Using a multimeter instead of this instrument ?

Discussion in 'General Electronics Chat' started by Externet, Aug 7, 2013.

  1. Externet

    Thread Starter AAC Fanatic!

    Nov 29, 2005
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    Hi.
    I do not want to buy this thingy; gardeners may find it convenient but they do not know how to use a multimeter nor have one.

    http://www.hydroponics-simplified.com/ec-meter.html

    So I want just to probe the solution resistance to obtain meaningful readings instead of pretty lights on a unitless scale.

    How would you suggest to equate ?
     
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  2. wayneh

    Expert

    Sep 9, 2010
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    It's not the meter that's important, it's the probe. You can't measure conductivity unless your design compensates for probe surface area, distance between the poles, surface corrosion and probably a bunch of other things. You can't just plunge your multimeter leads into a solution. If you want simple, use a refractometer.
     
  3. Externet

    Thread Starter AAC Fanatic!

    Nov 29, 2005
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    Thanks, Wayne.
    The bunch of other things may include temperature compensation but what about volume of the solution?
    A larger volume presents a bigger number of current paths in parallel than a small sample of solution; for a fixed electrode area and spacing.

    I thought a refractometer measured optical behavior of light trough a fluid, not electrical conductivity. :(
     
  4. BillB3857

    Senior Member

    Feb 28, 2009
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    Also, the metallurgy composition of the probes comes into play. Different alloys on each probe creates a simple battery with the solution becoming the electrolyte, in many of these instruments. A lab grade unit will have a precisely mixed calibration solution available to insure proper operation.
     
  5. GopherT

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    Nov 23, 2012
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    Scroll down on that page and they show three different 'wraps ' that put a value on each LED, so they are not unitless.

    Also, the sample size will not matter if you are measuring conductivity with very low currents (normal). Measuring with such high current that the sample will start heating enough to impact measurements, then you have other things to worry about.

    Finally, this just measures ionized dissolved solids and doesn't care if it is ammonium nitrate fertilizer (good fertilizer) or sodium chloride (not good for plants). It cannot identify urea - even though it is a good nitrogen nutrient source.

    I don't know what these are best used for but they only measure total dissolved salts.
     
    Last edited: Aug 8, 2013
  6. Externet

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    Nov 29, 2005
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    Then, what are the units for 'CF' and 'EC' ? ---Siemens/cm :confused:---Ω⋅cm :confused:---

    I did not say value less, I said unit-less.
     
    Last edited: Aug 8, 2013
  7. wayneh

    Expert

    Sep 9, 2010
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    True, but the deviation from normal refraction tells you how much of the solute is present. Grape buyers measure sugar content of grape juice this way, for instance. I agree refractive index is not so useful if you're looking at final dilutions of fertilizer in 100 ppm range. Too little deviation, and I would go with something else.

    As with conductivity you need to know something about the nature of the solute to relate the reading to actual composition. It's fairly common to compute concentration assuming sodium chloride if the composition is generic or unknown.
     
  8. LDC3

    Active Member

    Apr 27, 2013
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    Purified water has a resistivity of about 18MΩ·cm.
    If salt is added to reduce the resistivity to 0.1MΩ·cm, the solution can conduct a current of 10μS/cm.
     
  9. PackratKing

    Well-Known Member

    Jul 13, 2008
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    Unless...

    Using a Fluke 77 DMM, I determined there was enough straight-up difference in the conductivity of brine, simply by sticking probes into the solution, spaced 1/2" and watching the resistance change considerably as I diluted the brine... so on that note,

    I devised circuitry for a switching 2 wash-machine solenoid water valves based on brine concentration, for a Culligan reverse-osmosis water softener.
    The sensor, reads the resistance / conductivity of the full-strength brine at the start of the backwash cycle, remanding that to the sewer, and when the salt in solution is reduced enough, it switches the flow to a holding tank for watering the lawn and flowerbeds.

    Since the flush cycle uses copious amounts of water, I figured why waste it ...
     
    Last edited: Aug 8, 2013
  10. wayneh

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    Sep 9, 2010
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    The solution to pollution is dilution. ;)
     
  11. GopherT

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    Nov 23, 2012
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    I was trying to be funny in my post above because the units used are a bit of a mystery. The highlited box on the link you provided is reproduced below. The ppm cannot be calculated from conductivity unless you know the weighted average molecular weight of the ion mixture you are measuring. the product is based on some standards that have been developed over time in the horticulture industry (doubtful), or it points to"quackery" (more likely).


    =============================

    TERMINOLOGY

    EC= Electrical Conductivity (the measure of total dissolved solids in a solution)

    CF = Conductive Factor (ditto)

    ppm = Parts Per Million (same)

    TDS = Total Dissolved Solids (read in ppm)

    ====================
     
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