Understanding ORP and PPM
Some people use ORP (Redox) sensors and some people use ppm (parts per These sensors use electrochemistry to measure the free chlorine in the water. Oxidation Reduction Potential or Redox is the activity or strength of oxidizers and reducers in relation to their concentration. Oxidizers accept. Download scientific diagram | Relation between free chlorine concentration, ORP , and pH (Source: Santa from publication: Towards Identifying Disinfectants and.
The Basics: ORP and Free Chlorine Monitoring
Often, on salt chlorine installations, special metals must be used for the electrodes to prevent build-up on the sensor, making proper installation and care of the sensors especially important. Even sunlight affects ORP readings.
ORP will go down as the sun rises. UV rays begin hitting the pool water, causing the chlorine to combine with CYA. Is a high bather load introducing organics such as oils and sweat? Perhaps the chlorine is tied up with phosphates from source water, or large amounts of pollen were introduced suddenly. Many controllers use this form to provide a free chlorine measurement.
But it does provide an idea of how much free chlorine is available. This system uses a special membrane through which only free chlorine ions can pass and provide direct readings. These are true readings of free chlorine and not indirect values derived from ORP and pH. Selective membrane sensors are good at measuring free chlorine but are the most expensive option.
The term colorimetric is used to describe a measurement system where reaction-based chemical indicators are used to detect the presence of a specified chemical.
The indicator reacts with the chemical and produces a visible color change in the solution.
The darker the color, the more chlorine is present. DPD test kits, as an example, are familiar to those in the pool and spa industry.
However, studies have shown that there can be significant variations in readings from these kits due to differences in color perception between individual users.
- Understanding ORP and PPM
- Order Online or Call (800) 462-1926
- NSW Health
Essentially, a colorimetric system is an automated hand check. DPD colorimetric chlorine measurement systems automatically test as often as every 5 minutes. The higher the millivolt reading, the more powerfully the swimming pool water is able to oxidise and disinfect.
Oxidisers cause the millivolt value to increase and therefore increase disinfection. Typical oxidisers are hypochlorous acid a component of free chlorineozone, hydrogen peroxide, and potassium monopersulphate. Reductants cause the millivolt value to decrease and therefore decrease disinfection.
Typical reductants are the hypochlorite ion a component of free chlorinechloramines, cyanuric acid, organic matter dust and dirturine, perspiration, sputum, micro-organisms, cosmetics, and faecal material.Demonstration: The Effects of Chlorine and pH on ORP Readings in Water
A drop in the ORP indicates an increase in chlorine demand caused by reducing agents or contaminants entering the water. A decrease in ORP indicates that chemical reactions are about to occur.
Free Chlorine and ORP
Compared to amperometric control, ORP is considered to be a more accurate measure of disinfection rate. Also, ORP controllers can automatically add disinfectant according to demand. They therefore anticipate the disinfecting and oxidising chemical reactions that are about to occur. ORP is an indicator of micro-organism inactivation.
Studies on specific micro-organisms have found a direct correlation between increasing ORP and micro-organism inactivation as shown in Graph 6.
Drinking water is adequately disinfected at an ORP of mV. In swimming pools, an ORP of to mV allows for both a quick disinfection and for breakpoint chlorination destruction of chloramines where conditions permit.
Oxidation-reduction potential - Fact sheets
Eutech Instruments Pty Ltd The ORP value required in swimming pools is higher than that required for water supplies because contamination is constantly being added to swimming pools. An increasing pH decreases the concentration of HOCl and hence its disinfection power.
Similarly a decreasing pH increases ORP because the oxidative power of free chlorine increases. ORP measurement correlates weakly with free chlorine measurement because they measure two different entities.
ORP measures oxidative disinfection power not the concentration of free residual chlorine. Free chlorine measures the concentrations of hypochlorous acid HOCl and the hypochlorite ion OCl- not the oxidation disinfection power. Free chlorine is a variable component of ORP. Oxidative disinfection does not correlate well to free chlorine for two reasons: This will occur when the pH is high.