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Maximum safe voltage for light bulb

  1. Mar 12, 2013 #1
    Hi, I am trying to build a conductivity meter as part of a school project (I have to measure the conductivity, among other things, of local water systems). Unfortunately, I cannot buy one for this project, as it should be home-made for these purposes.

    If I wanted to create a meter that actually quantitatively measures the conductivity of water samples, would there be a way to create one at home using supplies from hardware stores? Or, would I simply have to buy one to get an actual qualitative reading and not have to guess conductivity based on how bright the bulb on my crude home-made one is?

    If there is no short way, how best might I go about measuring conductivity with a simple light-bulb based meter where I judge conductivity based on how bright the bulb is?

    Also, if I had to use a light bulb, what would be the maximum voltage I could put on the bulb, since using say a 9v battery would most definitely blow a 2.4v bulb from a flashlight? Is there a definite ratio between recommended voltage and maximum safe voltage?
  2. jcsd
  3. Mar 12, 2013 #2
  4. Mar 13, 2013 #3
    If you have to build the meter yourself from scratch, you could wind yourself an electro magnet and use it to move a needle. A simpler method would be to fill a test tube with water and a small amount of baking soda. Then you let a current flow through it and measure how much gas is being produced.
  5. Mar 13, 2013 #4


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    Even if you built a conductivty meter you would have to calibrate it against a "standard" to verify its readings.

    “Conductivity is measured with a probe and a meter. Voltage is applied between two electrodes in a probe immersed in the sample water. The drop in voltage caused by the resistance of the water is used to calculate the conductivity per centimeter. The meter converts the probe measurement to micromhos per centimeter and displays the result for the user. NOTE: Some conductivity meters can also be used to test for total dissolved solids and salinity. The total dissolved solids concentration in milligrams per liter (mg/L) can also be calculated by multiplying the conductivity result by a factor between 0.55 and 0.9, which is empirically determined (see Standard Methods #2510, APHA 1992).

    Suitable conductivity meters cost about $350. Meters in this price range should also measure temperature and automatically compensate for temperature in the conductivity reading. Conductivity can be measured in the field or the lab. In most cases, it is probably better if the samples are collected in the field and taken to a lab for testing. In this way several teams of volunteers can collect samples simultaneously. If it is important to test in the field, meters designed for field use can be obtained for around the same cost mentioned above.”
  6. Mar 13, 2013 #5
    See, it's the giant cost that I'm trying to avoid, while making something practical. Neither spending $350 or waiting and measuring gas created is going to be very practical for multiple testing occasions for a simple project. Neither is measuring current going to help much. If it helps, the conductivity meter is mainly to measure dangerous levels of salinity in fresh water systems around where I live. Thanks, though!
  7. Mar 13, 2013 #6


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    I don't know what would be considered dangerous levels of salinity. Would this $99 Handheld "low-concentration" Salinity Meter work?

  8. Mar 15, 2013 #7
    Well, the meter above would work, but I'm trying to avoid high cost, so I just made a simple light-bulb based meter and all the locations I tested were non conductive entirely.
  9. Mar 15, 2013 #8


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    A light bulb would require significant current but a small LED would do the job quite adequately with only a few mA and would vary in brightness in a convincing way that would roughly show variations in conductivity of the water sample.
    Two plates of a few cms square (stainless steel spoons?), separated by a cm or so could be your cell. Then put a Red LED (which operates with around 1.8V) and a fixed resistor of around 100 Ω in series a couple of AA batteries. If you start with tap water and gradually add salt and mix, I reckon you would see the LED gradually get brighter and brighter. Varying the separation of the plates will give you a control of the current / brightness, too.
    You can either do all the sums and calculate the resistance you'd expect for a given concentration of salt; this link tells you how the concentration will affect the conductivity of a salt solution. Or you can 'suck it and see' what you get. Have a spare LED with you in case you burn one out but that's the only harm you can do. Using an LED, which takes very little current will save on batteries, too.
  10. Mar 16, 2013 #9
    You could also build youself an analog to digital converter out of a comparator IC (e.g. LM339). Then you could have several LEDs that display the resistance in binary.
  11. Mar 16, 2013 #10


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    Sky's the limit, of course but this is a school project. Quick results are very desirable so one LED with varying brightness is favourite.

    Something just struck me. A second LED, fed through a fixed resistor, would give a good reference brightness to compare the varying / measuring LED.
  12. Mar 16, 2013 #11
    I'm not clear on whether cost is the primary hurdle or limits are imposed by an instructor that require you to DIY. Anyway, I know very little EE but it occurred to me that the following ultra cheap little device (this one's eight bucks) measures moisture according to the soil's conductivity. I wonder if it's calibrated, or could be recalibrated with a bit of tinkering, to roughly measure salinity in water samples. It doesn't even require a power source.
    Last edited: Mar 16, 2013
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