DIY conductivity meter

[H.crispa]

I just thought you guys might get a kick out of this thing I made a few years back. It measures the salt level in your tank through electrical conductivity. It requires you to mix up a solution to "calibrate" it and I don't even have time to post the instructions right now but this post is to remind me to do it when I get home. It's a fun little project.

 

[H.crispa]

Here is how it works:
Contrary to popular belief, water is actually a poor conductor of electricity. Distilled water is actually an insulator. However, when salts are dissolved in it-- such as in seawater, it becomes conducive and the degree to which it is depends on the amount of the salts dissolved in solution. Therefore, the saltier the water, the better a conductor it makes.

So, if you take an off the shelf volt/ohm meter and do a few simple mods, you can get an instrument that, once calibrated, will tell you instantly the density of the water when you dip the contacts in.

Without further delay:
1. Take the volt/ohm meter (get a cheapo at radio shack or Kmart or a flea market) and mount it to a dowell rod. I did this by removing the back, drilling two small holes in it and fishing a small wire through the holes. Then I drilled two holes in the dowell the same distance apart as the holes in the back of the case. Pull the wire through the dowell rod and twist them together.

2. Plug in the leads for the meter and either cut off or wrap the leads around the dowell and secure them with tape-- or just let them hang loose like I did.

3. Solder the long contacts back together if you shortened the wire and tape them to the dowell so that they are about an inch apart. Now dip the entire end into Plast-dip to seal it all off and let the plastidip dry.

4. Take a file and file off the plastidip from the tips of the contacts.

5. Turn the meter to the continuity setting. It should be marked with something like ohm X1K.

6. Dip the contact ends into your tank and you should get a reading.
This reading corresponds with whatever specific gravity your water is currently. Mark this and write whatever the SG is at that point.

7. Now, mix up a quart of seawater to 1.017 using a good hydrometer, reinsert the meter and mark this the same way you did above. Add more salt until the SG is 1.018 and do the marking process again. Continue this way until you have your meter marked at all the SGs you want. Now you have a brand new redneck EC meter!

Here is the result:

Here is what mine reads at 1.025 (yours will be slightly different):

 

[firefish2020]

This thing actualy works i got to see it demonstrated today :thumbsup:

 

[phljess]

Why do you need to cut the ends off and all that crap. Why could you not just put the leads into the water your testing without dipping them in plastic and using the dowel rod?

 

[firefish2020]

Not my post but I think it would'nt look as cool LOL

 

[H.crispa]

Because unless you reduce the area of exposure to the water, it will peg the meter every time and calibration is impossible. You have to space the contacts about 1 inch apart and only expose the tip. Otherwise, the reading will increase and decrease according to how deep you dip the contacts in the water-- and most often the meter will just peg. Feel free to experiment. That's just the way I did it.

 

[phljess]

I have heard about using volt/ohm meter to get SG before, but most meters are more expensive than hydrometers, etc. I figured you would have to space the tips of the meter but I am do not understand why you have to remove the clips. Anyway, it is an interesting way to calculate SG. It should also be fairly accurate.

 

[LtPiper]

I've wound up with about 6-7 hydrometers through various means. some of them are supposed to be the "good ones". I can take the same sample of water and put them in each of them and get a different reading. I'd love to have somebody around here provide a test solution so I could get a reading so I could calibrate and have an idea which one is right.

 

[mcox33]

For what it is worth I just bought Mike a new meter at Harbour Freight last week. They were on sale for either $2.99 or $3.99 can't remember which. Hopefully I will get time to go back and get me one now. I bet they are still on sale though. In case anyone is interested.

 

[coralreefer]

<a href=showthread.php?s=&postid=8183167#post8183167 target=_blank>Originally posted</a> by LtPiper
I'd love to have somebody around here provide a test solution so I could get a reading so I could calibrate and have an idea which one is right.

A standard basically equal to NSW is
3.7 g table salt in 96.3 g ro water= s.g 1.0265/35 ppt at 68 degrees.

You could also check you hydrometers against a calibrated refractometer.

 

[firefish2020]

This is why I use a Pinpoint salinity monitor
Just have to re-calibrate it when I change batteries.

 

[AcroSteve]

How timely. This is great.

I was looking into a way to very quickly determine if my kalk was saturated or not. Looks like conductivity is the best measure.

<a href=showthread.php?s=&postid=8280074#post8280074 target=_blank>Originally posted</a> by Randy Holmes-Farley
Conductivity is a far better gauge. Saturation is about 10.3 mS/cm. I show that here:

Now, I just need to get the conversion factor.

 

[Randy Holmes-Farley]

FWIW, conductivity meters do not work in the way this DIY meter works. The exact size of the electrode surface will greatly impact the measure conductivity in a DIY meter, and they do not apply a DC voltage. In fact, they use an AC voltage to avoid effects at the electrodes, and the best meters use 4 electrodes rather than 2.

I discuss how these work in these two articles:

What is TDS?
http://www.reefkeeping.com/issues/2004-04/rhf/feature/index.php

Using Conductivity to Measure Salinity
http://www.aquariumfish.com/aquariumfish/detail.aspx?aid=1804


from the first one:

Can I Just Use a Multimeter to Measure Conductivity?"

No. Several factors make it impossible to accurately measure conductivity with a standard multimeter. The size and shape of the electrodes are significant, but more important is what happens at those electrodes. If a DC current is applied to seawater, numerous reactions take place when the ions hit the electrodes. Some ions will plate out on the electrodes, some may bubble off as gases, and the electrodes themselves may dissolve. These and other effects all serve to change the nature of the solution at the electrode, impacting the measured conductivity.

So how do conductivity probes get around this problem? They use an AC current rather than DC. Using fields that oscillate very rapidly, there is no overall movement of ions toward one electrode or the other. The ions move one way for a tiny fraction of a second, and then back the other direction for the second half of the cycle. Overall, the solution and electrodes stay unchanged and the conductivity is accurately measured. Modern conductivity meters use complex AC waveforms to minimize additional complications such as capacitance, which can interfere with simple conductivity measurements.

In practice, commercial conductivity probes have either two or four electrodes, with the four-electrode version being more resistant to fouling and other effects that can cause degradation of the measurement. The electrodes are made of nonreactive materials such as epoxy/graphite, glass/platinum or stainless steel. The choice depends primarily on the nature of the solution to be tested, but nearly any commercial unit will be suitable for tap water.

 

[H.crispa]

I stand corrected. However, it still does work. You do have to clean the electrodes by wiping them off before each dip and take 3 to 4 readings to make sure it is reading consistently. This little gizmo is not perfect or anything but it is a cheap and fun little project just the same.

Interesting you should mention stainless steel. Many of the multimeters have SS probes or at least plated ones to resist corrosion. The plating probably has its own effects on conductivity, particularly when you run DC current through them and go from one type of solution to another. In any case, this "redneck 'ec meter'" is reasonably accurate when used in the above manner. I certainly would not use it for any type of chemistry experiment or anything else where if a reading is slightly off it can screw up the whole thing. Still, "redneck" is the operative word at this point. Perhaps I should change the title of the post???

Thanks for your input on this, I love to lean new stuff and you have given me some stuff to chew on for a while.

I am by nature a tinkerer, not a chemist. I just know a little about a lot and a lot about a little-- Just not a lot about a lot.

 

[AcroSteve]

I am kind of looking for a "go", "no go" kind of test for my kalk saturation. Basically to see if what I have left in the bottom of my resevoir is still effective.