615 High Purity pH Sensor

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615 High Purity pH Sensor

Pure-Water pH Sensor

Close spacing between sensor components sharply reduces high impedance and ion effects for stable pH readings. Small internals allow low-flow with fast pH response.



  • Stainless steel total-surround shielding defeats antenna effect
  • Large concentric reference assures stable ionic contact and stable readings
  • Cell is driven by analyzer to produce electron-rich environment for stable readings to 18 megaohm (option -26)
  • Design passes air and CO2 bubbles
  • Fast accurate response, even with varying flow
  • Handles 100 °C at up to 100 psi, -12 option up to 130 °C
  • High-purity temperature compensation with the IC Controls 655-T analyzer
  • Economical, easily replaceable pH sensor

Pure-Water pH Measurement

Due to the lack of conductive ions in high purity water, a number of interferences occur. These interferences show up as a drifting and spiked measurement that deviate from the correct value. Without ions providing a conductive circuit, the pH sensor, coupled to a modern ultra-high-impedance preamp, acts as a high-gain, high-efficiency antenna. Chloride ions escaping from the reference cell tend to partly short the antenna effect, causing the pH sensor to try and establish a new balance. When any changes in sample flow occur, the rate at which these ions are swept away is upset, resulting in a compensating adjustment in the pH sensor. This is often described as sweeping away the “ion cloud”. A related effect is the junction potential at the reference junction which is also influenced by the number of ions and the rate of sweeping away. As sample flow increases, the actual conductivity produced by the ions drops due to fewer ions left in the junction area. The junction, like a thermocouple, puts out a millivoltage relative to the two dissimilar conductors it is composed of. This, of course, changes if one of the conductors (ion concentration) is changing.

Selection Guide
User Manual
615-25 User Manual (Pure Water)
615-26 User Manual (Ultra-Pure Water)
Engineering Specifications

For Reliable pH Measurement of Low-Conductivity Water

  • High-purity pH measurement recommended for water with conductivity of 15 microsiemens/cm down to 1 microsiemens/cm
  • Ultra-pure pH measurement recommended for water with conductivity of 5 microsiemens/cm down to 0.055 microsiemens/cm (resistivity 2 megohm·cm to 18 megohm·cm)
  • Small-flow applications, such as in pilot plants
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pH Theory and Measurement

pH Measuring techniques, the sensing versus the reference electrode, etc
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Pure Water pH Measurement in Low Conductivity Samples

Pure water definition, symptoms and effects on low conductivity samples, laboratory & on-line solutions
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