5 ms·

Maybe someone with more experience than myself can chime in. At a previous startup we used regular pH sensors, from Atlas Scientific (exactly like the ones in t

Maybe someone with more experience than myself can chime in. At a previous startup we used regular pH sensors, from Atlas Scientific (exactly like the ones in the article), submerged in a tank of nutrient water. After a few days (a week tops) they would drift out of spec, and would need to be taken out, re-calibrated, and put back into the process. Short of a system that takes samples and feeds the probe, and can calibrate automatically, how do you measure pH in-system long term? Is there a special kind of probe that stays in calibration for longer?

the__alchemist 6y ago
I don't think there's a way around periodic calibration if you want precise measurements. At least not with normal glass electrodes. If the precision requirements are loose, you could go months between calibration. If you're trying to get within 0.1pH, every few weeks or so. For anything that's going in a scientific paper, calibration before every use is important. There might be a clever way around this, like with a tech other than glass electrode, or some type of automation system with motors that raise and lower the probe into calibration solutions. Something interesting: pH electrode response is nearly linear, and you can make the transfer function using entirely 2 (or 3 to compensate for non-linearity) calibration points. (voltage, pH, temp) No arbitrary coefficient required, other than maybe temp compensation. Edit: I wonder if your sampling idea could kill 2 birds with one stone, if implemented well: By sampling using a mechanical system, you could measure regularly, and calibrate. Auto calibration, and avoid the lifetime reduction from immersion. Eg some type of motor/pump system connected to the electrode.
- amerine 6y ago
  What about the 1yr+ claim on https://atlas-scientific.com/probes/industrial-ph-probe/ https://atlas-scientific.com/probes/industrial-ph-probe/ ?
  - the__alchemist 6y ago
    No idea. Sounds awesome. Might be a higher quality probe, or one designed with low drift rate in mind, given its marketed as an industrial probe. That length doesn't specify how much drift is expected, so it's unclear what tolerances that has in mind.
    - amerine 6y ago
      Ahh, it’s the tolerance details that I was missing. Thanks for the insight.
  - ComputerGuru 6y ago
    It may be suspended in water indefinitely, but the rated lifespan is four years. Hmmm..
bayesian_horse 6y ago
I don't think it's the quality of the probe so much as whatever is in the fluid you are measuring. There are simple reactions going on and much less simple ones, including biological stuff accumulating. An additional problem I've seen is that the temperature sensor for compensation is almost always supposed to be on the circuit rather than in the probe or external. Which is fine in some applications but useless for example for heated aquariums. If you don't want to recalibrate the probe all the time you probably need to figure out a solution to have the probe in a compartment that gets flooded with clean water vs sample fluid.
- the__alchemist 6y ago
  I'm wondering how feasible it would be to make a system with 3 pumps, and holders for ion-selective probes (pH, ORP etc). In an automated process, flood with sample; measure; evacutate; flood with water to clean; evacuate; flood with KCL to store. Probably too expensive to be practical. For temp compensation: I'm not sure the best approach. Temp dependence is loose, so you can get away with a reasonable temperature error for most applications. I went with on-circuit air temp sensor, with the ability in the drivers to feed in an offboard value. This is a reasonable approximation in many cases, but as you point out, for heated tanks, large ones, in-ground ones, or open bodies of water etc, you need to measure water temp directly.

the__alchemist 6y ago

I don't think there's a way around periodic calibration if you want precise measurements. At least not with normal glass electrodes. If the precision requirements are loose, you could go months between calibration. If you're trying to get within 0.1pH, every few weeks or so. For anything that's going in a scientific paper, calibration before every use is important. There might be a clever way around this, like with a tech other than glass electrode, or some type of automation system with motors that raise and lower the probe into calibration solutions. Something interesting: pH electrode response is nearly linear, and you can make the transfer function using entirely 2 (or 3 to compensate for non-linearity) calibration points. (voltage, pH, temp) No arbitrary coefficient required, other than maybe temp compensation. Edit: I wonder if your sampling idea could kill 2 birds with one stone, if implemented well: By sampling using a mechanical system, you could measure regularly, and calibrate. Auto calibration, and avoid the lifetime reduction from immersion. Eg some type of motor/pump system connected to the electrode.

amerine 6y ago
What about the 1yr+ claim on https://atlas-scientific.com/probes/industrial-ph-probe/ https://atlas-scientific.com/probes/industrial-ph-probe/ ?
- the__alchemist 6y ago
  No idea. Sounds awesome. Might be a higher quality probe, or one designed with low drift rate in mind, given its marketed as an industrial probe. That length doesn't specify how much drift is expected, so it's unclear what tolerances that has in mind.
  - amerine 6y ago
    Ahh, it’s the tolerance details that I was missing. Thanks for the insight.
- ComputerGuru 6y ago
  It may be suspended in water indefinitely, but the rated lifespan is four years. Hmmm..

bayesian_horse 6y ago

I don't think it's the quality of the probe so much as whatever is in the fluid you are measuring. There are simple reactions going on and much less simple ones, including biological stuff accumulating. An additional problem I've seen is that the temperature sensor for compensation is almost always supposed to be on the circuit rather than in the probe or external. Which is fine in some applications but useless for example for heated aquariums. If you don't want to recalibrate the probe all the time you probably need to figure out a solution to have the probe in a compartment that gets flooded with clean water vs sample fluid.

the__alchemist 6y ago
I'm wondering how feasible it would be to make a system with 3 pumps, and holders for ion-selective probes (pH, ORP etc). In an automated process, flood with sample; measure; evacutate; flood with water to clean; evacuate; flood with KCL to store. Probably too expensive to be practical. For temp compensation: I'm not sure the best approach. Temp dependence is loose, so you can get away with a reasonable temperature error for most applications. I went with on-circuit air temp sensor, with the ability in the drivers to feed in an offboard value. This is a reasonable approximation in many cases, but as you point out, for heated tanks, large ones, in-ground ones, or open bodies of water etc, you need to measure water temp directly.