ALTERNATIVE METHODS FOR HYDROMETER CALIBRATION

The calibration procedure based on volumetric dilution of a saturated NaCl solution is designed for use by those who do not have access to analytical equipment for weight and volume measurement. It provides accurate results with no equipment other than ordinary household items. The accuracy derives from two principal factors: (1) the ease of duplicating volume measurements to better than 1% using ordinary household containers of appropriate shape; and (2) the characteristic of NaCl that its saturation molality and molarity, and thus density, vary very little with temperature over useful ranges.

Those having sufficiently capable equipment may wish to just weigh out desired amounts of a solute (salt, sugar, or whatever) and measure a desired volume of water and prepare an unsaturated solution having known specific gravity. The procedure should be examined for sensitivity to errors in the weight and volume measurements. Many graduated cylinders are good only to 1% at full capacity.

One alternative procedure can be based on repeating a weight measurement rather than a volume measurement. For example, a given weight of sucrose (table sugar) can be dissolved in an equal weight of water to yield a solution having a specific gravity of 1.107 at 20C. In this procedure, the sugar is poured into a container on the scale until approximately the desired amount is reached, considering the tare of the container. The scale reading is noted, or the beam is brought into balance. Then the contents of the weighing container are poured into another container in which the solution will be mixed, and the weighing container returned to the scale. The weighing container must be free of any residual sugar. Then water is added into the weighing container until the same scale reading is obtained or until the beam just balances. No changes are made to the scale settings. The water thus weighed can then be mixed with the sugar to dissolve it. This procedure requires a scale with good repeatability. Its absolute accuracy does not matter. A kitchen scale capable of weighing five pounds (80 oz) may not be capable of repeating a weighing to one ounce. A repeatability of 1% should be the objective.

This author may be contacted for calculations for calibrations at other points, such as within the range of narrow-range hydrometers.

Stanley E. Prevost
sprevost@knology.net
1/31/99

© Copyright 1999, 2002 Stanley E. Prevost All Rights Reserved. This document may be copied and distributed for noncommercial use provided that this copyright notice is maintained with the document.