Arduino function to read a Vegetronix VH400 Soil Moisture Sensor. See http://www.vegetronix.com/Products/VH400/

read_VH400.ino

// MIT License (MIT)
// 
// Copyright (c) 2015. Michael Ewald, GeomaticsResearch LLC.
// 
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
// 
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
// 
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.
//
// Author: Michael Ewald (mewald@geomaticsresearch.com)
// Web: https://GeomaticsResearch.com
// Last-updated: 2015-07-04

float readVH400(int analogPin) {
  // This function returns Volumetric Water Content by converting the analogPin value to voltage
  // and then converting voltage to VWC using the piecewise regressions provided by the manufacturer
  // at http://www.vegetronix.com/Products/VH400/VH400-Piecewise-Curve.phtml
  
  // NOTE: You need to set analogPin to input in your setup block
  //   ex. pinMode(<analogPin>, INPUT);
  //   replace <analogPin> with the number of the pin you're going to read from.
  
  // Read value and convert to voltage  
  int sensor1DN = analogRead(analogPin);
  float sensorVoltage = sensor1DN*(3.0 / 1023.0);
  float VWC;
  
  // Calculate VWC
  if(sensorVoltage <= 1.1) {
    VWC = 10*sensorVoltage-1;
  } else if(sensorVoltage > 1.1 && sensorVoltage <= 1.3) {
    VWC = 25*sensorVoltage-17.5;
  } else if(sensorVoltage > 1.3 && sensorVoltage <= 1.82) {
    VWC = 48.08*sensorVoltage-47.5;
  } else if(sensorVoltage > 1.82) {
    VWC = 26.32*sensorVoltage-7.89;
  }
  return(VWC);
}






struct VH400 {
  double analogValue;
  double analogValue_sd;
  double voltage;
  double voltage_sd;
  double VWC;
  double VWC_sd;
};

struct VH400 readVH400_wStats(int analogPin, int nMeasurements = 100, int delayBetweenMeasurements = 50) {
  // This variant calculates the mean and standard deviation of 100 measurements over 5 seconds.
  // It reports mean and standard deviation for the analog value, voltage, and WVC.
  
  // This function returns Volumetric Water Content by converting the analogPin value to voltage
  // and then converting voltage to VWC using the piecewise regressions provided by the manufacturer
  // at http://www.vegetronix.com/Products/VH400/VH400-Piecewise-Curve.phtml
  
  // NOTE: You need to set analogPin to input in your setup block
  //   ex. pinMode(<analogPin>, INPUT);
  //   replace <analogPin> with the number of the pin you're going to read from.

  struct VH400 result;
  
  // Sums for calculating statistics
  int sensorDNsum = 0;
  double sensorVoltageSum = 0.0;
  double sensorVWCSum = 0.0;
  double sqDevSum_DN = 0.0;
  double sqDevSum_volts = 0.0;
  double sqDevSum_VWC = 0.0;

  // Arrays to hold multiple measurements
  int sensorDNs[nMeasurements];
  double sensorVoltages[nMeasurements];
  double sensorVWCs[nMeasurements];

  // Make measurements and add to arrays
  for (int i = 0; i < nMeasurements; i++) { 
    // Read value and convert to voltage 
    int sensorDN = analogRead(analogPin);
    double sensorVoltage = sensorDN*(3.0 / 1023.0);
        
    // Calculate VWC
    float VWC;
    if(sensorVoltage <= 1.1) {
      VWC = 10*sensorVoltage-1;
    } else if(sensorVoltage > 1.1 && sensorVoltage <= 1.3) {
      VWC = 25*sensorVoltage-17.5;
    } else if(sensorVoltage > 1.3 && sensorVoltage <= 1.82) {
      VWC = 48.08*sensorVoltage-47.5;
    } else if(sensorVoltage > 1.82) {
      VWC = 26.32*sensorVoltage-7.89;
    }

    // Add to statistics sums
    sensorDNsum += sensorDN;
    sensorVoltageSum += sensorVoltage;
    sensorVWCSum += VWC;

    // Add to arrays
    sensorDNs[i] = sensorDN;
    sensorVoltages[i] = sensorVoltage;
    sensorVWCs[i] = VWC;

    // Wait for next measurement
    delay(delayBetweenMeasurements);
  }

  // Calculate means
  double DN_mean = double(sensorDNsum)/double(nMeasurements);
  double volts_mean = sensorVoltageSum/double(nMeasurements);
  double VWC_mean = sensorVWCSum/double(nMeasurements);

  // Loop back through to calculate SD
  for (int i = 0; i < nMeasurements; i++) { 
    sqDevSum_DN += pow((DN_mean - double(sensorDNs[i])), 2);
    sqDevSum_volts += pow((volts_mean - double(sensorVoltages[i])), 2);
    sqDevSum_VWC += pow((VWC_mean - double(sensorVWCs[i])), 2);
  }
  double DN_stDev = sqrt(sqDevSum_DN/double(nMeasurements));
  double volts_stDev = sqrt(sqDevSum_volts/double(nMeasurements));
  double VWC_stDev = sqrt(sqDevSum_VWC/double(nMeasurements));

  // Setup the output struct
  result.analogValue = DN_mean;
  result.analogValue_sd = DN_stDev;
  result.voltage = volts_mean;
  result.voltage_sd = volts_stDev;
  result.VWC = VWC_mean;
  result.VWC_sd = VWC_stDev;

  // Return the result
  return(result);
}

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