Calibration for the Watermark 200SS Soil Water Potential Sensor to fit the 7-19-96 “Calibration #3” Table from Irrometer
The calibration data from calibration table “#3” of Irrometer utilize a linear relationship between resistance (R) and Potential (P) for P > -10 kPa. A curvilinear relationship with much flatter initial slope is used for greater (more negative) potentials. A similar, steep relationship between P and R was observed experimentally by Shock et al. (1998).
The best procedure for reproducing the Irrometer data table, which represents a sensor temperature of 24°C (75°F) was to divide the resistance range into 0 £ R £1 kOhm, 1 < R £ 8 kOhm, and 8 kOhm < R. A linear function is used for 0 £ R £ 1 kOhm, an equation (no. 8) by Shock et al., (1998)  is used for 1 < R £ 8 kOhm, and a quadratic equation developed here is used for resistance greater than 8 kOhms.
The equation 8 by Shock fit the 1 < R £ 8 kOhm range better than any other equations developed and tested. However, the Shock equation, developed using experimental data in the range of –10 ³ P ³ -75 kPa, deviated from the Irrometer table (the Irrometer table included data in the range of 0 ³ P ³ -200 kPa) at values of P less than about –100 kPa.
The 0.018*(T-24) term represents the 1.8% shift in resistance reading per one degree C change in temperature from the 24°C base that is recommended by Irrometer. The 1.8% per C is equivalent to 1% per degree F.
For 1 kOhm £ R £ kOhm, equation 8 of Shock et al. (1998) is used:
For measured resistance > 8 kOhm, the following quadratic equation is used:
where, again, the 1+0.018*(T-24)) adjusts for sensor temperature. Equation 3 was determined by least squares regression from the Irrometer table for the range of 10 kPa > P > -200 kPa. The equation had a coefficient of determination r² = 0.9996 and standard error of estimate of 1.07 kPa.
 From Irrometer Company, Inc., Riverside, CA via M.K. Hansen Company, E. Wanatchee, WA, October, 2000.
 Shock, C.C., J.M. Barnum, and M. Seddigh. 1998. Calibration of Watermark soil moisture sensors for irrigation management. Proceedings of the 1998 Annual Meeting of the Irrigation Association. p. 139-146.
Figures showing the fit of the various calibration equations for different graphical scales (these are all of the same plot), where the "segment" curve is the combination of equations 1-3.