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Posted By: NJwirenut Submersible limit switches? - 06/23/06 01:43 PM
I am looking for some limit switches for an underwater application. They need to withstand continuous immersion in fresh water at a depth of 6-7 feet. I need NC contacts, for operation at 24VAC.

Cutler-Hammer has their E50 6P+ series, which are rated NEMA 6P/IP67, but for continuous immersion, I really think I need an IP68 rating. Does anyone know of a source for these?
Posted By: e57 Re: Submersible limit switches? - 06/23/06 11:32 PM
limit switches? Underwater?

Whats the application? Float switch for water level, or some other type of mechanism that just happens to be in the water. If the later, it may be wise to remote the switch above water by use of push rods, cams or other means. Just for a maintenance point of veiw.

Did one on a bicycle brake cable once..... Not in water, but sure you can get teflon lined stainless cable . Just an idea.



[This message has been edited by e57 (edited 06-23-2006).]
Posted By: NJwirenut Re: Submersible limit switches? - 06/24/06 12:26 AM
Application is a hydraulically operated lift mechanism to position a large (4x8 foot) mirror for underwater photography in a towing tank used for scale model testing. The mirror normally sits flush with the tank floor, but is hinged along one of the long edges, and can be raised up to a 45 degree angle by a hydraulic motor and acme screw "scissor jack" mechanism recessed into a shallow pit under the mirror. The tank itself is essentially a swimming pool 8'deep, 16'wide, and 312'long.

Linkages to the surface will be difficult, as nothing can be run through the water column above the mirror, as it would interfere with the tests being run. There is a 3" PVC conduit between the pit and the outside of the tank (above waterline), to be used as a chase for the hydraulic hoses and limit switch wiring.

I need limit switches to detect the end of travel in both directions, to avoid overtravelling the device and twisting the frame/cracking the mirror.

This mirror mechanism is just ONE of the strange projects I am involved with these days. The hydrodynamics laboratory I work for is in the process of a major renovation/upgrade, and myself and our machinist are responsible for just about all the specialized apparatus needed. This includes a large servohydraulic wavemaker, 125 HP towing carriage drive with .01% speed repeatability, 300' long flexible cable carriers for power and instrumentation to a moving carriage, and racks of support electronics/signal conditioners, all tied together with literally miles of low voltage signal and control cabling.

Hope to get some pics of the neater stuff and post them here on ECN.


PS, as far as maintenance is concerned, we have SCUBA divers on-staff for the minor stuff, and the tank gets drained about once a year for regular PM.

[This message has been edited by NJwirenut (edited 06-23-2006).]
Posted By: JoeTestingEngr Re: Submersible limit switches? - 06/24/06 01:37 AM
Wow, sounds like you get to play with some fun stuff! Was some of this used in prior attempts to kill a Roadrunner? Is there any chance of coupling position pulses off of the screw drive? Wouldn't there be pressure spikes in the hydraulic lines at either end of travel. I believe that is one way that some systems determine when tools of various lengths contact the work surface.
Joe
Posted By: NJwirenut Re: Submersible limit switches? - 06/24/06 01:45 AM
I considered pressure switches on the hydraulic lines, but I would rather not have the mechanism bind against the stops, but come to a repeatable, controlled stop by bumping a limit switch.

Magnets and reed switches are another possibility, but I just don't trust them as much as a straight mechanical switch in this type of heavy machinery application.
Posted By: LarryC Re: Submersible limit switches? - 06/24/06 02:28 AM
A few options spring to mind.

First: Fasten an inclinometer to the backside of the mirror. This will give you an analog signal proportional to mirror tilt. Then use any standard process controller for set point control for upper and lower limits. This also allows the option of ramping the travel speed up or down for gentler starts and stops, and shorter travel time.

Second: Use any IP68 sealed proximity switch adjusted to operate at desired location.

Third: Clippard along with the rest of the pneumatic and hydraulic component manufacturers make manually operated control valves. Use a piston style switch to supply or release fluid pressure when the mirror carriage is at its desired position.

Fourth: Attach a sealed shaft encoder to the acme-threaded shaft and generate pulses to count revolutions to determine upper and lower travel limits. Omega has all sorts of controllers to handle the pulse counting and limit set points.

LarryC
Posted By: Rick Kelly Re: Submersible limit switches? - 06/24/06 03:17 AM
Can you replace the hydraulic motor with a hydraulic cylinder?

If so... why not duplicate the internal cylinder with a cylinder on the outside of the tank. Pipe the two together, fill the system with oil and bleed all air out. Apply force to the cylinder outside of the tank and you will get a corresponding motion inside the tank. Apply your control sensors to the cylinder outside the tank and you are good to go.

