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!

Dave B.

Dear Moderators,

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,

Francesco

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!

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?

Many thanks,

Francesco