If there is not enough electrolyte drawn into the hydrometer, squeeze the bulb slowly to remove the liquid and try to compress the rubber bulb more completely to draw up more electrolyte from the cell. Once the reading of the SG is complete, be sure to return the electrolyte to the same cell that it was removed from.
Since the density of a liquid changes with its temperature, the SG measured with a hydrometer needs to be adjusted, or compensated. Quality hydrometers have a built-in thermometer with an easy-to-use scale for making this adjustment. The thermometer’s temperature is referenced to a value that is either added or subtracted from what is indicated on the float. Verifying a hydrometer’s accuracy should be done regularly by taking a reading of distilled water—which should measure 1.000. Be sure to adjust for differences in water temperature.
Sometimes, a battery cell’s electrolyte level is too low to allow a hydrometer to draw up enough electrolyte. Distilled water will need to be added but, before taking a SG reading, you will need to charge the battery for several hours to ensure that the electrolyte is fully mixed. Otherwise, the acidless water will float on the top of the more dense electrolyte, giving an inaccurately low SG reading.
After completing all readings, be sure to rinse off the hydrometer in clean water and allow it to dry before putting it away. Be especially careful of setting down the hydrometer on a dirty surface, since dirt in the hydrometer can interfere with an accurate reading and contaminate the battery electrolyte. Keeping the hydrometer inserted into a bottle of distilled water between uses is the best way to keep it clean, protect it from being dropped or knocked over, and to reduce the amount of acid that gets on your clothing.
While a hydrometer is an off-the-shelf tool to measure SG, it has some shortcomings. First, the scale on the float can be difficult to see in poor light. Second, because the surface of the liquid is curved, it’s possible to get inaccurate readings depending on what part of the meniscus is used when reading the scale. The float can also be tilted in the tube, causing it to hang up on the side and making it hard to read accurately. And finally, the relatively large volume of electrolyte required to fill the hydrometer makes it easier to end up with acid on your hands and clothing, or on the outside of the battery.
Refractometers, however, only require a small drop of the electrolyte to measure the SG, making them much less messy and easier to use. The level on the scale is much easier to read and they can be used in low-light conditions. They also have excellent accuracy and provide consistent results, unlike float type hydrometers.
A refractometer uses the “angle of refraction” of light through a small drop of the liquid being tested. The angle will vary depending on the density of the solution.
To measure SG, use the sampling dropper to obtain some electrolyte from the cell. Then, open the refractometer’s illuminator flap and put a drop or two of electrolyte on the measurement prism’s surface. After closing the flap, look through the eyepiece. Read the value where the shadow boundary line meets the scale. It may be necessary to point the refractometer toward a light source (the sun or a lamp), but during the day, ambient light is usually sufficient. When you’re done, wipe the prism and flap with a clean, soft cloth and rinse the dropper with distilled water. Store the refractometer in its protective case as it’s a somewhat fragile device that can have its calibration affected if dropped or jostled. Use all of the same safety precautions and equipment as previously discussed in the hydrometer section.
The refractometer will retain its accuracy as long as it is kept close to the nominal temperature used as its reference (77ºF). Many refractometers also include a built-in temperature compensation mechanism. However, allow the instrument to equalize with the environment before using and protect it from temperature extremes.
Like a hydrometer, you can verify the accuracy of the measurement of a refractometer in the field—all you need is some distilled water. The reading should equal 1.000 or 1,000, depending on the scale of the instrument.
Christopher Freitas is an engineer and project manager for international RE projects. He was a cofounder of OutBack Power Systems and was the director of engineering at Trace Engineering.