Spray application manual
31 January 2025
Module 8: Calibration of the spray system
8.7 Tank calibration
Published 24 January 2025 | Last updated 20 January 2025
Preferred method
Tank calibration
15 January 2025Tank calibration. Another video from GRDC's Spray Application GROWNOTES™ series.
| Craig Day: We have a clear sight glass here with no markings, so we need to ensure the sprayer is level. As we fill it, we should mark either 50- or 100-litre increments. A useful final calibration check is knowing the volume of your hopper. For example, if your hopper holds 61 litres, you can verify this when using a different water source to ensure your inflow calibration is accurate. If you measure 61 litres from another source, you can confirm there’s no restriction in supply affecting your inflow metre’s performance. For the spray unit’s flow metre, it’s a good practice to check its calibration a few times a year. Adjustments might be necessary when switching between different water rates, such as from 50 to 100 litres per hectare. It’s worth verifying the flow metre calibration at these times. To start, level the spray unit to ensure it’s sitting on an even surface. Then, turn on the pump and let water flow through. Record the flow rate in litres per minute displayed by the flow metre. Use this figure to mark increments on the sight glass in 100-litre steps, starting from zero. This process is important to ensure the correct amount of water is flowing into the tank. However, note that tank markings can be misleading. While they’re evenly spaced, tank bulges in the centre can make the markings less accurate. The readings are usually reliable at the top and bottom of the tank, but for partial loads, they might not provide the precision needed to measure the correct amount of water. |
Before starting your tank calibration, make sure the spray unit is on level ground.
Use a calibrated inflow meter to check and re-mark tank graduations.
This is a two-person job – a constant supply is critical.
Using the same supply-pump RPM, water source and volume per minute, add water to the spray tank.
One person should mark the tank as the other calls out the volume from the flow meter in 100-litre increments.
Use the inflow meter to check the capacity of the fresh water tank as well.
Tip
When connecting to different water sources, inflow meters may vary in their accuracy. A simple check is to flow water to the point of run-off into your calibrated induction (mixing) hopper. This will quickly check flow meter accuracy.
Alternative method (where an in-fill meter is not available)
If the sprayer flow meter is accurately calibrated, fill the tank to a known point (full mark) and add a mark to the tank or site gauge at this point. As you pump out liquid, record marks on the tank or site gauge for every 100L through the rate controller.
Once the tank is empty, note the total litres measured through the controller and record this volume at the full mark. Work your way down the tank or site glass, recording the volume at each mark by reducing the previous figure by 100L each time.
The tank capacity at the full mark can also be checked using an accurate weighbridge. Check the weight when the sprayer is empty and when it has been filled to the full mark and subtract the difference. Remember, one litre of water is equal to one kilogram.