ORP/EC meter probes and ranges

[Sral]

Hi everybody.

As you might know I have been working on an EC circuit and have also suggested to @robsworld78 that his pH circuit might actually be also suited to measure ORP.
Now two questions about that, since I have no practical experience about saltwater tanks or aquariums in general:

• How well do plastic vs glass bodied probes hold up to saltwater in your experience ?
  • what kinds of plastic hold up better or worse in your experience ?
• What are the ranges of ORP (in mV) that you would personally be interested in ?
• What are the ranges of EC or TDS (in µS/cm or ppm) that you would personally be interested in ?

 

[theatrus]

Hi everybody.

As you might know I have been working on an EC circuit and have also suggested to @robsworld78 that his pH circuit might actually be also suited to measure ORP.
Now two questions about that, since I have no practical experience about saltwater tanks or aquariums in general:

• How well do plastic vs glass bodied probes hold up to saltwater in your experience ?
  • what kinds of plastic hold up better or worse in your experience ?
• What are the ranges of ORP (in mV) that you would personally be interested in ?
• What are the ranges of EC or TDS (in µS/cm or ppm) that you would personally be interested in ?

Correct. ORP probes are very similar to pH probes and usually you can just use the same high impedance buffer circuit. It helps if you have some programmable gain as well.

That said, I don't monitor ORP at all. It can probably pinpoint some interesting effects so maybe I should

As for EC, I'm either in salt water or in "0" TDS RODI water and nothing in between.

 

[Sral]

Correct. ORP probes are very similar to pH probes and usually you can just use the same high impedance buffer circuit. It helps if you have some programmable gain as well.

That said, I don't monitor ORP at all. It can probably pinpoint some interesting effects so maybe I should

As for EC, I'm either in salt water or in "0" TDS RODI water and nothing in between.

What exactly does „salt water“ and „RODI“ mean for you in terms of interesting range and acceptable accuracy or resolution ?

 

[theatrus]

What exactly does „salt water“ and „RODI“ mean for you in terms of interesting range and acceptable accuracy or resolution ?

My source water is very clear, so the interesting ranges are 0-60ppm TDS on the fresh water side. I’d be interested in actually looking at one more decimal place in low PPB but I haven’t checked the reality of doing that (resolution of 0.1ppm). Most cheap meters are limited to 1ppm resolution with accuracy of a few ppm.

Salt is more salinity tracking at around 30-40ppt TDS. Resolution of 100ppm is plenty in this range. Accuracy specs of +/-300ppm would be amazing but could be worse and still be very useful.

 

[oreo54]

What exactly does „salt water“ and „RODI“ mean for you in terms of interesting range and acceptable accuracy or resolution ?

The TDS of saltwater will be high because of the salinity of the water. Saltwater usually has a TDS of at least 10,000 mg/L, and some conditions can go up to 35,000 mg/L or higher

I assume it is rather difficult to design a 0-35000 ppm probe with .1 ppm accuracy .

What is Acceptable TDS for a Reef Tank? -

The standards for tap water are made so that any water running into a home will be safe for consumption by humans, but this does not mean that it will be safe

sealifeplanet.com

 

[theatrus]

I assume it is rather difficult to design a 0-35000 ppm probe with .1 ppm accuracy .

What is Acceptable TDS for a Reef Tank? -

The standards for tap water are made so that any water running into a home will be safe for consumption by humans, but this does not mean that it will be safe

sealifeplanet.com

Correct. If anything, a selectable range setup (part loading, dip switches) would be required. This is too many orders of magnitude for one sensing system.

 

[Sral]

Correct. If anything, a selectable range setup (part loading, dip switches) would be required. This is too many orders of magnitude for one sensing system.

That is nigh impossible, indeed.

A Precision/Resolution of 0.1 ppm at 1 ppm is comparatively easy, since that's 10%. At 35,000 ppm this would correspond to 3,500 ppm, so not that taxing.
I think 0.01 ppm at 1 ppm and 100 ppm at 35,000 ppm should be manageable. Either by changing the probe, or by changing a setting on the sensor.

Accuracy would be another matter, but I think something like 2-3 times the above values should be manageable as well.

What is your experience with probe health and/or other plastic parts in salt water ? Would glass be much better that a price difference of 50$ to 80$ would be warranted for example ?

 

[theatrus]

That is nigh impossible, indeed.

A Precision/Resolution of 0.1 ppm at 1 ppm is comparatively easy, since that's 10%. At 35,000 ppm this would correspond to 3,500 ppm, so not that taxing.
I think 0.01 ppm at 1 ppm and 100 ppm at 35,000 ppm should be manageable. Either by changing the probe, or by changing a setting on the sensor.

Accuracy would be another matter, but I think something like 2-3 times the above values should be manageable as well.

What is your experience with probe health and/or other plastic parts in salt water ? Would glass be much better that a price difference of 50$ to 80$ would be warranted for example ?

The only issues I’ve had with plastics in water is:

- PLA. It will decompose slowly.
- Some plastics aren’t UV stabilized. I run a little bit of 385nm and these plastics disintegrate after a year.

I’ve started prototyping with more UV cure resin but haven’t tried them in a tank environment yet. The resins in liquid form are pretty toxic.

The other issue is plastic wants to absorb water so it isn’t dimensionally stable the way glass is. Different plastics have different absorption (e.g. delrin is pretty good, acrylic is actually pretty bad), Glass on electrodes will be better sealed over time, but most pH electrodes are PC body with glass bulb without issue.