Thermo Scientific Orion Versa Star Pro User Manual page 138

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Appendix A Channel Specific Settings and Features
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138 Orion Versa Star Pro Meter User Manual
Periodic Electrode Testing
Periodically check the pH electrode resistance following the steps above and record resistance
values in the provided table. Diagnostic testing at 25 °C is ideal, due to the resistance stability
at this temperature. It is important to keep as many sampling variables constant as possible to
get the most out of the periodic resistance testing. The resistance reading is fully dependent on
temperature. When testing, use the same solution at a known, stable temperature each time
and measure around the same temperature each time.
Small Variances in Resistance – Resistance will typically increase as the pH-sensing bulb
ages. This is no cause for concern. Over time, this can lead to slower response rates, slope
degradation and reduced immunity to noise. By keeping track of the change in resistance
(delta, as listed in the table as an example), this information may be used to determine when to
replace the pH electrode before failure occurs, eliminating down time.
Large Decreases in Resistance – When the pH-sensing glass membrane resistance is more
than ten times lower than the expected resistance value at a given temperature, it is often an
indication of a crack in the bulb or shorting of the electrode, leading to a total failure in function.
If you see this type of change in the resistance reading, most likely no further testing or care will
resolve the problem and a new pH electrode will be needed.
Increases in Resistance – As the electrode ages, the resistance of the glass pH-sensing bulb
increases. Eventually, the aging of the electrode, and visible increase in resistance, manifests
in a slower response rate, low slope, and reduced immunity to noise. At some point, the
resistance becomes "super high". The pH electrode diagnostics from the Versa Star Pro meter
with pH/LogR measurement module help pinpoint causes of reduced pH electrode performance
and quickly show the effectiveness of cleaning and care processes.
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If the resistance values are not out of expected range, but electrode response continues to
be slow or erratic, these effects may also be seen because of reference clogging or
contamination. Follow the cleaning procedures in the pH electrode user manual.
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If the resistance value between checks increases substantially higher, the fill solution may
be depleted. Follow the cleaning and filling procedures in the pH electrode user manual.
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If the resistance values show a "super high" resistance, cleaning and care may improve
performance, but will not be able to return the probe to like new condition. Consider
purchasing a new electrode.
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