Electrical way of testing water parameters?

[ryanjury]

Hi I am looking at doing a uni project designing a fish tank monitor with a screen displaying temp PH, nitrates and nitrites etc.

I was wondering if anyone on here knows of an electrical way (or has seen this done already) of testing these things.

Obviously temp is the easy one but I haven't seen the others done yet..

Just want to find out if its too hard so I can replan if it is.

Thanks

[lduncan]

It's not hard. But it will be expensive.

ISE's (Ion Selective Electrode) are what you're looking for. They are similar to pH probes. You also need to watch out for interference ions. They range in price from about USD $200 for the cheapest ones, up to thousands of dollars for higher quality ones. They also vary slightly depending on the ion you need to monitor.

Layton

[ryanjury]

I was wondering if there was anyway to use components and design a circuit to do it to save $...

[lduncan]

Not really. The more expensive devices use a component call an ISFET

Ion Selective Field Effect Transistor. It's a pretty interesting component. But I doubt you can buy them as a discrete component, and the circuitry required to drive them needs to be fairly sensitive from what i've seen.

The key component to almost any ion sensor is the production of a semi permeable polymer membrane. They allow the ion you want to measure pass through and exclude all others. This causes a charge to build up on the other side of the membrane which can then be sensed by the gate of the ISFET, allowing current proportional to the concentration to flow.

These polymers I believe are the expensive part.

I don't think it's something you can DIY, or do inexpensively.

Layton

[ryanjury]

Hey Layton,

Thanks alot for that... I was wondering how hard it would be im glad someone knows I am quite confident programming micros and doing circuit design etc.. But all this chemistry sounds like its over my head..

Thanks alot for your input

Ryan

[lduncan]

No problem.

A pH probe wouldn't be too expensive. Using a high input impedance opamp buffer. Fed to a ADC on a micro would work.

Standard pH probes typically output a voltage of 0v at a pH of 7 then 58mV per decade (for each pH point) in either direction (positive and negative).

The probe pretty much acts as a high output impeadance voltage source with output varying between -406mV to +406mV

[suphew]

You could add a TDS probe as well, you can buy units from the states with 2 probes for testing RO/DI filters for about $50, so the probes can be too hard to make or cost too much. Also what about water level?

Rather than just a monitor have you considered building controller? Temp probes could turn on heaters, fans, chiller, auto top up or even water changes, lights, etc.

[puttputt]

Theres a good idea, threaten to build a controller, market and sell it and get everyone really excited about it, then.....................

[ryanjury]

I have every intention of building at the very least a system to moniter some of the things, if I can monitor it then it wont be hard at all to do a controller as well..

We were thinknig along the lines of using a buffer and feeding it into the ADC like you do with temperature.. Any idea what a standard PH probe is? How does it detect the PH? It sounds like what were after and im sure if we figure out how it works we can build one..

"Standard pH probes typically output a voltage of 0v at a pH of 7 then 58mV per decade (for each pH point) in either direction (positive and negative)."

[lduncan]

Well a pH probe works like all other ISE's. It's an ISE which is sensitive to H+ ions, so you need to have some sort of semi-permiable membrane.

They basically operate like a battery. You have two electrochemical half cells. One electrode is called the reference, which is a tube filled with potassium chloride with a silver wire in it, and a porous end which comes into contact with the test solution. The second is called the measurement electrode, which is a silver/silver chloride wire in a potassium chloride solution, held in tube made of lithium doped glass (this glass acts as the selective membrane for H+ ions). The potential difference (voltage) is measured between the two half cells, and that corresponds to the concentration of H+ ions (pH).

Most pH probes are combination probes which means they house the two half cells in a single "stick".

The probes are something you buy. You can't really make them. The meter on the other hand is relatively easy to design and build.

Layton

[malawi_man]

soo..........who's gonna make me a ph meter??

[Robert Smith]

This is an interesting problem. I was considering how to automate all the monitoring of the tank, but when I saw how much the sensors cost got sticker shock. Most of these sensors are designed to be very selective. It may be possible to get away with much less selectivity in an aquarium if the parameters don't vary much. ie. you know that only a certain number of chemicals can be present and can probably then take this into account.

Can anyone give a list of all the ions and molecular species that would need to be monitored in a freshwater tank?

- Nitrate ([NO3]+)

- Nitrite ([NO2]-)

- Dissolved oxygen (O2)

- pH (H+)

- Amonnium ([NH4]+)

- etc?

[ryanjury]

Yeah I was hoping to make my own sensors if they were diyable I mean ive got quite a strong background in electronics but it looks as if they are fairly specialised and even if a company does publish how it works the likelyhood of getting the specialized components is very slim..

[Robert Smith]

Ryan,

I think this is more of a physics/chemistry problem than electronics. The control system would be easy enough to knock-up, but you'd have had to spend a small fortune on off the shelf sensors.

I was considering a cheap & dirty UV/Vis/IR spectrometer based on LEDs and CMOS detectors (possibly with a prism/grating). I'm not so sure about the required sensitivity. Solid state opto-electronics have advanced so much and got so cheap that I'm sure there must be a simple way of doing this.

Bob