A Salinity Meter from a Battery, Two Nails and a Multimeter: Cell Design, a Weighable Salt Calibration, and the Crop Decisions It Can Certify

Salinity decisions on a small farm — whether irrigation water is usable, whether a soil is too salty for beans but acceptable for barley — are decisions about electrical conductivity (EC), and commercial EC meters are often the one instrument a smallholder does not have. This paper designs a conductivity cell from a 1.5 V battery, two steel nails and a handheld multimeter, and computes what it can certify. The nails (3 mm diameter, 50 mm immersed, surfaces 20 mm apart in a drilled template block) give a nominal cell constant of 0.424 cm⁻¹ and a resistance range from 4244 ohms at 0.1 dS/m to 26.5 ohms at 16 dS/m, read as a voltage across the cell in series with a 330 ohm resistor. The dominant error is temperature, not electronics: conductivity rises 2.0 percent per degree Celsius, so an uncorrected sample at 15 °C reads 20 percent low — a soil extract exactly at the bean salinity threshold of 1.0 dS/m reports as 0.8 dS/m. Electrode polarisation under direct current is controlled by taking opposed-polarity readings within 2 seconds and averaging (stated residual 1 percent, against 12 percent for a reading held 10 seconds). Calibration uses a two-step table-salt standard originated for kitchen equipment — 5.00 g of salt made to 1 litre, then diluted tenfold — giving a 0.500 g/L standard of 1.01 dS/m with 3.46 percent uncertainty, where weighing 0.5 g directly on the same scale would carry 20 percent. The assembled error budget for liquids is ±5.3 percent (root-sum-square), which separates all four stated crop thresholds (bean 1.0, tomato 2.5, wheat 6.0, barley 8.0 dS/m) with non-overlapping bands when the cell measures water or a saturation extract directly; through a 1:1 soil:water extract, the soil-dependent conversion factor widens the budget to ±25.6 percent and the wheat and barley bands then overlap, so that route screens but does not certify. This study is simulated, not measured.