V2eps: Convert sensor voltage to epsilon and v.v. using...
In TillF/FDR2soilmoisture: Processing data of FDR-measurements to obtain soil moisture

View source: R/v2eps2v.R

V2eps

R Documentation

Convert sensor voltage to epsilon and v.v. using manufacturer's specifications.

Description

Convert sensor voltage to epsilon and v.v. using manufacturer's specifications. Defined for Theta Probe and PR2 (DeltaT).

Usage

V2eps(V, type)
eps2V(eps, type)

Arguments

`V`	(corrected) sensor voltage [V]. Raw sensor output should be corrected first using `correct_sensor_voltage`.
`eps`	permittivity epsilon [-].
`type`	Type of sensor. Must be `"Theta Probe"`, `"Theta Probe polynomial"` OR `"PR2"`. See details for explanation.

Details

Applies the manufacturers conversion functions or its inverse. V: voltage in [V]; eps: epsilon [-]

Values for type:

"PR2": PR2 default equation according to manual (PR2\_user\_manual\_version\_5.0.pdf, eq. 2), (PR2\_SDI-12-\_User\_Manual\_version\_4\_1.pdf, eq. 2):

\sqrt{\epsilon} = 1.125 - 5.53*V + 67.17*V^2 - 234.42*V^3 + 413.56*V^4 - 356.68*V^5 + 121.53*V^6
"Theta Probe polynomial": Theta Probe polynomial* (ThetaProbe user manual, eq. 3.):

\epsilon = 1.07 + 6.4*V - 6.4*V^2 + 4.7*V^3

Don't use for voltage > 1 V or theta > 0.5, see below!
"Theta Probe": interpolated lookup data table from ThetaProbe user manual table (p. 14), see details below

Value

Permittivity value (epsilon) [-] or sensor voltage [V].

Warning

*"Theta Probe polynomial" from ThetaProbe user manual:

p. 10 states: “In the range 0 to 1 Volt (corresponding to a soil moisture range 0 to ~0.55 by volume), this relationship can be fitted very precisely by a 3rd order polynomial”
p. 11 states: “For very high moisture contents (\theta \,>\,0.5\, m^3 \cdot \, m^{-3}), the polynomial equation should be used. This is usually only necessary for organic soils.”

The statement on p. 11 is obviously wrong (see 'example 1', below): The polynom obviously differs considerably from the points in the linearization table, especially for voltage > 1 V or theta > 0.5, so only the statement on page 10 seems correct.

-> For U > 1 V, use "Theta Probe" (i.e. based on the table) rather than "Theta Probe polynomial".

Author(s)

Till Francke

References

Delta-T Devices Ltd, 1998. ThetaProbe SOIL MOISTURE SENSOR TYPE ML2 USER MANUAL (ML2-UM-1). Delta-T Devices Ltd, Cambridge, UK.

Delta-T Devices Ltd, 2016. User Manual for the Profile Probe type PR2 (PR2-UM-5.0). Delta-T Devices Ltd, Cambridge, UK.

Examples

V2eps(V= 1.1, type = "Theta Probe") #convert to epsilon, eq. 1 of Theta-Probe user manual

  eps_air =1.00059     #permittivity of air
  eps_h2o =81          #permittivity of water at 20 degree C
  #display voltages that would be expected when measuring in air and water
  eps2V(eps_air, type = "Theta Probe")  
  eps2V(eps_h2o, type = "Theta Probe") 


# example 1: reproducing the figure on p.11,  Theta-Probe user manual (voltage vs. sqrt(eps)) ####
  V = seq(from=0.0, to=1.3, by=0.02)
  eps = V2eps(V, type = "Theta Probe polynomial") #polynomial equation (eq. 1)
  
  plot(1, xlim=c(0, 1.4), ylim=c(0,9), type="n", 
  xlab="voltage [V]", ylab="sqrt(epsilon) [-]", 
  main = "Theta Probe, voltage-to-epsilon conversion")
  lines(V, sqrt(eps), ylim=c(0,9), type="l", lty=2, col="red")
  abline(a=1.1, b=4.44, col="green") #linearized version (eq. 2)
  eps = V2eps(V, type = "Theta Probe") #polynomial equation (eq. 1)
  lines(V, sqrt(eps), ylim=c(0,9), type="l", lty=1, col="red")
  
 
  abline(v=1, lty=2) #show  extent of the original figure
  abline(h=6, lty=2)
  
  legend("topleft", legend=c("polynomial", "linearized", "lookup, linear interpolation",
  "extent of orig. figure", "calibration data"),
         lty=c(2,1,1,2,0), pch=c(NA,NA, NA, NA, 21),
         col=c("red", "green", "red", "black", "blue"))
  


# example 1b: add examples from device-specific calibration to plot ####
  #replace this by your own calibration file
  calib_file = system.file("example", "sensor_calibration.txt", package = "FDR2soilmoisture")
  #load the file
  calib_data = read.table(calib_file, nrow=-1, sep="\t",
  stringsAsFactors = FALSE, header=TRUE, na.strings = c("NA",""))  
  
  calib_data = calib_data[calib_data$type=="Theta Probe",]
  
  points(calib_data$air_measurement/1000, rep(1, nrow(calib_data)), col="blue")
  points(calib_data$water_measurement/1000, sqrt(rep(81, nrow(calib_data))), col="blue")

TillF/FDR2soilmoisture documentation built on Dec. 6, 2024, 9:30 a.m.