Jar Test Says 18 GPA: Fix It with Speed, Pressure, or New Tips

First, the math.

• On 20-inch spacing, 1/128 acre per nozzle is a 204-foot course, so the ounces caught per nozzle equal the gallons per acre.
• Average catch = (17 + 19 + 18) / 3 = 18 oz → 18 GPA, which is 20% above the 15 GPA target.

Option A - speed up. Flow per nozzle is fixed at a given pressure, so GPA changes inversely with speed:

• New speed = 6.5 × (18 / 15) = 7.8 mph. Simple, requires no parts, and stays within the nozzle's pressure range. Verify with a second catch and check that the higher speed doesn't produce a poor pattern on the boom.

Option B - drop pressure. Flow varies with the square root of pressure:

• (15 / 18)² = 0.694, so 40 × 0.694 = 27.8 psi - below the catalog's 30 psi pattern minimum. Reject: you would trade a rate error for a streaking, poorly overlapped pattern and more drift-prone fines.

Option C - smaller tips. Correct, but requires buying and swapping 36 tips mid-season.

Decision: run at 7.8 mph, or change tips if the tractor cannot hold that speed reliably in the field.

Second issue: three nozzle catches do not calibrate a 36-nozzle boom. Catch every nozzle or at least a representative spread across the boom and replace anything more than ±10% off the average. The 17-to-19 spread suggests a partly worn or partially plugged tip.

Full tank:

• 500 gal ÷ 15 GPA = 33.3 acres per tank
• Product: 2 pt/acre × 33.3 acres = 66.6 pt ÷ 8 pt/gal = 8.3 gal of product
• Surfactant at 0.25% v/v of the finished spray volume: 0.0025 × 500 gal = 1.25 gal

Field total: 88 acres ÷ 33.3 = 2.64 tanks → two full tanks plus a partial.

• Acres remaining after two full tanks: 88 − 66.6 = 21.4 acres
• Water: 21.4 × 15 = 321 gal
• Product: 21.4 × 2 pt = 42.8 pt = 5.35 gal
• Surfactant: 0.0025 × 321 = 0.8 gal

Judgment points:

• Mixing a full 500 gallons when only 21 acres remain creates a leftover-disposal problem, which is a legal problem. Size the last load to the acres left.
• Measure the partial load against the water volume, not against a full tank, or the surfactant and product will be over-applied on the last pass.
• Acres per tank depends on the corrected GPA. If the boom is still running 18 GPA, 500 gal covers only 27.8 acres and the product rate per tank must be recalculated - an off-rate caused by a calibration error is still an off-label application.

Before anything goes in:

• Read both labels for tank-mix prohibitions and for adjuvant requirements; run a jar compatibility test with the actual carrier water if this combination is new or the water source has changed. Nebraska water varies enough in hardness and pH that a mix that worked 20 miles away can curdle here.
• Confirm the tank and lines are clean of the last product - a residual herbicide from the previous job is one of the most common causes of crop injury.

Order of addition (with this tank's products):

1. Fill the tank about half full with water and start agitation.
2. Add dry formulations first if any are used (water-dispersible granules, then wettable powders). None here.
3. Add the emulsifiable concentrate - the liquid formulation goes in before the adjuvant.
4. Add the nonionic surfactant last, after the tank is nearly full, with agitation running.
5. Top to the final volume, keep agitation going, and check for an even, milky mix with no oil film or lumps.

Handling:

• Load on a pad or with containment; never at the edge of a ditch or within 50 feet of a well.
• Triple-rinse or pressure-rinse the jugs and pour the rinseate into the tank, then rinse the tank in the field if the label permits.
• Wear the label PPE for the concentrate - gloves and eye protection at minimum, apron if the label says so. Mixing is the high-exposure step even with a "low-toxicity" product.