Hi

I've designed a flow cell made up of 2 pieces of plexiglas with fluid
flowing in between. The top and bottom pieces are separated by a gasket
and pressed together by fasteners. The thickness of the gasket therefore
defines the depth of the flow channel. I would like to vary the
thickness from 0.5 mm to 2mm (after compression) by using gaskets of
different thickness. When fluid is flowing through the flow cell,
depending on the flow velocity, the internal pressure can vary from 0 to
5 atmosphere. Right now, I'm sourcing for a suitable gasket material.
Apart from the main criterion that it mustn't leak, it is also
imperative that its thickness be relatively constant after a certain
compressive stress. This requirement is necessary as the fasteners are
hand tightened and so compression forces can vary by quite alot.
Finally, the material should resists acids and alkalies. What would you
recommend?

Thanks.

From:	Uncle Al
Subject:	Re: What gasket material should I use?
Date:	Wed, 29 Dec 2004 09:02:06 -0800

Philip wrote:
>
> Hi
>
> I've designed a flow cell made up of 2 pieces of plexiglas with fluid
> flowing in between. The top and bottom pieces are separated by a gasket
> and pressed together by fasteners. The thickness of the gasket therefore
> defines the depth of the flow channel. I would like to vary the
> thickness from 0.5 mm to 2mm (after compression) by using gaskets of
> different thickness. When fluid is flowing through the flow cell,
> depending on the flow velocity, the internal pressure can vary from 0 to
> 5 atmosphere. Right now, I'm sourcing for a suitable gasket material.
> Apart from the main criterion that it mustn't leak, it is also
> imperative that its thickness be relatively constant after a certain
> compressive stress. This requirement is necessary as the fasteners are
> hand tightened and so compression forces can vary by quite alot.
> Finally, the material should resists acids and alkalies. What would you
> recommend?

A spacer that is a rigid core (e.g., a square loop of Plexiglas) with
a bead (o-ring) of gasket sitting in a milled groove in the spacer and
the plate on each side. Use a torque wrench to tighten over several
cycles of opposite fastener adjustment or just squish down the bead.

How acidic or basic? What about leaching? Ordinary vulcanized rubber
ought to do it. A suitable durometer Kraton rubber if you want to get
clean and fancy about it.

--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/qz.pdf

From:	Stackclimber
Subject:	Re: What gasket material should I use?
Date:	Wed, 29 Dec 2004 18:45:55 -0600

I was inclined toward a hollow silicone rubber oring. It will deform, but
have good rebound.

"Uncle Al" wrote in message
news:41D2E30E.2D199E@hate.spam.net...
> Philip wrote:
>>
>> Hi
>>
>> I've designed a flow cell made up of 2 pieces of plexiglas with fluid
>> flowing in between. The top and bottom pieces are separated by a gasket
>> and pressed together by fasteners. The thickness of the gasket therefore
>> defines the depth of the flow channel. I would like to vary the
>> thickness from 0.5 mm to 2mm (after compression) by using gaskets of
>> different thickness. When fluid is flowing through the flow cell,
>> depending on the flow velocity, the internal pressure can vary from 0 to
>> 5 atmosphere. Right now, I'm sourcing for a suitable gasket material.
>> Apart from the main criterion that it mustn't leak, it is also
>> imperative that its thickness be relatively constant after a certain
>> compressive stress. This requirement is necessary as the fasteners are
>> hand tightened and so compression forces can vary by quite alot.
>> Finally, the material should resists acids and alkalies. What would you
>> recommend?
>
> A spacer that is a rigid core (e.g., a square loop of Plexiglas) with
> a bead (o-ring) of gasket sitting in a milled groove in the spacer and
> the plate on each side. Use a torque wrench to tighten over several
> cycles of opposite fastener adjustment or just squish down the bead.
>
> How acidic or basic? What about leaching? Ordinary vulcanized rubber
> ought to do it. A suitable durometer Kraton rubber if you want to get
> clean and fancy about it.
>
> --
> Uncle Al
> http://www.mazepath.com/uncleal/
> (Toxic URL! Unsafe for children and most mammals)
> http://www.mazepath.com/uncleal/qz.pdf

From:	Uncle Al
Subject:	Re: What gasket material should I use?
Date:	Wed, 29 Dec 2004 17:25:48 -0800

Stackclimber wrote:
>
> I was inclined toward a hollow silicone rubber oring. It will deform, but
> have good rebound.

