What is water-sensitive paper?
Water-sensitive paper is a rigid paper with a specially coated, yellow surface which will be stained dark blue by aqueous droplets impinging on it. It has been developed for field use by Syngenta for the quick evaluation of LV sprays. For droplet assessment aqueous sprays no longer need the addition of dye. Just place the papers in the target area before spraying. Following exposure to the spray the water-sensitive papers will be stained. Retrieve the papers as soon as they have become dry. Check the droplet pattern. For a quick estimate compare the exposed collectors with a known standard or count the droplets either using a hand lens or an automatic image analyzer.
Water-sensitive paper before exposure to the spray.
Water-sensitive paper after exposure. (Courtesy Spraying Systems Co.)
Where to use it
Water-sensitive paper can be used for checking spray distribution, droplet density from aerial and ground spray applications and droplet sizing.
Overdosing is a waste of product. With herbicides it might result in crop damage and claims. With insecticides underdosing might not kill the pest. Calibrate the sprayer and check the spray pattern. Water-sensitive paper helps you to keep the environment clean.
In aerial LV applications: place cards at 2 to 3 m intervals over three run widths. Fix cards on a rigid horizontal support slightly above the ground or just above the crop canopy.
Airblast sprayers in orchards: staple water-sensitive paper directly onto leaves at the periphery and inside the canopy at the top, in the center and lower part of the trees.
Number the collectors consecutively before placing them on the supports. This will help you to spot irregularities in your spray system when evaluating the exposed cards.
The correct boom height can also be determined with water-sensitive paper. Insufficient overlapping of the spray pattern can be corrected by raising the boom. Excessive overlapping is leveled out by lowering the boom.
Field sprayer applying LV sprays: staple water-sensitive paper onto a wooden lath and place it across a single run width. Leave gap for tractor wheels.
Making safe and optimal use of water-sensitive paper
- Cards should not be retrieved while the deposit is still wet;
- Appropriate personal protective equipment should be worn when collecting and handling exposed samples;
- Gloves should always be used when handling the paper to avoid staining and contamination;
- Water-sensitive paper should be stored before and after spray exposure under dry conditions in airtight bags or boxes;
- Exposed water-sensitive paper should be disposed of responsibly after use.
Visual assessment of droplet densities
Compare your spray samples with some known standard. The standard cards below and on the following page cover the range of acceptable droplet densities for coarse and medium LV spray. The droplet density in the target area should not be less than:
For routine checking of sprays you might also prepare your own standard cards by selecting spray cards with known droplet densities from previous spray operations.
Standard cards with a known droplet density per cm square
Computer-plotted standard cards displaying the expected number and sizes cards spraying at 3 different volume rates (20, 30, 40 l/ha) and using 3 different droplet spectra (VMD, 200, 300, 400 μm) assuming waters sprayed and the spread factor is two.