The shelf life of a pH or ORP sensor is similar to that of an unplugged battery. It will not decay much when not in use and at moderate temperatures. When the pH or ORP sensor is stored at normal room temperatures (70 F +/- 8 F) with its original storage cap and solution we estimate the following degradation rate:
  • One year in storage: < 5% loss of useful lifetime
  • Two year in storage: < 12% loss of useful lifetime
  • Three year in storage: < 20% loss of useful lifetime

    If the storage solution is allowed to dry or leaks out due to storage at high temperatures, freezing, or other causes, the lifetime may be significantly reduced or failure may occur.
  1. Store sensors in a controlled environment (70F)
  2. Visually inspect the sensor cap every 6 months to 1 year. If you observe either air in the clear sensor cap or significant amounts of salt (potassium chloride filling solution) outside of the seal, then remove cap, rinse, re-fill with diluted 4 pH buffer (1 part buffer to 4 part water) and re-seal with electrical tape.
  3. If pH sensor has been in storage for > 6 months, we recommend:
  4. (a) Remove cap and clean sensor with tissue or toothbrush.
    (b) Exercise in 4 and 7 buffer (go back and forth) for a few minutes prior to calibration and use.
The pH of the process itself, is unaffected by the process flow-rate, however, the measurement of pH with a pH sensor can be slightly, to moderately effected, by flow-rate.
  • For Barben Analytical Performance Series industrial pH sensors we expect negligible effects, <0.05 pH, over 0 to 10 ft/s, due to the large Teflon reference electrode junction and the solid, internal construction, of the Axial Ion Path reference.
  • For most conventional applications with other industrial pH/ ORP sensors there will be little influence of the flow-rate on a pH measurement, with variations of the magnitude, +/-0.1 pH for flow velocities 0 to 10 ft/s. This variation is due to small electrical potential effects at the reference electrode junction (diaphragm).
  • For ultra-high-purity (UHP) water, the flow-rate can have a significant effect on the measurement due to UHP style reference junction and the electrons passing the glass without a path to ground. Consult the manufacturer for this type of application.
If you believe your sensor is experiencing pH changes due to changes in the flow velocity, it may be dirty or coating on the glass and/or reference junction. Please consult "Cleaning pH/ORP Sensors" tech note.
The answer is entirely dependent on the process conditions in which they will by used. For mild processes such as municipal drinking water or effluent applications, the sensor generally lasts for several years, and in some cases up to 8 years. For "tough" industrial applications that have high temperature (e.g. 90 C) and high caustic concentrations (e.g. 10.8 pH), lifetimes of 9 to 12 months may be more typical for our Axial Ion Path sensor.

When answering this question for customers, I like the analogy of the pH sensor being like a battery. If you take a pair of batteries and put them into your remote control for your TV, they will last years. If you take the same batteries and put them into a remote control race car, you will find they last only a few hours.

Unlike a battery, there is no clear formula to compute the expected "amp-hours," since the reference half-cell chemistry is open to the process chemistry, leaving a large unknown. We can only state, that in general, higher temperatures, higher cycling pressures, higher chemical concentrations and higher pH sensor poisons (H2S, NH3, Pb, etc.) lead to a shorter sensor lifetime.

We would like to point out, that our patented Axial Ion Path reference technology will best protect the reference half-cell from poisoning by slowing the mass transfer between the reference and process chemistries while still allowing the necessary electrical communication. This results in higher accuracy between calibrations (less "zero drift,") and a longer lifetime than other competitive sensor technologies. The tougher the process, the bigger the difference.
The answer is entirely dependent on the process conditions in which the sensor will be used and the technical requirements of the application.

We recommend to begin by calibrating frequently and then gradually going to longer intervals as the stability of the sensor is determined from "before and after calibration values."

In general, pH sensors can be calibrated as frequently as once a week, to more commonly once per month, and in some cases only once per quarter. Sensors can be calibrated in-situ, via one-point method (compared to a grab sample) or removed from process, cleaned and calibrated via a 2-point buffer method.