Unknown - I'm testing them, but can't provide data longer than the time since start. They're double-junction, which helps with logevity by slowing contamination of the internal KCl solution. My intent is to have them used for continuous immersion.
The latest data: Comparing two probes manufactured in the same batch, one immersed in a hydroponic solution, and one with periodic measurements. 3 months in, the immersed one has drifted by about 0.3 pH after the first 2 months; fixed by calibration. Another 0.1 in the third month. I don't have a good answer for long-term life, but hope an immersed one will last at least a year, and one used for periodic measurements for several. Older probes require more frequent calibration, have slower response, and might experience jitter.
Neither of these storage fluids are an actual pH buffer.
Stored separately like this to maintain readiness while unpowered, the electrode pair is repeatedly rinsed and handled using referee technique to freshly update its electronics and/or software to reflect the response of that particular pair to agreed aqueous pH values by comparison to standard buffers.
This can be considered a _neutral_ baseline from which dissimilar sample matrices can give a most reproducible lab reading, regardless of the functional interference of the specific matrix with absolute accuracy over the short term or the long term.
Ideally under laboratory conditions the referee electrode pair's response shows insignificant drift beyond the needed precision over a longer-than-needed term, and the electronics an order of magnitude better at least.
It's not impossible to have good reliable readings over a year's period with clean samples in industrial use, without need to recalibrate.
A combination electrode can be handled in a referee way for good performance too, but they are usually stored in a buffer, so instead a separate glass sensor living most of its life in DI water is the theoretically better baseline device to achieve commonality between labs, especially with unknown or matrix-affected samples.
Once your laboratory technique, referee electrodes, and calibrated electronics can be operated as your most reliable _reference instrument_, then representative samples of the process fluid can be taken to the reference lab the regular way.
It is not usually preferred to remove a process electrode and calibrate for it the laboratory way.
Just correct for any deviation the continuously immersed electrode shows at that time when responding to the same particular fluid as the Reference Instrument does to a representative sample.
The Reference Instrument is calibrated against the Primary Standard (the Standard buffers).
The working process instrument is calibrated against the Secondary Standard (the Reference Instrument). Specific provisions can be made to validate beyond the process range.
Routine continuous service process electrodes do not have to be the high-dollar research kind, and referee lab work can be be done with less costly electrodes than Metrohm.
An aged continuous process electrode, well stabilized by a limited-harm matrix can be more stable than a brand new replacement even if, or sometimes especially if its responsivity has been reduced compared to a new replacement. However there needs to be enough quality analog gain to compensate, and to further reduce jitter appropriate high-impedance shielding and vulnerable removable connections having electrochemically compatible metals or platings without harmful corrosion. Metrohm electrodes and their removable cables are both gold plated at the electrode and have o-rings which protect from laboratory vapors once connected. The other end of the cable where it conects to the instrument matters too. Lag from old electrodes can still sometimes be a problem but also sometimes the least of your problems.
the__alchemist|5 years ago
The latest data: Comparing two probes manufactured in the same batch, one immersed in a hydroponic solution, and one with periodic measurements. 3 months in, the immersed one has drifted by about 0.3 pH after the first 2 months; fixed by calibration. Another 0.1 in the third month. I don't have a good answer for long-term life, but hope an immersed one will last at least a year, and one used for periodic measurements for several. Older probes require more frequent calibration, have slower response, and might experience jitter.
fuzzfactor|5 years ago
https://metrohm.blog/2020/01/27/common-mistakes-ph/
https://metrohm.blog/2020/07/06/best-practice-sensors/
Notice the Separate pH Electrode is properly stored in distilled water:
https://www.metrohm.com/en/products-overview/60150100
The corresponding Separate Reference Electrode is properly stored in the same fluid as its internal electrolyte:
https://www.metrohm.com/en/products-overview/60733100
Neither of these storage fluids are an actual pH buffer.
Stored separately like this to maintain readiness while unpowered, the electrode pair is repeatedly rinsed and handled using referee technique to freshly update its electronics and/or software to reflect the response of that particular pair to agreed aqueous pH values by comparison to standard buffers.
This can be considered a _neutral_ baseline from which dissimilar sample matrices can give a most reproducible lab reading, regardless of the functional interference of the specific matrix with absolute accuracy over the short term or the long term.
Ideally under laboratory conditions the referee electrode pair's response shows insignificant drift beyond the needed precision over a longer-than-needed term, and the electronics an order of magnitude better at least.
It's not impossible to have good reliable readings over a year's period with clean samples in industrial use, without need to recalibrate.
A combination electrode can be handled in a referee way for good performance too, but they are usually stored in a buffer, so instead a separate glass sensor living most of its life in DI water is the theoretically better baseline device to achieve commonality between labs, especially with unknown or matrix-affected samples.
Once your laboratory technique, referee electrodes, and calibrated electronics can be operated as your most reliable _reference instrument_, then representative samples of the process fluid can be taken to the reference lab the regular way.
It is not usually preferred to remove a process electrode and calibrate for it the laboratory way.
Just correct for any deviation the continuously immersed electrode shows at that time when responding to the same particular fluid as the Reference Instrument does to a representative sample.
The Reference Instrument is calibrated against the Primary Standard (the Standard buffers).
The working process instrument is calibrated against the Secondary Standard (the Reference Instrument). Specific provisions can be made to validate beyond the process range.
Routine continuous service process electrodes do not have to be the high-dollar research kind, and referee lab work can be be done with less costly electrodes than Metrohm.
An aged continuous process electrode, well stabilized by a limited-harm matrix can be more stable than a brand new replacement even if, or sometimes especially if its responsivity has been reduced compared to a new replacement. However there needs to be enough quality analog gain to compensate, and to further reduce jitter appropriate high-impedance shielding and vulnerable removable connections having electrochemically compatible metals or platings without harmful corrosion. Metrohm electrodes and their removable cables are both gold plated at the electrode and have o-rings which protect from laboratory vapors once connected. The other end of the cable where it conects to the instrument matters too. Lag from old electrodes can still sometimes be a problem but also sometimes the least of your problems.