11.3 Calibration Worked Scenarios

Scenario A — 60-foot wheat boom at 8 mph

Quick Answer: With 0.4 GPM nozzles, 8 mph, and 20-inch spacing, GPA = 14.85. A 300-gallon tank mixed at 15 GPA covers 20 acres and, at 1.5 pints per acre, needs 3.75 gallons of product. A 7A 2-gallon backpack at 1 percent takes 2.56 fl oz of product. Mixing for more acres than the tank will actually cover over-applies.

North-central Oklahoma winter wheat. Self-propelled sprayer, 60-foot boom, 20-inch spacing, travel 8 mph, catalog tips 0.4 GPM at the pressure you will use. Compute GPA, then prove it a second way.

Path 1 — per-nozzle formula

GPA = (GPM × 5,940) / (MPH × W) GPA = (0.4 × 5,940) / (8 × 20) 0.4 × 5,940 = 2,376 8 × 20 = 160 GPA = 2,376 / 160 160 × 14 = 2,240 Remainder 136 136 / 160 = 0.85 GPA = 14.85 gallons per acre

Path 2 — whole boom

Nozzles = (60 × 12) / 20 = 720 / 20 = 36 nozzles Total flow = 36 × 0.4 = 14.4 GPM Speed = 8 × 88 = 704 feet per minute Area per minute = 704 × 60 = 42,240 sq ft/min Acres per minute = 42,240 / 43,560 43,560 × 0.97 = 42,253.2 (0.002 acre high) 42,240 / 43,560 = 42240 ÷ 43560 = 4224 / 4356 = 117.333 / 121 = 0.9697 acre/min GPA = 14.4 / 0.9697 = 14.85 GPA

Both paths agree. If this is the catch-tested number, mix from 14.85 GPA, not from a catalog guess of 10 or 20.

Compare with the 6 mph, 0.3 GPM example in Section 11.1 (also 14.85 GPA). Same GPA can come from different speed-and-tip pairs. What you may not do is keep 0.4 GPM tips, speed up from 8 mph to 10 mph, and still claim 14.85 GPA:

GPA at 10 mph = (0.4 × 5,940) / (10 × 20) = 2,376 / 200 = 11.88 GPA

11.88 / 14.85 = 0.80, a 20 percent underdose of both carrier and product if the tank was mixed for 14.85 GPA and you simply drove faster.

Scenario B — mix the 300-gallon wheat tank at 15 GPA and 1.5 pt/acre

After calibration you decide to mix at 15 GPA (a 1 percent rounding from 14.85 that you write on the tank chart) and the product rate is 1.5 pints per acre.

Step 1. Acres per fill Acres = tank gallons / GPA = 300 / 15 = 20.00 acres

Step 2. Product per fill Pints = 1.5 pt/acre × 20 acres = 30 pints Gallons of product = 30 / 8 = 3.75 gallons 3.75 gallons = 3 gallons + 0.75 gallon = 3 gallons + 3 quarts (because 0.75 × 4 = 3 quarts)

Step 3. Water (carrier) Carrier ≈ 300 − 3.75 = 296.25 gallons of water, plus the 3.75 gallons of product, to make 300 gallons of mixture. (For a 1.5 pint/acre EC this volume replacement is small; still add product to a partly filled tank and then bring to volume so it disperses.)

Step 4. Whole-field check for 80 acres Tank loads = 80 / 20 = 4 fills Total product = 4 × 3.75 = 15 gallons of product Check: 1.5 pt/acre × 80 acres = 120 pints; 120 / 8 = 15 gallons. Matches.

If you had mixed from the unrounded 14.85 GPA instead: Acres per 300 gal = 300 / 14.85 = 20.20 acres Product = 1.5 × 20.20 = 30.30 pints = 3.7875 gallons The 15 GPA rounding changes product by 0.30 pint per fill, about 1 percent. That is a documented rounding. It is not the same as mixing for 10 GPA.

Scenario C — Oklahoma turf hose-end, 1,000 sq ft units

Category 3 lawn care, Oklahoma City bermuda, hose-end or pump sprayer. Measured lawn 80 ft × 75 ft.

Square feet = 80 × 75 = 6,000 sq ft Thousands of sq ft = 6,000 / 1,000 = 6.0 Acres = 6,000 / 43,560 = 0.1377 acre (43,560 × 0.1377 ≈ 5,998)

The label says apply 2.0 fl oz of product per 1,000 sq ft in 1 gallon of water per 1,000 sq ft.

Product = 2.0 × 6.0 = 12.0 fl oz of product 12.0 fl oz = 12 / 128 = 0.09375 gallon of product, or 12 / 16 = 0.75 pint Carrier water = 1 gal × 6.0 = 6.0 gallons of water (plus the small product volume in the hose-end jar, following the product's mixing order)

GPA equivalent of 1 gallon per 1,000 sq ft: 43,560 / 1,000 = 43.56 GPA Check: 6.0 gallons / 0.1377 acre = 43.6 GPA. Same delivery, different unit.

