The orifice chart on a TeeJet CP4916 package is honest about one thing in small print: the flows are for water. 28% UAN weighs 10.65 lb/gal, 32% weighs 11.06, and 10-34-0 weighs 11.65 — all a good deal heavier than water's 8.34. Heavier liquid flows slower through the same hole at the same pressure, so a plate picked straight off the chart under-applies. On a sidedress pass that's nitrogen you paid for and didn't put down. The fix is one square root. Here's the method, or skip it and use the Fertilizer Orifice Plate Calculator, which does the correction against the plates we have in stock.
1. Flow per orifice, corrected for row setup
Same formula as a spray tip, but the spacing is per orifice, not per row:
GPM per orifice = (GPA × MPH × effective spacing) ÷ 5,940
where effective spacing is row spacing divided by nozzles per row. One orifice per 30-inch row is 30 inches. A y-drop or a dual-outlet body with two orifices per row is 15. Get this wrong and you're off by a factor of two before you've started.
Example: 15 GPA of 28% at 6 mph, 30-inch rows, one orifice per row = (15 × 6 × 30) ÷ 5,940 = 0.45 GPM of product per orifice.
2. Convert to a water-equivalent flow
Water-equivalent GPM = product GPM × √(lb/gal ÷ 8.34)
For 28% UAN the factor is √(10.65 ÷ 8.34) = 1.13. For 32% it's 1.15, and for 10-34-0 it's 1.18. So our 0.45 GPM of 28% needs a plate that flows 0.51 GPM of water at the pressure you'll run. That's the number you look up on the chart.
The intuition: the chart says how much water a hole passes. To push the same volume of a heavier liquid, you need a bigger hole (or more pressure) by the square root of the density ratio. Ignore it and a 28% pass runs about 12% light; 10-34-0 runs about 15% light.
3. Pick the plate from the middle of the chart
The CP4916 chart runs 5 to 60 psi. Several plates will hit 0.51 GPM somewhere in that range; pick the one that hits it between roughly 15 and 45 psi. At the bottom of the chart a small pressure change is a big flow change, and orifice streams break up and blow around at the top. In our example a #68 plate lands at about 31 psi; a #75 would do it at about 21, and a #63 would need about 43. Any of those work — the #68 leaves the most room for the rate controller in both directions.
Every plate on our site lists the full 5–60 psi chart, and the calculator shows the same chart re-computed in GPA of your product at your speed and spacing, with the matching cells shaded.
4. Strainers and orientation
Orifice plates plug. Match the tip strainer to the plate: #15 and smaller → 200 mesh; #16–39 → 100 mesh; #40–70 → 50 mesh; #72 and up → no strainer needed. Install the plate with the stamped number facing the outlet (toward the ground). Backwards, the sharp edge is on the wrong side and the flow is off from the chart.
5. When to run a VR cap instead
A fixed plate holds rate over roughly a 1.5× speed range before the pressure leaves the usable band. If your sidedress rig runs 4 mph on contour ends and 8 in the open, or if one applicator has to cover several rates in a day, look at a TeeJet SJ7A-VR or SJ3-VR variable-rate StreamJet cap. Its orifice opens as pressure rises, so one size covers 2–3× the flow range of a fixed plate between 30 and 80 psi, and the rate controller holds the target with no plate swaps. The calculator shows the speed window each VR size covers at your rate. They're more money per outlet than a $2.75 plate; they earn it back the first time you don't stop to change plates.
6. Calibrate anyway
Charts are for a clean plate, into open air, at 70°F. Fertilizer temperature changes density and a coated plate changes the hole. Before the season, catch one outlet for 30 seconds at your working pressure, weigh or measure it, and compare to the corrected target. Do three or four across the bar. More than about 5% off across the board means your solution weight is different from the label — adjust the input, not the plates. One outlet off means a plugged strainer or a worn plate.
We stock CP4916 stainless plates in the full range and both VR cap sizes. Run the Fertilizer Orifice Plate Calculator, or call (509) 838-2209 with your rate, speed, spacing and product and we'll size it on the phone.
Spray Center Electronics is an authorized TeeJet dealer in Spokane, WA. Solution weights are typical values; UAN and polyphosphate density varies with concentration and temperature, so confirm with your supplier's spec and a catch test.