[NAFEX] pH meters

Jwlehman at aol.com Jwlehman at aol.com
Fri Dec 9 09:17:43 EST 2005


In a message dated 12/8/2005 6:26:11 PM US Eastern Standard Time, 
list at ginda.us writes:> 
> 
> I had one of those cheap pH meters, too.  I scrubbed it with steel wool 
> and stuck it in my soil.  It read 7.0.  I scrubbed it some more and wet 
> down the soil more.  It read 7.0.  I brought it inside and filled a 
> glass with one part vinegar and four parts water.  It read 7.0.  So I 
> threw it away.


Some different metals will generate an electric current when in an acid 
environment. Clean a penny and nickel, then place them together on one side and 
lightly pinch the tip of your tongue between the other side. The strange taste 
you experience is actually a very small current passing through the tongue tip. 
The acid in your saliva between the two metals generates the current. (Grade 
school experiment)   

My meter is made of two dissimilar metals. The shank one and the tip another 
which I believe is silver or a silver alloy. There is an insulator between 
them. The meter is actually reading the voltage generated between these two 
metals with the meter calibrated to pH rather than micro-volts. The higher the soil 
acid the higher the generated voltage. The instructions with mine says to 
keep the tip polished with a clean cloth and not use an abrasive such as steel 
wool to clean it. I believe some of the iron will coat the metal tip alloy 
throwing the calibration off, and if both the tip and shank get contaninated with 
iron may even render it totally useless. Also the instructions with mine says 
they will not work properly if the soil is too wet nor in a solution and mine 
doesn't.  And if too dry will not work either. 

Theoretically a cheap one should work as well as the expensive models, 
especially if the bimetal tips are made of the same metals and they likely are. The 
main difference may be the quality of the meters. An expensive meter may have 
jeweled meter mounts. A cheap one spring taunt. The spring taunt model will 
not take much physical shock abuse as it might experience in field use. In other 
words, DON'T DROP IT.  Either meter type should read just as accurately. 

My background is electronics and meter calibration for 6 years of it at 
Magnavox. 

My pH meter seems to work just fine. It is maybe 8 years old and cost about 
$15.00. I dry and polish it between every reading as some of the acid will 
remain on the tip and contaminate (influence) the following reading. 

Jerry
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