SOWACS: Reading gypsum blocks with a Watermark???

[Flavio Arruda]

Flavio Arruda writes to the sowacs list:
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Dear Tom Thomson

You can use Watermark meter as an AC "ohm-meter" if you calibrate your
blocks for measurements of electrical resistance as a function of water
potential.

An simple equation (not published yet) to convert your readings from a
Watermark meter to electrical resistance is:

 Resist = 1.118 - 0.11349*T + 0.22713*Read + 0.00199*T^2 -
0.0001831*Read^2 - 0.00207*T*Read

where Resist is in kohm;  Read is the Watermark reading (indicated as
"cbar") and T is the temperature adjusted in the Watermark meter (indicated
as degree C).

The equation is good for readings above 20cbar (R^2=0.993, n=63). I have a
better and very long equation that adjusts
well the data for the full range of the instrument. Let me know if you need
it.

Regards

Flavio Arruda

**************************************************
Dr. Flavio Bussmeyer Arruda
Instituto Agronomico (IAC)
Centro de Ecofisiologia e Biofisica
S. Irrigação e Drenagem
P.O. Box 28
Campinas, SP, Brazil 13001-970
Mobile: 55 (19) 9797-4776
Tel/fax: 55 (19) 3241-5188 ramal 417
E-mails: farruda@iac.sp.gov.br, farruda@mpc.com.br
**************************************************

-----Mensagem original-----
De: Bruce -owner of sowacs [mailto:bruce@sowacs.com]
Enviada em: Terça-feira, 1 de Julho de 2003 06:32
Para: Bruce Metelerkamp
Assunto: SOWACS: Reading gypsum blocks with a Watermark???

Tom Thomson writes to the sowacs list:
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I use Delmhorst blocks for annual crops like corn and beans and Watermarks
for perennials. My Delmhorst meter is nearing its lifes end and I hate to
have to buy another especially since I have a new Watermark meter. I read
the blocks with the Watermark meter and there appears to be a pretty good
correlation in the midrange but less so at the dry end. I was wondering if
there is a utility out there to convert Watermark readings to Delmhorst
readings???

Thanks for the time....Tom

=====================================================
Tom Thomson                          Northwest Agricultural Consulting
1275 Oak Villa Road                                  tomt@teleport.com
Dallas, Oregon 97338                            Phone/FAX 503-623-0468
====================================================

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