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Taking Proper Current Reading

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Digital:
I am a bit confused on how to properly hook up a multimeter to take the current reading in the cell. I have a current limiter based on a lm317 that is capable of about 3.5ma to 21ma. How I found this current range was by connecting the positive of the power supply to the positive of the multimeter and the negative of the power supply to the negative of the meter. Apparently this is an incorrect way to measure the current in a circuit and Kephra shows the proper way in his notes, you connect the positive of the power supply to positive of the meter, negative of the power supply to the positive of the cell (silver anode) and the negative of the power supply to cathode (copper wire in my case).

I am confused because I am unsure which current reading to use when doing the PPM calculations, further to this confusion depending on where I connect the positive of the meter I have get a reading that can vary by +1ma and if the solution is stirring, it can also reduce the current reading by about +2ma. Also, when I take the reading in the proper manner, the current limiter maxes out depending on the depth of the cathode and can’t be adjusted higher unless the cathode is deeper in the solution. The odd part of this was that before when I was doing this and I lowered the cathode the voltage drop would be quiet significant, dropping from say 21v to 8v, now it doesn’t really move past 21v regardless of the depth of the cathode, but the current does increase. Although in the past runs I did, I heated up the water, which does increase conductivity. For the readings I mentioned I just used cold water, so this could be the case for the voltage not dropping as much.

With all that said, my question is which current reading should I use to do calculation with? And if it fluctuates and drops with stirring, do I use the current reading when the solution is resting or the one when it’s in motion?

Thanks

Gene:
No current source is dead nuts accurate unless you have a fine adjustment to tweak it dead on.  If its a current limiter though, it should be dead nuts stable regardless what current its designed to operate at or what you set it to if its adjustable.

Connect your positive power supply terminal to the limiter positive input.  Connect the limiter output (or negative) terminal to the positive meter probe (yep). Connect the negative meter probe to your power supply ground terminal. Turn it on and read the current.  This is what its set to and even connected to a cell it should be rock solid stable at this current (not splitting hairs, obviously).

You do need a minimum of about 3V headroom for an LM317 to function properly meaning your power supply needs to be 3V higher than the highest cell voltage you're shooting for at a minimum though I'd go at 6-9v higher for enough safe area (margin).

For an given electrode arrangement (depth of both which should be 1.5" apart), the lower the current, the lower the cell voltage and the higher the current, the higher the cell voltage. This is why you need to adjust the cathode because though stable at a given depth, the cell voltage may not be greater than 10V without changing the depth of the cathode.  By changing the depth of the cathode, given a fixed depth for the silver anode, you're changing the resistance of the cell - less submerged, higher resistance, more submerged less resistance. Given a fixed current, if you up the resistance, the voltage goes up and if you reduce the resistance, the voltage goes down.

The LM317 is only spec'd to limit to a minimum current of 10ma.  Depending on process spread for the chips, it may be anywhere between 0ma and 10ma but usually when they say 10ma, its rare to see it go below half this though it does happen.

When you stir, you lengthen the path the electrons have to traverse which increases cell resistance where this raises the cell voltage for a fixed current where you most likely would have to readjust the cathode depth to compensate to get the cell voltage in the range you want.  Your limiter, assuming you have enough headroom, should be rock solid stable until your cell voltage gets within 3V of your power supply voltage. If not, something is wrong.

After measuring the limiter current without the cell attached, set your cell up, add your electrolyte (1ml of 1 molar sodium carbonate per liter of water - 20 drops) and hook up your limiter which means your power supply positive terminal connects to the positive terminal of the limiter, the limiter negative terminal connects to the anode (silver) and the cathode of the cell connects to the power supply ground terminal.  Yes I meant exactly what I said.  Limiters are series devices - they're connected in series with the cell, not across it.

Measure the cell voltage and adjust the cathode until its over 10V (I usually shoot for 11-12V for a little safe area).

Now lift the negative limiter lead off the anode and connect it to the meter positive probe (yep), put the meter in current reading mode and hold the negative probe on the silver anode connection of the cell.  Read the current. THIS is the current you should use in Faraday's law of electrolysis to compute run time BUT if everything is working properly and your cell voltage isn't any closer than 3V to your power supply voltage, the current (not splitting hairs) should be the same as you measured with the limiter not connected to the cell.  If it isn't, something is wrong.

Digital:
I just realized my first paragraph in the first post was a bit confusing, when I said "positive of the power supply" and "negative of the power supply" I meant positive of the current limiter and negative of the power supply.

The current limiter doesn’t have a negative output, only positive coming from the power supply. I don’t see how it could have a negative as the LM317 and the potentiometer only allows for voltage in. Lm317 Vin from the power source, Vout from the LM317, (still positive) to the potentiometer and resistors and from there to an alligator clip. So I am not sure how I can connect it to the meter as you described. The only way without connecting it to a cell is; positive from the current limiter --> positive probe of the meter and negative from the power supply --> negative of the meter. Maybe I am confused about the terminology here.

