Volume measurements are needed to calculate the proper concentration of most chemicals which are applied to water and to calculate holding or transport densities.
Tanks:
Most tanks used for holding and transporting fish are rectangular. Rectangular volume is calculated by the formula:
Volume = length x width x depth
When measuring a tank, take inside measurements of length and width and the depth at the appropriate water level. If a standpipe or other type of overflow drain is present then the height to the overflow should be the depth measurement.
If the bottom of the tank is sloped toward the drain an average depth measurement should be used. To get average depth of the tank take three measurements: at the shallow end, in the middle, and at the overflow. Add these depths together and divide the total by 3.
For example, a rectangular tank, without a sloping bottom, has a measured inside width of 36 inches, a length of 72 inches and a depth at the standpipe overflow of 24 inches. The calculated volume is 62,208 cubic inches (36 x 72x 24).
In many cases it will be necessary to convert cubic inches (in3 ) to either cubic feet (ft3 ) or gallons. Table 4 gives simple ways to make these conversions. Cubic inches are converted to cubic feet by multiplying by 0.000579 (or by dividing by 1728). Cubic inches are converted to gallons by multiplying by 0.00433 (or by dividing by 231).
A volume of 62,208 in3 is the same as 36 ft3 (62,208 x 0.000579 or 62,208 + 1728) and 269 gallons (62,208 x 0.00433 or 62,208 ÷ 231). Circular tank volume (Figure 4) is determined by the formula:
Volume = 3.14 x radius2 x depth The radius is measured as 1/2 the inside diameter of the tank. The radius is squared or multiplied by itself.
For example, a circular tank with an inside diameter of 72 inches and a standpipe depth of 24 inches has a volume of 97,667 cubic inches (3.14 x 36x36x 24). Using Table 4 the volume can be
converted into cubic feet (97,666.56 ÷ 1728 = 56.52) or gallons (97,666.56 ÷ 231= 422.8).
Ponds:
Pond volumes can be calculated using the formula:
Volume = surface area x average depth
Calculating surface area was presented in the first section of this fact sheet. Calculate the average depth by measuring the depth at intervals around the pond. A boat and weighted cord (marked in feet) are used to take depth measurements.
Measurements can be done in a grid pattern or in a crisscross pattern.
The number of depth measurements taken affects the accuracy of the estimate. Increasing the number of measurements increases the accuracy, so take as many measurements as possible. Record all depth measurements, add them together and divide the total by the number of measurements taken. For example, in Figure 5, the sum of the depth measurements totals 93 feet.
Divide 93 by 16 (the number of measurements) to get an average depth of 5.8 feet. The pond volume in this example (taking the surface area as 6.58 acres from previous example) would be 38.16 acre-feet (6.58 acres x 5.8 feet).
An alternative but accurate method to calculate pond volume is by adding salt and testing for chloride. This method is based on the fact that 1 acre-foot of water weighs 2.71 millions pounds and, therefore, 2.71 pounds of active ingredient dissolved in 1 acre-foot of water gives a solution of 1 part per million (ppm) or 1 milligram per liter (mg/l).
If 2.71 pounds of salt are added to a pond of exactly 1 acre-foot, then the sodium chloride concentration will increase by 1 ppm. To determine volume by this method requires a chlorine test kit and uses the following procedure.
- Test a pond water sample for chloride. Record this concentration. As an example, a pond with a surface area of 4 acres tests 0.1 mg/1 of chloride.
- Broadcast 50 pounds of salt per surface acre of the pond; record the total pounds of salt added. In this example, broadcast 200 pounds of salt (4 acres).
- Wait at least one day to allow the salt to dissolve and distribute evenly.
- Take several water samples from different areas and depths (Figure 5) and test them for chloride concentration. In this example, six samples tested 8.1, 8.3, 8.9, 8.2, 8.5, and 8.6 mg/1 of chloride.
- Calculate the average chloride concentration by adding all the concentrations together and dividing by the number of samples. In this example, average concentration = 8.43 mg/1.
- Calculate the change in chloride concentration by subtracting the beginning concentration from the average concentration after salt treatment. In this example, 8.43 -0.1 = 8.33 mg/1 is the change in chloride concentration.
- Calculate the pond volume using the formula:
Volume = (weight of salt applied x 0.6) + 2.71 (acre-ft) change in chloride concentration In this example, (200 x 0.6 ÷ 2.71) ÷ 8.33 = 14.77 acre-feet.
The 0.6 in the formula is the proportion of chloride in sodium chloride (NaCl), Measure chloride accurately and double-check your calculations! As a word of caution, this method will not work in deep ponds that are thermally stratified.
Keep good records of your pond area( s) and volume(s). Do not rely on your memory. The water level and volume in watershed ponds may vary from season to season with rainfall, evaporation, siltation and other factors.
Pond managers should calculate the volume of ponds at different water levels, so chemical treatments can be applied properly under any condition. Do not guess the area or volume of your pond because the consequences could be costly.
Authors:
Michael P. Masser and John W. Jensen
