Often growers need to dose products to the irrigation system using injectors. How do you know how much sanitizer or product to add to the water to reach the desired concentration? In this article we give examples to show calculations to determine how much sanitizer to use with or without injectors.

Making these calculations is easy and you only need to know the following equation:

Ci × Vi = Cf × Vf

• Ci is the initial concentration of the active ingredient in the sanitizing product.
• Vi is the volume of the product that will be added to make the final solution.
• Cf is the desired concentration in the final solution.
• Vf is the final volume which includes the volume of product plus the water used to mix the product
• Tip: The units for the concentration and volume must be same on both sides of the equation.

For the examples below, we will assume that we need a final concentration of chlorine dioxide (ClO2) of 2ppm and that the original concentration of the product is 2%. While chlorine dioxide is used as an example, the calculations can be used to determine proper dosage of other products such as pesticides and plant growth regulators.

1. Batch solutions with no injector

Assume you need to make 10 liters of water with a concentration of 2ppm Chlorine Dioxide (ClO2). Usually, ClO2 comes in tablets and the manufacturer will provide instructions on how much water to dissolve the tablets in to reach a desired concentration. Assume that the ClO2 product gives a 2% product concentration when dissolved according to the manufacturer's instructions. To convert from % to ppm we use the conversion ratio of 1%=10,000ppm. Also, remember that 1ppm is the same as saying 1mg dissolved in 1 liter of water (1mg/L). We can start inserting our known variables to the previously shown equation.

• Ci: 20,000mg/L ClO2 (from manufacturer's instructions to dissolve the tablets, 2% in this example)
• Cf: 2mg/L ClO2 (our target concentration)
• Vi: ? (we don't know how much of the concentrated product we need to use)
• Vf: 10L (the final volume of product that we want to make)
• Equation: 20,000mg/L x Vi = 2mg/L x 10L

In this case we do not know how much of the sanitizing product to add to water to reach the final concentration. We need to solve the equation for Vi, which is the right side of the equation divided by Ci.

Vi = (2mg/L x 10L) ÷ 20,000mg/L
Tip: always do the calculations inside the parenthesis first

Vi = 0.001L or 1mL

Add 1mL of the product to 9,999mL of water.

2. Proportioner injector set at 1:100

Injectors take solution from a concentrated stock tank and inject the concentrated solution into the irrigation system. In this example the proportioner is set to 1:100, meaning that of 100 parts of a final solution 1 part came from the concentrated solution. This tells us that the final volume is 100 and the initial volume of concentrated solution is 1. In this case we need to figure out two things, 1- what is the concentration in the stock tank (concentrated solution)? and 2- how much product to dissolve to make the concentrated solution in the stock tank? We make the calculations in two steps:

Step 1: Stock tank concentration

Input the variables in the equation:

• Ci: ?
• Cf: 2mg/L
• Vi: 1L
• Vf: 100L
• Equation: Ci x 1L = (2mg/L x 100L) → Ci = (2mg/L x 100L) ÷ 1L → Ci = 200mg/L

We just calculated that the stock tank that will feed the injector needs a concentration of 200mg/L. The next step will tell us how to make the stock tank solution.

Step 2: Product to add to make the stock tank solution (assume that the stock tank has 10L)

Input the variables into the equation. Notice that this one is similar to the batch solution example discussed previously. Your stock tank is your final solution, and you need to know how much of the product to add:

• Ci: 20,000mg/L
• Cf: 200mg/L (result from previous step)
• Vi: ?
• Vf: 10L (the stock tank volume)
• Equation: 20,000mg/L x Vi = (200mg/L x 10L) → Vi = (200mg/L x 10L) ÷ 20,000mg/L → Vi = 0.1L or 100 mL in 9.9L of water

3. Peristaltic or positive displacement pump

These types of injectors operate using a different principle than mechanical proportioners or venturi injectors. The peristaltic pump pushes concentrated stock solution into a pressurized system. The volume added in each injection stroke will vary depending on the pressure inside the pipes. Peristaltic pumps also have a rated limit on how many injection strokes they can do per minute, and some allow you to change this value. The manufacturer must provide documentation showing how much volume the pump injects per minute at different pressures (this is pump specific). Also, it is recommended that you test and calibrate this value on your farm. To do so, have a known volume of stock solution, run the system and injector for 10 minutes, measure how much stock solution is used, and divide the used volume by 10 minutes which will give you the injection rate per minute.

For this example, assume that your irrigation system has a flow rate of 10L per minute and 29psi. For this example, the pump's documentation shows that the pump can deliver 0.56 to 7.92 mL per minute (depending on system's pressure) and that it can do 120 strokes per minute. The pump injects water to a pressurized system and the higher the pressure the lower the volume it delivers. For this example, we will use the manufacturer charts:

Figure 1 Peristaltic pump flow rate at different pressures.

For this specific pump, at 29psi it delivers 6L per hour or 0.1L per minute. Now that we have collected information, we can make the calculations the same way we did with the proportioner injection.

Step 1: Stock tank concentration (assume a 1-minute run time)

Input the variables in the equation assuming a one-minute run time:

• Ci: ?
• Cf: 2mg/L
• Vi: 0.1L (from the pump manufacturer or the calibration step)
• Vf: 10L (from the system's flow rate, the system's flow rate is assumed to be 10L per minute)
• Equation: Ci x 0.1L = (2mg/L x 10L) → Ci = (2mg/L x 10L) ÷ 0.1L → Ci = 200mg/L

Step 2: Product to add to make the stock tank solution (assume a 10L stock tank volume)

• Ci: 20,000mg/L
• Cf: 200mg/L (result from previous step)
• Vi: ?
• Vf: 10L (the stock tank volume)
• Equation: 20,000mg/L x Vi = (200mg/L x 10L) → Vi = (200mg/L x 10L) ÷ 20,000mg/L → Vi = 0.1L or 100 mL in 9.9L of water

Consult with the manufacturer about compatibility of the injector with certain chemicals. Abrasive materials can increase wear of injector components.

4. Checking injector performance

Injectors wear out over time, impacting their accuracy. Manufacturers will provide replacement parts to maintain the injectors in shape. It is a good practice to check injector performance. The following steps show how to check the performance injector:

• Turn off water flow through the injection system
• Set the injector to 1:100. See table below for different injection ratios
• Disconnect the injector outlet from the downstream irrigation system
• Attach a hose to the injector outlet and place it in an empty 5-gallon bucket
• Measure 6.5 ounces of water in a cup
• Remove the strainer from the edge of the suction line
• Insert the suction line to the cup with the 6.4 ounces of water
• Turn on water flow through the injector
• The injector should suck the 6.4 ounces when the 5-gallon bucket is full

Table 1 Ounces of stock solution injected at different injection ratios for every gallon and 5 gallons of solution exiting the injector.

Service the injector if the values deviate from expected values. Injector manufacturers sell maintenance kits and gaskets to service the injectors.