Hello
I programmed 6 CS655 TDR on the same com port (C7) , in clay soil (5cm,10cm,25,cm,35cm,50cm,80cm) , and they give me (50cm and 80cm) value of volumetric water content and permittivity of the orde of 9999. it's too late to buy an A200 because i have already installed it, but we did well to take soil samples with each sample CS655 TDR measurements for calibration, can i use the time think back to each TDR to calculate volumetric water content (as for the CS616 , VWC = C0 + C1 + C2 × × TIME TIME ^ 2 ) ?
If your CS655 are not using OS2, updating the OS on them might be enough to get valid readings. OS version 2 increased the range of EC over which the CS655 could give valid readings. The A200 is the simple and recommended way to update the OS. You would need to connect to the RS232 leads on the sensor, which I understand could be in a junction box depending on your installation.
Are your time period readings also 9999?
If you aren't reading time period, it is a simple program change to a different SDI12 command.
There is a well documented method to calibrate to your samples in a lab. Refer to chapter 8.2 of the CS616 manual. The section isn't in the CS655 manual, because users generally don't have to calibrate them.
for now, we can not buy and wait for the A200 .. we don't have problem (9999) in time period, it ok .. for the moment, can i juste use the time period for each TDR to calculate volumetric water content (as we do for the CS616 in CRBasic , VWC = C0 + C1× time period + C2 × time period ^ 2 ) ?
You will need to do a soil specific calibration to get coefficients for the equation. The conductivity of your soil is too high for the standard coefficients found in the CS616 manual and stored internally in the CS655.
Chakir,
As Jacob said, you can do a soil specific calibration that converts period directly to water content. You will need at least three data points covering the entire range of the soil. Five data points is better. A quadratic fit as you described above should be fine. A best fit with a power function should also work well.
If your soil is extremely electrically conductive then it is possible that the period value will not change much over the full range of water content. That would make calibration difficult. In that case it is possible to estimate water content using the bulk EC measurement obtained from your calibration work. The relationship between water content and bulk EC is a soil specific power function of the form:
EC = C1 * (VWC)^C2 + C0
or solving for VWC:
VWC = EXP((LN((EC - C0)/C1))/C2)
where EC is bulk EC measured by the CS655, VWC is fractional volumetric water content measured by oven drying of samples, and C0, C1, and C2 are soil specific fitting parameters.
Hello Notso
You have right about the period value, it change not much over the full range of water content. but what do you mean by "it is possible to estimate water content using the bulk EC measurement obtained from your calibration work" ?
In dry soil vwc=0 that mean that
C0= EC (dry soil) ... Is this what you want to mean Notso by " using the bulk EC measurement obtained from your calibration work" ?
Chakir,
Sorry for not answering sooner. I have not checked the forum for a while. Yes in dry soil where VWC = 0, C0 is the bulk EC reading. It will be very close to zero. To determine C1 and C2 you will need to measure bulk EC at two other water contents. Normally you should use saturated soil for one of those measurements and some water content between dry and saturated for the other measurement. Use oven drying to determine the actual volumetric water content for the soil and then solve for the values of C1 and C2.