If the same 12 fl oz of product were mixed into only 3 gallons and still sprayed over 6,000 sq ft, the area dose of product would still be 2 fl oz/1,000 sq ft, but the coverage (wetting) would be half as much water — a calibration/coverage problem, not a product-math freebie. Hose-end jobs go wrong when someone scales product to the lot but forgets to scale water, or scales water and pours a whole bottle of concentrate.

Scenario D — Category 7A two-gallon backpack, percent mix

Structural / general pest technician, 2-gallon backpack, label calls for a 1 percent by volume spray for the listed site.

Ounces of product = (tank gallons × percent × 128) / 100 Ounces = (2 × 1 × 128) / 100 = 256 / 100 = 2.56 fl oz

Check from gallons: 1 percent of 2 gallons = 0.02 gallon; 0.02 × 128 = 2.56 fl oz Check from the 1.28 oz/gal rule: 1.28 × 2 = 2.56 fl oz

A 2 percent mix in the same tank: (2 × 2 × 128) / 100 = 512 / 100 = 5.12 fl oz

A 1.25 percent mix: (2 × 1.25 × 128) / 100 = (2.5 × 128) / 100 = 320 / 100 = 3.20 fl oz

Measure with a graduated cylinder, not a "capful." On a 2-gallon tank, doubling 2.56 to 5.12 because the job "looks dirty" is a doubled dose on a perimeter that may border a sidewalk or play area. Percent mix is still a labeled rate.

Scenario E — pasture ATV boom

Eastern Oklahoma bermuda pasture, ATV with a 20-foot boom, 20-inch spacing, 0.2 GPM tips, 10 mph in fourth gear on a measured course.

Nozzles = (20 × 12) / 20 = 12 nozzles GPA = (0.2 × 5,940) / (10 × 20) = 1,188 / 200 = 5.94 GPA

Whole-boom check: Total flow = 12 × 0.2 = 2.4 GPM Speed = 10 × 88 = 880 ft/min Area/min = 880 × 20 = 17,600 sq ft/min Acres/min = 17,600 / 43,560 = 0.4040 acre/min GPA = 2.4 / 0.4040 = 5.94 GPA

15-gallon ATV tank: Acres per fill = 15 / 5.94 = 2.525 acres If the pasture herbicide is 2 pints per acre: Product per fill = 2 × 2.525 = 5.05 pints ≈ 5 pints + 0.8 fl oz (0.05 pint × 16 fl oz/pint = 0.8 fl oz)

Rounding 5.94 GPA to 6 GPA for mixing: 15 / 6 = 2.50 acres; product = 2 × 2.50 = 5.00 pints. That 1 percent rounding is honest. Skipping the catch test and mixing as if the ATV threw 10 GPA would put 2 pt × (15/10) = 3 pints in the tank that actually covers only 2.525 acres — an underdose — or, if someone mixed for 4 GPA, an overdose. The next subsection is that trap in the overdose direction.

Granular pasture work on the same ATV does not use 5.94 GPA. Catch pounds on a known swath as in Section 11.2.

The over-application trap: too much product in too few acres

Product per tank is computed from acres the tank will cover. Acres the tank will cover = tank gallons / actual GPA. If actual GPA is higher than the GPA you mixed for, the tank empties on fewer acres, and every extra pint you added is now concentrated on that smaller acreage.

Worked overdose (wheat tank): You mixed the 300-gallon tank as if the boom applied 15 GPA, so you added product for 300 / 15 = 20 acres. Catch tests later show the boom is actually applying 20 GPA (slower pass, worn tips, or a pressure increase you did not recalculate).

Acres actually covered = 300 / 20 = 15 acres Product in the tank was for 20 acres Dose factor = 20 / 15 = 1.333 That is a 33 percent overdose (one-third extra product per acre).

Slower speed alone can do this. Scenario A at 8 mph was 14.85 GPA with 0.4 GPM and 20-inch spacing. Slow to 6 mph without changing tips:

GPA = (0.4 × 5,940) / (6 × 20) = 2,376 / 120 = 19.8 GPA 300 gallons now cover 300 / 19.8 = 15.15 acres instead of about 20.2 acres at 14.85 GPA 20.2 / 15.15 ≈ 1.33, again about a 33 percent overdose if the product was already in the tank.

The other direction is an underdose: mix for 14.85 GPA, then speed up to 10 mph (11.88 GPA). The tank covers more acres than you bought product for. That fails the pest, and it tempts a second, off-label pass.

Rule: mix after you know GPA. If speed, pressure, nozzles, or boom height change, recompute acres per tank before the next fill. Do not open the valve to "make it look right" and keep pouring 3.75 gallons of product into 300 gallons as if nothing changed.

Practice checks below use new numbers. They are not these five jobs restated.