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I have an application where I need a solenoid to control a valve at the bottom of a tank. For unimportant reasons the valve needs to be inside the tank at the bottom.

I was wondering are all solenoids inherently waterproof?

Current will follow the path of least resistance (kirchhoff's current laws prove this).

I did some basic internet searches and turned up that on the lowest end the resistivity of water is 20 Ohms per meter. If my solenoid is 24V and draws .6 amps that means the coil (plus added resistor) is 40 ohms.

Therefore the current would prefer to go through the water. I tested this theory by dropping my solenoid in my tank and running the power. It worked perfectly. But it should be noted my tank is plastic and is not grounded.

If I had grounded the water in the tank would my results have been different? or is there something else making the solenoid's coil have a lower resistance than the water in the tank?

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    \$\begingroup\$ "Current will follow the path of least resistance (kirchhoff's current laws prove this)." - This is certainly not the case. Are you actually familiar with Kirchhoff's laws? \$\endgroup\$ Oct 12, 2014 at 6:14
  • \$\begingroup\$ @user272845: The only Kirchhoff's Current Law I know says "The sum of all currents in a node is zero". What other KCLaws do you know? \$\endgroup\$
    – Curd
    Oct 12, 2014 at 13:46
  • \$\begingroup\$ @pericythion "the sum of all currents in a node is zero" is exactly the law I am referencing... the lower the resistance the more current will take that path. If the resistance of the coil is significantly smaller than the sum resistances of all other paths (as in the application of my question) then the current will for all intents and purposes follow the coil. nitpicky semantics really \$\endgroup\$
    – user272845
    Oct 14, 2014 at 7:17
  • \$\begingroup\$ @user272845: You misunderstand, so I'll paraphrase: It's Ohm's law which states that for a fixed voltage across a resistance the current through the resistance will vary inversely with the change in resistance, while Kirchoff's current law states that if a battery is pumping 1 ampere into the junction of the battery and a resistor (+) then there must be one ampere leaving that junction (-) to get back to the battery, making the algebraic sum of the currents in that node (junction) equal to zero. \$\endgroup\$
    – EM Fields
    Oct 14, 2014 at 9:23

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No they are not generally waterproof. The current will corrode the copper away over time and your coil will fail, even though the current flow will be much less than you think ( probably < a few mA ).

Even a small pinhole in the insulation of the copper wire will be enough to cause failure eventually.

If you get a solenoid with a molded sealed coil and waterproof the leads you should be able to make this work. Or, better, keep the electrics away from the water and run an actuator down into the water.

Edit: With regard to your specific question about "grounding the water in the tank", that does not make a lot of sense. The water is a bulk material with a relatively high resistivity. If you placed a metal plate within the tank and grounded the metal plate, and if the power to the solenoid was at some voltage with respect to ground, current would flow through the water. The details are actually quite difficult to calculate simply by hand because the electric field will not be simple. It's pretty easy to simulate with suitable software.

However, suppose you had a 1cm x 1cm x 10cm long rectangle of water with plates at either of the long ends and the resistivity of the water was 40 ohms-meter, then the current would be:

R= \$\rho L \over A \$ = \$40 \Omega-m \cdot 0.1m \over {0.01m \cdot 0.01m} \$ = 40k\$\Omega\$

so with 24V, 600uA would flow. This is kind of a "spherical horse in a vacuum" theoretical calculation but it should serve to illustrate that the currents involved will not be anything like the coil current (well, unless you get salt or other ionic contaminants dissolved in the water).

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  • \$\begingroup\$ Ah I didn't think about the corrosive effects. Good point. However this didn't quite answer my question, I'm more interested in what would have happened if I grounded the water. Are you suggesting all solenoids have an insulated coil? \$\endgroup\$
    – user272845
    Oct 12, 2014 at 6:10
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    \$\begingroup\$ Solenoids use layers of copper wire wound in turns that lie next to each other. If the wire was not insulated a whole layer of the wiring would become a short circuit. The common insulation used is an enamel coating on the wire. \$\endgroup\$ Oct 12, 2014 at 6:36
  • \$\begingroup\$ See edit regarding conductivity etc. \$\endgroup\$ Oct 12, 2014 at 7:32
  • \$\begingroup\$ @SpehroPefhany That was exactly the answer I was looking for. I came up with similar numbers for de-ionized water, my lower limit was for very ionized water so that explains why our numbers are different by a factor of 1000! Thanks for the answer \$\endgroup\$
    – user272845
    Oct 14, 2014 at 7:07
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1) Solenoids are not inherently waterproof.

2) The resistivity of the water isn't given in ohms per meter, it's given in ohm-meters, where the resistance is measured between any two parallel faces of a one meter cube.

3)Re. Kirchoff; from here: "At any node (junction) in an electrical circuit, the sum of currents flowing into that node is equal to the sum of currents flowing out of that node."

4) The resistance of your coil would not change, but it would be paralleled by the very much larger resistance of the water between the solenoid terminals paralleled by the water surrounding the terminals.

5) if your tank and one side of the supply were electrically connected, (whether the tank was "grounded" or not), the current from the supply would increase very slightly.

6) The resistivity of the copper in the coil being about a billion times lower than the resistivity of water is what makes the coil have a lower resistance than the water surrounding it.

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  • \$\begingroup\$ But doesn't that mean they are waterproof? The substantially higher impedance of the water makes the solenoid "waterproof" because the current will continue to follow the coil whether or not it is submerged. \$\endgroup\$
    – user272845
    Oct 14, 2014 at 7:18
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    \$\begingroup\$ "Waterproof" implies the capability of mitigating all of the deleterious effects of water, a property not inherent in all solenoids. \$\endgroup\$
    – EM Fields
    Oct 14, 2014 at 8:51

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