[This message has been edited by Rick Kelly (edited 06-23-2006).]
Posted By: Tesla Re: Submersible limit switches? - 06/24/06 07:37 AM
Do it optically with a laser beam.

The laser would bounce down and up from above the water.

You could place photo cells at certain sweet spots or use a mark.

I'd use an off the shelf laser from PLS.

It's not like you're swinging this thing up and down all the time, or are you?
Posted By: winnie Re: Submersible limit switches? - 06/24/06 02:04 PM
I also need reliable continuously submersible switches, for a model robot submarine. We have been using some IP-68 rated switches sold by McMaster ( http://www.mcmaster.com )as part# 6944K11. These have proven to be reliable, and cheap enough, but don't have much in the way of mounting options.

I've been wondering if 'float type' liquid level switches could be used, with the float removed and a different mechanical actuator in place. Many of these switches seem to be rated to several atmospheres, but I don't know the details of their operation or sealing. Judging by the pictures in the mcmaster catalog, many of these look magnet based, but when you have a well defined and enclosed mechanical system using magnets I bet they are pretty reliable.

What I would really like to buy is a completely waterproof lanyard switch, where you have a NO switch held closed by some sort of pin or clip, with the pin or clip on a lanyard, so that you can open the switch with a yank on the lanyard.

-Jon
Posted By: NJwirenut Re: Submersible limit switches? - 06/24/06 02:56 PM
I just found these:
http://www.seacon-usa.com/Products/giannini_switches.htm

It looks like they might have something that will fit my application, and maybe yours, Jon. No idea about price yet...

Replacing the motor and screw with a simpler cylinder isn't an option because of the limited depth of the pit under the mirror (about 8"). There may be an inclinometer installed on the mirror for setting intermediate angles, but I wouldn't want to rely on that type of electronic system as a failsafe limit device.




[This message has been edited by NJwirenut (edited 06-24-2006).]
Posted By: gfretwell Re: Submersible limit switches? - 06/24/06 04:35 PM
We use those float switches that just hang on the cord in our well aerators and they seem to last forever in some pretty nasty well water.
Posted By: mhulbert Re: Submersible limit switches? - 06/26/06 12:55 AM
Will a proximity sensor work instead? Many of them are at least IP67, I'm sure there are IP68 models out there, but I can't seem to find a specific one yet.

Also, what about an optical sensing unit, with a fiber optic strand run down to the sensing location? Might make maintenance easier.
Posted By: NJwirenut Re: Submersible limit switches? - 06/26/06 01:35 AM
Since we are building both the mechanism and the control system for it, a proximity switch is certainly an option. But they add the complications of providing DC power to the units and adding relays to convert the open collector outputs to NC relay contacts. Never having used prox switches underwater, how is their range and repeatability affected by sensing a target through water, rather than air?

For stuff like this, I'm a big fan of the KISS principle. And it doesn't get much simpler than a pair of switch contacts interrupting power to the motor starter coil.
Posted By: JBD Re: Submersible limit switches? - 06/26/06 01:54 AM
Many heavy duty industrial limit switches,like Square D Class 9007 Type C, are rated NEMA 6P submersible.
Posted By: LarryC Re: Submersible limit switches? - 06/26/06 04:06 AM
"And it doesn't get much simpler than a pair of switch contacts interrupting power to the motor starter coil."

I really hope you mean interupting the signal to the hydraulic valve(s). Controlling a hydraulic system by turning the pump on and off isn't a reliable method of maintaining position or control. Use two adjustable stops on the threaded shaft. Set the prox switch to generate the stop signals at each end of desired travel, and use the shaft stops to hit hard stops to limit travel in case of end of travel switch failure.

Set the hydraulic relief valve to lift when the motor driven frame hits the hard stop.

LarryC

[This message has been edited by LarryC (edited 06-26-2006).]
Posted By: NJwirenut Re: Submersible limit switches? - 06/26/06 11:14 AM
JBD:

If you read the actual specs for NEMA 6P classification, they specify TEMPORARY immersion in water of LIMITED DEPTH (1 meter, IIRC). My application involves continuous immersion at 6-7 feet.

Larry:

Actually, the plan is to cut power (using a pair of "ice cube" relays) to both the directional control (spool) valve AND the hydraulic pump starter.

Yes, the relief valve is going to be the backup safety device in case of a switch failure.


