Re: Watermark or gypsum blocks?

[John Manwaring]

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Dear Demetrios,

You said:
> I am a research plant pathologist working at the present time on the
> epidemiology of a cucumber disease caused by a fungal parasite. In my study
> I would like to investigate the effect of osmotic and matric potential of
> water on fungal growth, survival and disease development. 

I would recommend psychrometry for your type of study (and not 
just because we make them).  Psychrometers are used by plant 
physiologists and pathologists more than gypum blocks and 
Watermark sensors primarily because of their extended range; a 
psychrometer will measure water potential in the range of about
-0.1 to -7 MPa.  Psychrometers do not need to be coupled to the 
liquid phase of water, as do gypsum and Watermark sensors 
(someone recently posted that this was not entirely true; 
comments?).  Psychrometers measure water in the vapor phase, 
hence the extended range.

The same technology is used for vapor pressure osmometers, 
which output in osmolality units (mmol / kg).  They measure the 
depression of the dew point temperature due to water being bound 
by solute (osmotic) and / or particulate matter (matric).

For more information on psychrometry, see the Sowacs sensor 
page: http://www.sowacs.com/sensors/psychrometry.html
and the link to Wescor.


John Manwaring
Applications Scientist
Wescor Environmental Division
Logan, UT 84323
(435)753-8311
(435)753-8177 fax