Industrial RS485 Water Quality Ammonia Nitrogen & pH Sensor

Both sensors use Modbus register writes for calibration — no physical trim pots.
For the SEN0711 ammonia nitrogen sensor, it's two-point via function code 0x10. Soak the probe in your low standard (say 1 mg/L) for at least 15 minutes until the reading stabilizes, then write 0x0001 to register 0x1200 and 100× the standard value to 0x1201. Rinse, repeat with the high standard (say 10 mg/L), writing 0x0002 and 100× that value. The two standards should be about 10× apart. pH works the same way through registers 0x1100 and 0x1101 — first point writes 0x0003 plus 100× your buffer pH, second point writes 0x0004 plus 100× the second buffer pH.

For the SEN0680 DO sensor, you need a zero-oxygen solution and a 100 % saturation environment. Mix 5 g of anhydrous sodium sulfite into 95 g of distilled water for the zero point, let it sit for an hour. For the 100 % point, bubble air through distilled water for an hour then let it rest 30 minutes. Once the readings stabilize in each environment, write 0x0001 to register 0x1010 for zero calibration and 0x0002 for the 100 % point. Keep the fluorescent film about 1 cm above the water surface during calibration so it stays moist without water droplets.
The full register tables and example frames are on the SEN0711 and SEN0680 wiki pages.