[This message has been edited by NJwirenut (edited 06-26-2006).]
Posted By: JBD Re: Submersible limit switches? - 06/26/06 03:09 PM
The "P" stands for prolonged submersion versus the temporary rating of just plain NEMA/TYPE 6. I do not know the actual depth.
Posted By: FountainGuy Re: Submersible limit switches? - 06/26/06 07:40 PM
I like Tesla's idea best; photo optics is a good option, or use a pressure transmitter or pressure switch. Even though you're talking about 6P submersible enclosures, keeping electricity out of water is always a good idea. I'm all too familiar with reed switches. They fail too easily; moving parts. I try to design fountains as simple as possible. Whenever electronic water level controls are necessary, conductivity sensors are the simplest to maintain, modify and understand.
Posted By: feather Re: Submersible limit switches? - 06/27/06 01:19 AM
In the old days we would just solder a mercury switch to a couple of leads, seal with tar and mount them. o'course mercury is a bad word nowa days.
Posted By: Mike Wescoatt Re: Submersible limit switches? - 06/27/06 02:49 PM
What we did for our lifts underwater is used cable type position encoders. the encoders give us high resolution feedback on the position of the cable on a calibrated spool. The encoder sits above water and the only thing that is underwater is the stainless steel aircraft cable attached (by way of pulleys) to the lift. This gives us absolute (not realitive) feedback on position to 0.01" which we round off for error correction. We also used prox sensors mounted in the wall of a waterproof box mounted next to a magnet on a cylinder mount for "home" positioning. You could use a force home position if you need to.
Posted By: Trumpy Re: Submersible limit switches? - 06/27/06 03:11 PM
IP68 of IEC 639 states "that this rating is for immersion in water of depth of at least 20 metres, continuously".
Want to argue??.
Posted By: Trumpy Re: Submersible limit switches? - 06/27/06 03:23 PM
A few questions Kenny.
What sort of load will the sensors be running?.
Could you do this with a PLC?.
All sensors I've worked with have a rating of IP 68, capacitive or inductive.
Posted By: NJwirenut Re: Submersible limit switches? - 06/27/06 07:00 PM
It's Bob, not Kenny...

Switches are in series with the coils of small relays, which in turn operate a dual coil solenoid hydraulic valve (to control the direction of oil flow), and a motor starter which runs the hydraulic pump (1.5 HP).

A PLC would seem to be extreme overkill to me for such a simple application. The control setup is simply a center-off SPDT switch which selects up or down motion, which pulls in one of 2 relays (through the associated limit switch) depending on desired direction. The selected relay then applies power to the appropriate solenoid, as well as the pump starter.

No argument at all about the meaning of IP68. That's exactly what I am looking for. But I haven't found any limit switches rated IP68, only IP67/NEMA 6P. If a proximity switch will work properly underwater (as opposed to simply surviving immersion), they might be an option. But they will require a DC power supply, and additional relays to interface them to the system. A pair of mechanical limit switches would be simpler, if I can get some that will tolerate life underwater.

[This message has been edited by NJwirenut (edited 06-27-2006).]
Posted By: gfretwell Re: Submersible limit switches? - 06/27/06 08:33 PM
Bob, those float switches I am talking about live virtually forever underwater. I have one that came with my house that is at least 30 years old and they are cheap ($25-30)
Posted By: NJwirenut Re: Submersible limit switches? - 06/27/06 09:02 PM
I think I know the switches you mean, and they are great for float switches. Damn near indestructable. But i need something to sense the position of a mechanism very repeatably and with fair precision.

Not sure how I would do that with a float switch....

[This message has been edited by NJwirenut (edited 06-27-2006).]
Posted By: Mike Wescoatt Re: Submersible limit switches? - 06/28/06 01:00 PM
Try a magnet and prox sensor mounted in a waterproof box with only the prox head sticking out. Most of the big names have harsh environment heads that will deal with heat, water, cold, chemicals...

Using a prox sensor means you could use 24VDC (preferred) or 24VAC in the pool for safety, then use that to pilot a relay to do whatever on the topside of the pool away from the water.
Posted By: John Crighton Re: Submersible limit switches? - 06/28/06 10:23 PM
In 2003, Honeywell upgraded the entire 914CE limit switch line to fluorocarbon seals complying with IP68. See 914CE16-3, in stock at Allied:
http://www.alliedelec.com/catalog/pf.asp?FN=846.pdf
Posted By: NJwirenut Re: Submersible limit switches? - 06/28/06 11:19 PM
Thanks John! That looks like EXACTLY what I had in mind. I never thought of looking at an electronics catalog for these, but instead went to the electrical suppliers. Newark Electronics stocks those Honeywell units, as well, and we have a standing account with them.
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