Sampling Core Question

[RickV84]

I am attempting to analyze interstitial water chemistry, and for that I'll be using sediment core sample with overlying water from the estuary. To ensure the water doesn't become unnaturally stratified, I want to gently perturb the water column. I was thinking of using a the engine and propeller from a toy boat, but I haven't come across anything that would work. Does anyone have any ideas?

It shouldn't be much longer than four inches and should be able to be completely submerged in water while still being able to operate. I only want to make a small incision to the core top to run the electrical; I'd rather not have to make a hole so large for a driving axle.

Any help you folks might be able to offer would be greatly appreciated!

[studiot]

Sounds interesting, but I'm not exactly clear what you want to do.

 

Perhaps if you explained what you want to do in (slightly) more detail?

 

[CharonY]

Not my field, but have lots of people who did it a floor down, who I could ask if necessary. What type of corer do you use and what precisely is the main concern?

[RickV84]

I will have simple gravity cores that penetrate the benthic layer (bay floor) of the estuary. The core will contain mostly sediment, but ~20% of the core will contain the overlying water. While I'm waiting for the probes in the sediment to equilibriate, I want the overlying water to stay well-mixed. With that, I'll need some sort of stirring mechanism that is on the underside of the core cap, in the overlying water, but not on the sediment.

 

My thought was to adhere a small RC boat or submarine motor and propeller to stir the water with a small hole in the core cap would be cut to run the electrical. Unfortunately, the closest thing I came across was a completely sealed unit that only runs for an hour on its battery. I would need it to run for about 24 hours.

[StringJunky]

Just throwing out there, but what about a magnetically-coupled stirrer - no need to make any holes? Basically you want a motor with a magnet on a spindle and the another magnet inside your sample.

[RickV84]

I actually tried this very thing working under a research professor before to poor avail for several reasons:

1. The magnetic plate was often too weak to spin the stirrer through the plastic core cap

2. The magnetic stirrer that would fit under the cap wasn't large enough to mix the water closer to the sediment

3. A mesh was still required to hold the magnetic stirrer close to the cap. If we left any gap between the stirrer and core cap, the plate was never strong enough to stir the rod; when we secured the mesh tight enough so the stirrer was close to the core cap, it was so often too tight it prohibited the rod from stirring.

 

Nonetheless, this still may be the route I go if I can't find a better substitute.

 

Thanks to everyone that has asked questions and contributed, by the way!

[CharonY]

Let me ask around. I vaguely remember that someone had a similar problem (I think, I tend to let the outdoorsie people just do their thing and only look at it once I get it in the lab). I could also have misremembered and they had the opposite problem in areas which more dynamic flow.

[StringJunky]

Neodymium magnets are the strongest. If you pursue that route again, and if you haven't used them yet, then they might be worth a look.

[studiot]

The impeller / propeller bit is easy, I'm sure you can improvise.

 

Driving it for 24 hours or so, have you considered compressed air as a power source?

 

The air could drive a fan that drove the impeller in the water.

 

SJ's comments are one way.

The actual fan need not have any physical connection to the impeller.

That is how my central heating pump works.

There is an electrically driven impeller in one chamber that is spearated from the impeller in the water chamber by a membrane.

Both impellers are magnetic.

 

A shaft passing through your cap via a sealing gland could also be spun by an air impeller, driving a water impeller in your sample tube.

 

Yo need to dismantle some old central heating pumps.