How to Calibrate Soil Hydrometers

Fill a 1,000 milliliter graduated cylinder with distilled or deionized water to the 500-milliliter mark on the cylinder.

Pour your measured 500 milliliters of distilled or deionized water in a 1-liter plastic bottle with a lid.

Place a weighing paper on a digital laboratory scale. Press the "TARE" button to zero the scale or follow scale manufacturer instructions for zeroing your scale.

Measure 40 grams of sodium hexametaphosphate onto the weighing paper.

Hold the paper carefully in a U-shape over your 1-liter plastic bottle with the bottom of the U directly above the mouth of the bottle. Tilt the opposite end of the weighing paper up and allow your weighed 40 grams of sodium hexametaphosphate to spill into the bottle.

Cap the bottle tightly and shake it for one minute to dissolve the sodium hexametaphosphate. Fill the bottle with deionized or distilled water up to the 1-liter mark on the bottle.

Pour the sodium hexametaphosphate solution you have made into a 1,000 milliliter graduated cylinder up to the 125-milliliter mark on the cylinder. Use the bottom of the curve of the liquid at the top of the cylinder as the level of the liquid in the graduated cylinder. This curve is called a meniscus. For example, the bottom of the meniscus of the liquid in your graduated cylinder is even with the 125-milliliter mark on the cylinder.

Fill the graduated cylinder with deionized or distilled water up to the 1,000-milliliter mark on the cylinder. Allow the graduated cylinder and its contents to reach room temperature.

Place your soil hydrometer in the solution. Wait until the hydrometer stops moving.

Read the scale on the hydrometer stem scale where the meniscus of the water in the graduated cylinder crosses it. The scale indicates measured grams per sample liter (g/l). For example, you read "1."

Note that if the value you read is greater than 0 on the stem scale, you must subtract this value from all subsequent sample readings using this hydrometer; if the value you read is less than 0, you must add this value to all subsequent sample readings using this hydrometer. For example, you read a value of 1 during your calibration procedure. You subsequently read a sample value of 3. Your experimental value for the sample is 2 g/l (3 - 1 = 2).

Measure the room temperature at which you have calibrated your soil hydrometer using a digital home weather station or thermometer. Write this temperature on a piece of paper. For example, you write "68 degrees Fahrenheit."

Recalibrate your soil hydrometer if you use it in temperature conditions different than those at which you originally calibrated the hydrometer. For example, you calibrated your soil hydrometer yesterday at 68 F. The temperature in the laboratory today is 63 F. You must recalibrate your soil hydrometer before using it.