The power supply is 24 volts, so it does have that 3V headroom needed, which is why when I connect it directly to the current limiter and take a voltage reading it shows as 21-22 volts.

That was the odd thing with the last run I did, I was expecting to see the voltage change as I moved the depth of the cathode. But it didn’t have any really noticeable changes like it did before. Before I could barely put the cathode in the water and it would drastically drop the voltage. But this might be because the room temperature water has more resistance then water at 60-80c. Which, as a side note, is an odd property of water and conductivity, I thought generally as you cool electrical connections resistance is lowered. Which is why superconductors need to be supercooled with helium or nitrogen etc.

The particular LM317 I have can go as low as 2.5ma according to the spec sheet (if I recall correctly), but its more likely to be around 3ma. And it definitely can go below 10ma, because I tested that by taking a reading with the meter in the cell with it hooked up properly, the lowest I got was 4ma, briefly playing around with it.

The limiter is fairly stable as it doesn’t adjust much at all unless the properties of the cells solution change, temperature, stirring, heat etc. I’m just confused as to what current to use for the calculations.

The way you describe hooking up the limiter to the cell is how I’ve been doing it, output of the LM317 (which I thought was positive, when attached to the meter – limiter output --> positive meter probe – power supply negative --> meter negative probe, it shows a positive voltage). So attached to the cell it’s, power supply positive --> limiter input --> limiter output --> anode (silver) and negative of power supply --> cathode (copper).

So the problem I am seeing is, when the limiter is connected to the cell it gets a different reading as to when it’s connected to just the meter. For example, I can get a stable 15ma with it connected to just the meter, but when its connected to the cell and the meter is attached the way you described I get 6-7ma, which can then further drop 0.5-1ma with stirring. This has lead me to question if I was able to correctly take an initial current reading of the limiter or if I was reading the current in the cell correctly in the first place, which I wasn't.

Sorry for the long reply, I really do appreciate the time, help and well thought out responses you are giving me and have given me in the past.
Thank you

Gene:
http://www.ti.com/lit/ds/symlink/lm317.pdf

Look at table 6.3 - Recommended operating conditions - Output current.

Its stated as 0.01-1.25 amps.  0.01 amps is 10 milliamps.

Then look at table 6.5, minimum load current to maintain regulation. Its stated as 3.5typical, 10 max meaning the amount of current needed to keep the regulator regulating varies here. Typical for this is a bad number to use because its related to the process spread of the chips where the lowest they're guaranteeing always works is 10ma.

If you use the proper amount of electrolyte, as long as there is enough headroom, the current through the cell should be constant regardless the cell voltage.  If its not, you have some kind of problem.

You need a relatively thin cathode.  The thicker it is, the less depth you need in the cell because of the amount of surface area and the voltage adjustment becomes really touchy. I use a piece of 24 gauge wire from a piece of CAT-3 networking cable with the insulation stripped back a good 1.5".  This affords you plenty of fine control of cell voltage.  I originally used a piece of ROMEX electrical wire which I think was 14 gauge and the length of wire in the cell was barely glancing the surface to get the voltage up where we need it.  There was no fine control and worse yet, if even a smidge of water in the cell evaporated (which is a much bigger problem at higher temps), the cathode could come out of the water due to the lower level in the cell.  With 24 gauge, you'll easily be 1/2", maybe more into the water and if that much water evaporates, you have other problems.

As you heat the water in the cell, the conductivity does increase a little (lower resistance) but its minimal.  If you stir, the path the current takes is longer since its now arc'd and that increases cell voltage.

In all cases, as long as you have at least 3V of headroom for the limiter to do its job, the current through the cell should be constant (not splitting hairs).  If its not, you have some kind of issue with the limiter.

The current you use for the calculation is the actual current flowing through the cell because thats the current thats pulling silver ions into the water and the rate this happens at is what dictates run time, but whether you measure the current just across/through the limiter when not connected to the cell or by putting the meter in series with the limiter output and cell anode, the current, as long as there's enough voltage headroom for the limiter to remain functional should be the same.  If its not, there is some kind of issue.

Digital:
So if I am understanding you correctly, for the limiter to maintain proper regulation the load on the cell needs to be at least 10ma or it'll become unstable. And If I understand you correctly about hooking up the multimeter to the cell and directly to the limiter, I was doing it correctly when I was hooking it up directly to the limiter but I wasn't doing it properly when I originally took a reading of the cells amperage. That's a good start and I can play with it more to trouble shoot it.

But I was running it at 15ma, so I don't see why I would have a different reading when the meter was hooked up to the cell in series, unless the limiter is damaged. It showed a reading of 15ma when hook directly across the terminals, but when it was in series with the cell it showed 6-7ma depending on the depth of the cathode or if the water was stirring. I should mention that the current was stable, apart from it jumping around 0.xx mA when stirring. It was just the drastic different in the current depending on how I took the measurement that's confused me.

I am using some copper wire as the cathode (from a previous recommendation by you) that came from some old telephone wire, I measured the gauge of it with a wire gauge tool and it is 24 gauge.

Thanks for the help again, I will need to trouble shoot it more when I have the time.

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