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12 January 2011 at 7:53pm
I have many questions about the 5TE sensor.
I am using the 5TE sensor with a wireless station that gives an excitation voltage of 2.5 volts. I try to take readings in the same medium and I get different readings with different sensors. The readings are off by 7-9%. is this the variability between sensors? or what could be the reason?
My other questions is about Coir; what is the latest calibration equation for coir?
What are the limits on measurement for the sensors? I noticed that EC5 reads up to 50% while the 5TE in a really wet media can display 70-90% water content.
I also would appreciate it if you could send me the latest and greatest of the calibration equations that you have.(email@example.com)
and lastly do you have any information on converting the Bulk EC to pore water EC while taking into account the moisture level and soil temp? or is it possible to convert Bulk EC to EC of the solution?
Thank you for your answers and apologize asking too many questions
13 January 2011 at 11:46am
Don't worry about asking too many questions, that's one of the reasons why I'm here. The voltage requirements for the 5TE are 3.6 - 15 VDC. It is underpowered at 2.5 V. What type of datalogger are you using? If your logger has a SW-12 V port we recommend using that for the 5TE.
The limits on our sensors are primarily determined by the calibration you are using. The 5TE uses the Topp equation which essentially gives you the entire possible range of volumetric water content (VWC) from 0 to 1. Our standard mineral calibration for the EC5, on the other hand, maxes out at 0.62 m3/m3. The EC5 is capable of measuring VWC up to 1, but the upper end of that range is nonlinear so a different calibration is required. A 2nd or 3rd order polynomial will fit best from 0.60 to 0.85 m3/m3.
We implement the Hilhorst model for estimating pore water EC from bulk dielectric and bulk electrical conductivity. It does take VWC, EC, and temperature into account. Please refer to the 5TE manual page 7 (http://www.decagon.com/assets/Manuals/5TE-Manual.pdf) for more information on implementing this. It is a model based on assumptions for which there are some ins and outs, a lotta what-have-you's, but it's a way to estimate in situ pore water EC. You can get the citation for the published work on this in the 5TE manual.
The online manuals for the individual sensors have the most extensive information about the available calibrations, but you might check out this application note that summarizes calibrations for some of our sensors: http://www.decagon.com/assets/Uploads/13392-03-AN-Calibration-Equations-for-the-ECH2O-EC-5-ECH2o-TE-and-5TE-Sensors.pdf
We do not have a Decagon calibration for coir. We have worked with it a little bit, it is possible to use Decagon sensors with coir, but we are not sure how reproducible it is. I recommend that you perform a custom calibration with your specific substrate and potting method to get the best results. You can find our procedure for calibrating here (http://www.decagon.com/education/calibrating-ech2o-soil-moisture-sensors-13393-04-an/). We do provide a lab service where we will generate a calibration curve to your specific substrate and packing density. Please contact firstname.lastname@example.org for pricing.
I hope this helps...
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