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What
is ORP?
ORP is a term used frequently in
the water treatment & food processing industry.
ORP stands for Oxidation-Reduction Potential. So what is that? The
best definition I can give is that “ORP is a measure of the cleanliness of the
water & its ability to break down contaminants”.
It has a range of –2,000 to + 2,000 and units are in “mV”
(millivolts). Since ozone is an oxidizer, we are only concerned with
positive ORP levels (above 0 mV).
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ORP
Level (mV)
|
Application
|
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0-150
|
No
practical use
|
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150-250
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Aquaculture
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250-350
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Cooling
Towers
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400-475
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Swimming
pools
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450-600
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Hot
Tubs
|
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600
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Water
Disinfection *
|
|
800
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Water
Sterilization **
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ORP
sensors work by measuring the dissolved oxygen.
More contaminants in the water result in less dissolved oxygen
because the organics are consuming the oxygen and therefore, the lower the ORP
level. The higher the ORP level,
the more ability the water has to destroy foreign contaminants such as microbes,
or carbon based contaminants. The
chart on the left identifies ORP levels for various applications.
| ORP
level can also be viewed as the level of bacterial activity of the water because
a direct link occurs between ORP level and Coliform count in water.
The chart to the right lists ORP levels and relative Coliform counts. |
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ORP
Level
|
Coliform
count in 100
ml of water |
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200
|
300
|
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300
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36
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400
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3
|
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600
|
0
|
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How does
an ORP meter work?
An
ORP meter measures very small voltages generated with a probe placed in
ozonated water. The electrode
is made of platinum or gold, which reversibly looses its electrons to the
oxidizer. A voltage is
generated which is compared to a silver (reference) electrode in a silver
salt solution, similar to a pH probe.
The more oxidizer available, the greater the voltage difference
between the solutions. |
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Disadvantages
ORP cannot be used as a direct indicator of dissolved ozone residual, except in
very clean water applications.
Advantages
ORP is a convenient measure of ozone’s ability to perform a chemical task.
ORP is valid over a wide pH range, and ORP probes are rugged enough to be place
into processes for in-line measurement. The probes can withstand process
pressures in excess of 30 PSI.
ORP
and Ozone
In
a clean water system, using ORP to measure the dissolved ozone status works
well. In our experience, however,
even the strong oxidizing power of ozone in moderate turbidity (cloudiness)
systems can result in ORP values far below expected and even negative (reducing)
values. In general, monitoring
ozone with ORP at the generator source works well but measuring dirty water
becomes unreliable. Ozone detection
kits are available and work well at the limit of approved concentrations for
your application.
*
Disinfection is destruction of specific pathogenic microorganisms
** Sterilization
is the destruction of all microbial life
Contact Us to
learn more.
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