Going back into my archives, I found the following discussion that did not bode well for the use of psychrometry as a routine method of soil water content (potential actually) measurement! See what you think... >>Bruce-- So far, I have seen no mention of psychrometry in your discussion group; any plans to in the future? I don't really need any technical information, since I'm already quite familiar with them, but am curious about other user's experiences/applications. --John Manwaring << From old messages of John Manwaring of Research Associate Utah State University Logan, UT 84322-5230 Date: Dear Bruce, " We have been using soil psychrometers coupled with Campbell Scientific data-loggers (model CR7X) for nearly ten years. We have found psychrometers to be quite useful for studying plant roots' movement of water from deeper to shallower layers of the soil. They are of limited usefulness in very wet soils or at shallow soil depths (because of thermal gradient problems). Another potential drawback is that the area of soil they are sensing is quite small (when compared to neutron probe or TDR soil moisture content methods). Depending on how they are set up, one can measure down to about -6 to -7 MPa. We monitor up to 140 sensors per data-logger per hour at peak. An additional benefit to psychrometers is that each sensor yields a very accurate soil temperature at that site." Here is some more input on psychrometers for you. >I had originally ruled out the use of psychrometers due partially>to the problems of temperature and sphere-of-influence you>mentioned. But I also had some indirect experience of them during>my undergrad years. (I studied under Prof Michael Savage who says>he has worked with Martyn in the past, and did his PhD on the>matter.) They [were] not cheap items and seemed not to last more>than a year or less in the soil. (These were the ceramic or>stainless steel mesh-covered units supplied (I believe) by>Decagon devices. They are still not cheap, much of this due to the composition of the lead wire; the sensor itself is about $20-25 (w/1 m lead) plus $1.20 per meter of additional lead. If you add a Viking connector (optional quick connect system), another $15 is needed. As far as their usable lifetime goes, we are still measuring some psychrometers that were installed ten years ago. There are no guarantees though; some psychrometers break upon installation or a year or two later. We install two sensors at each depth/site to better ensure having at least one working. I don't know what effect soil pH might have on longevity; ours is fairly alkaline (7.5-8.0). Both major psychrometer manufacturers are here in Logan; J.R.D. Merrill Specialty Equipment and Wescor.>The CR7 is somewhat more powerful than the CR10 (although the>CR10 has proven itself to be most adaptable and durable in>general applications). Do you know if the microvolt resolution of>the CR10 would be sufficent to measure down to 1.5 MPa? Perhaps>this would be easiest answered by either referring me to someone>who has used psychrometers with a CR10, or by telling me what>kind of voltages have to be measured.>For a CR10 the finest voltage range for a differential measurment>is specced at +-2.5mV with a resolution of 0.33 microVolts. (For>single ended it is double this). I am using (or have available)>an AM416 multiplexer. We had a post-doc who investigated whether a CR21 could be used instead of a CR7X. She thought that you could rig something up, with a loss of resolution (but also at a substantial savings). The big advantage to the CR7X is the programmable (both time and intensity) cooling current. You should be able to accomplish something similar with a lesser data logger, but with more programming.>I asked Prof Savage his opinion on some of these points two and>he replied: >========================================================== >In 1984 I gave a seminar on psychrometer usage to Martyn>Caldwell's group in Logan. Psychrometers are regarded as the>standard for soil water potential measurement. Psychrometers are>quite expensive ($120 each) and one needs a CR7X datalogger - my>Ph.D. was on psychrometers. They can be made to last in soil with>some careful installation procedures that I have never had the>time to investigate further.>Psychrometers are definitely not for routine usage but are the>best in the range -80 to -4000 kPa (-0.08 to -40 bars) - the>"drier" end. Tensiometers are fine up to about -80 kPa. What is>needed is something that can work on a CR10 - the heat>dissipation sensor may be OK and may be better than the>Watermark. >================================================================= I agree with virtually all of the above; definitely better at the drier end of the spectrum, relatively expensive (although less than the quoted amount, depending on what lead length you require). I'm not sure what he means by "not for routine usage", as we use them routinely. I know virtually nothing about the heat dissipation sensor you're describing; sounds kind of interesting though. Hope some of this helps. I am going through a similar process to yourself of trying to find out everything applicable about TDR. Hope this helps. --John Manwaring -.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-. Bruce Metelerkamp SOIL WATER RESEARCH OFFICER Institute for Commercial Forestry Research, University of Natal, PO Box 100281 Scottsville, ZA3209 Rep. of South Africa Voice:27 331 62314 E-mail: bruce@icfr.unp.ac.za FAX:27 331 68905 URL http://www.icfrnet.unp.ac.za/~metele /SoWaCS.html /RR.html Maintainer of the SoWaCS list.