed temperature of 77°F (25°C). This same standard temperature is recommended when using these calibrated cups. Volume expansion or contraction of both the cup and sample will cause errors if the temperature is not kept under control. In determining an absolute density value both the cup and its contents must be brought to the standard calibration temperature of 77°F or 25°C. However, in determining the relative density values between different samples, all that is necessary is that the cups and its contents be brought to the same temperature before weighing.