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|Author||Topic:ECTM and 5TM outputs||3064 Views|
26 January 2011 at 9:02am
I've got a question about the data that these probes output. As I read the manuals, it appears that if they are read (by a Campbell CR1000 data logger for example) using the "serialin" command, they output dielectric constant and temperature values that have been scaled. On the other hand, the SDI-12 on-line documentation seems to say that if they are read using the SDI-12 "D0" command, the dielectric constant and temperature are directly output (unscaled). Am I crazy, or is this actually what is happening? I find that the users manuals are very un-helpful in this matter. Thanks!
26 January 2011 at 4:20pm
Dave, you are not crazy. This has to do with the evolution of our probes. The EC-TM, which is the predecessor of the 5TM, uses the empirical observation of probe output to known water contents to arrive at a calibration. The 5TM (and 5TE) is now calibrated to a 5 point dielectric curve and gives bulk dielectric (Eb) as a raw output that can then be used with the published Topp equation or a calibration of the researcher's choice. We also added SDI-12 capabilities to the 5TM/TE to give the probes more versatility and a more flexible programming option.
We needed to scale the serial output of the 5TM (as Eb *50) to maintain the probe's optimal resolution. Scaling the serial output keeps the output simple to parse out as a numeric serial string without sacrificing resolution. SDI-12, however, allows us to report Eb (and temperature) to the appropriate decimal without scaling or requiring more complex code.
I apologize that the manual was unsatisfactory in this matter. I will take a look at it and see if we can make it more clear. In the meantime, you can get more information (well, all the information) about the communication protocols of Decagon digital sensors here: http://www.decagon.com/assets/Uploads/5TE-5TM-integrators-guideRev5.pdf
15 March 2011 at 10:50am
I need an explanation related to this topic of the 5TM output. It concerns the calibration of the sensors. Following the instructions you provide on this website and working in SDI 12 mode (then using as RAW data the dielectric permittivity), if I have well understood what I should find at the end of the calibration points collection is a linear relationship between the raw data and the volumetric water content (gravimetric measurements). But looking at the Topp's curve shape, that is not linear, what I do not understand is why shouldn't I find a straight line as best fit for my points instead of maybe a "calibrated" Topp's relation? If I find a polynomial or exponential or whatever that fits very well my points, do you suggest to use the same a linear regression or not? I have not started yet my calibration but this is a doubt I have and I hope to have been clear about it.
As usual thanks in advance,
16 March 2011 at 8:22pm
Good question! The calibrations are sometimes, but not always, linear. And you're right; the Topp equation is not at all linear. If you have done your own custom calibration and the best fit of the curve is a polynomial, then I would suggest using a polynomial. If you haven't done a custom calibration and you are working in typical soils (something between clay and sand) in typical EC ranges, then the Topp equation is a great calibration curve to use as well. Feel free to post your data if you still have questions or thoughts. What sort of data are you working in?
1 June 2011 at 8:31am
I've got another question about these probes. I'm working with a group that has A LOT of ECTM probes installed, and they are swapping units out with 5TM ones as the ECTM ones die. I'm putting together a system to keep track of things and keep their system up. Currently, all sensors (ECTM and 5TM) are being read in SDI-12 mode. I'm playing around with one of each in my lab, and I think that I understand the 5TM pretty well (reading in both SDI-12 and serial ASCII modes). The ECTM probes, on the other hand are kind of confusing. There seems to be about a factor of 2 difference between the "RAW" water output when reading them in SDI-12 and serial modes (SDI is bigger). I understand that these numbers are related to the dielectric constant of the medium, but can someone explain to me what that relationship is, and what kind of scaling is done between SDI-12 and serial ASCII mode? I've tried applying the relationship on pg. 16 of the users manual to both numbers to try and get the actual dielectric constant, but the numbers don't make sense (I've used damp paper towels, wet sand, and air as the medium) Thnaks!
2 June 2011 at 8:43am
The EC-TM is not calibrated to dielectric. The raw value that you get from the EC-TM output is just an analog to digital conversion (12 bit I think) that scales to the excitation. The EC-TM is calibrated to a range of mineral soils to correlate the raw ADC directly to volumetric water content. The EC-TM should have a raw value of ~ 435 ADC counts in air and ~ 1330 ADC counts in water.
The EC-TM was our fist attempt at implementing SDI-12 in our sensors and, to be honest, we didn't get it right in the EC-TM. The probes will work fine in SDI-12 if they are isolated from each other, but we do not recommend using these probes on a common bus line. There is not a different scale between SDI-12 and serial for the EC-TM to my knowledge and the engineer that I talked to confirmed this. I will dig into this a little deeper and hook an EC-TM probe up to an SDI-12 logger and see what I see. I will post again shortly when I have some more info for you...
24 February 2012 at 5:45am
Dear moderators, after one year I know a bit more about the mystic soil I am working with. It is apparently a sandy soil (sand content is about 60%) but, presenting a super high concentration of fine particles, it behaves much more like a loam or clayey-loam (especially for thermal properties).
We did a specific calibration using 7 probes 5TM. We used identical buckets (10.5cm diameter x 11cm height), one for each calibration point (we almost covered the entire saturation scale, from the driest value till very close to saturation). We filled each bucket the same way, checking weight and volume, and we managed quite well to reach the same dry bulk density everywhere. Then we stuck one probe per bucket, vertically and in the middle, burying only the needles but not the thermistor (it was very tough in dry soil to push them down, then we stopped at the beginning of the black plastic), we measured for 5 minutes, we removed the probes and we re-stuck them but each in a different bucket, after having adjusted a bit the soil with the fingers, in order to fill the holes left by the removed probes. We continued this way till each probe measured in every bucket. Finally we oven dried the soil in the buckets to cross-check the goodness of the volumetric water content we supposed to have put in every bucket (we obtained very good results).
What we finally found is a strong linear dependence of the dielectric permittivity measured by the 5TM on the volumetric water content. A straight line seems the best fit linking diel_permittivity to volumetric water content, but this means that toward saturation, where standard Topp equation flattens, the gap between the values measured by the 5TM and the real water content in the buckets becomes quite huge (even 15%), with the former values underestimating the latter.
Now my questions are:
- Are you familiar with calibration curves providing LINEAR dependence of the water content on the dielectric permittivity? Perhaps in loamy soils?This seems in any case strange to me..
- Could there be an error in the calibration procedure I am following?Maybe the probes are "feeling" and then measuring the air outside the bucket or left by the probes removed before. This could actually cause a biased underestimation (smaller dielectric permittivity) of the water content.
-In case I managed to explain well the situation, any suggestion?Maybe avoid to stick the probe vertically, or use larger and shallower buckets?
8 May 2012 at 3:59pm
On the subject of calibrating soil moisture sensors. 1) I have never been able to convince myself that my lab calibrations were valid for field use (i.e. within plus/minus 2 vol percent. 2) Field attempts at calibration have done little better. Based on these frustrating results I concluded that soil is not a good media to work with because it is plastic, swelling, shrinking, electrically variable and non-constant material. So I calibrate under as close to the field conditions where the data will be used, and suffer the criticisms as they arise.
I hope that this note does not trivialize the problem in the least, it is just the frustration of working with materials like soil.
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