1) Don't top post.
2) Silicone is a sponge for everything.
3) A hollow silicone o-ring? Send some money to Uncle Al.
4) Buna will do it. Get a catalog.

> "Uncle Al" wrote in message
> news:41D2E30E.2D199E@hate.spam.net...
> > Philip wrote:
> >>
> >> Hi
> >>
> >> I've designed a flow cell made up of 2 pieces of plexiglas with fluid
> >> flowing in between. The top and bottom pieces are separated by a gasket
> >> and pressed together by fasteners. The thickness of the gasket therefore
> >> defines the depth of the flow channel. I would like to vary the
> >> thickness from 0.5 mm to 2mm (after compression) by using gaskets of
> >> different thickness. When fluid is flowing through the flow cell,
> >> depending on the flow velocity, the internal pressure can vary from 0 to
> >> 5 atmosphere. Right now, I'm sourcing for a suitable gasket material.
> >> Apart from the main criterion that it mustn't leak, it is also
> >> imperative that its thickness be relatively constant after a certain
> >> compressive stress. This requirement is necessary as the fasteners are
> >> hand tightened and so compression forces can vary by quite alot.
> >> Finally, the material should resists acids and alkalies. What would you
> >> recommend?
> >
> > A spacer that is a rigid core (e.g., a square loop of Plexiglas) with
> > a bead (o-ring) of gasket sitting in a milled groove in the spacer and
> > the plate on each side. Use a torque wrench to tighten over several
> > cycles of opposite fastener adjustment or just squish down the bead.
> >
> > How acidic or basic? What about leaching? Ordinary vulcanized rubber
> > ought to do it. A suitable durometer Kraton rubber if you want to get
> > clean and fancy about it.
> >
> > --
> > Uncle Al
> > http://www.mazepath.com/uncleal/
> > (Toxic URL! Unsafe for children and most mammals)
> > http://www.mazepath.com/uncleal/qz.pdf

--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/qz.pdf

From:	Philip
Subject:	Re: What gasket material should I use?
Date:	Wed, 29 Dec 2004 23:13:02 -0800

Uncle Al wrote:
> Stackclimber wrote:
>
>>I was inclined toward a hollow silicone rubber oring. It will deform, but
>>have good rebound.
>
>
> 1) Don't top post.
> 2) Silicone is a sponge for everything.
> 3) A hollow silicone o-ring? Send some money to Uncle Al.
> 4) Buna will do it. Get a catalog.

Thanks everyone for the response. The use of spacers/washers should do
the trick. Why didn't I think of that!

On the gasket material, from my web searh, it seems that EPDM is more
resistant toward acids (eg. 0.1% H+) and alkalis (0.1% OH-). Or is
teflon/goretex a better choice?

From:	MM
Subject:	Re: What gasket material should I use?
Date:	Thu, 30 Dec 2004 15:42:00 GMT

Philip wrote:

> Uncle Al wrote:
>
>> Stackclimber wrote:
>>
>>> I was inclined toward a hollow silicone rubber oring. It will deform,
>>> but
>>> have good rebound.
>>
>>
>>
>> 1) Don't top post.
>> 2) Silicone is a sponge for everything.
>> 3) A hollow silicone o-ring? Send some money to Uncle Al.
>> 4) Buna will do it. Get a catalog.
>
>
> Thanks everyone for the response. The use of spacers/washers should do
> the trick. Why didn't I think of that!
>
> On the gasket material, from my web searh, it seems that EPDM is more
> resistant toward acids (eg. 0.1% H+) and alkalis (0.1% OH-). Or is
> teflon/goretex a better choice?

be sure to control your squeeze (initial compression) and extrusion gap
(the space between the 2 flanges) to make sure you don't leak. but
then, 5 atm isn't that much. squeeze will be your main concern, to
activate the seal before operation. a starting reference is at:
http://www.parker.com/ead/cm2.asp?cmid=3903&
the book to download is:
http://www.parker.com/o-ring/Literature/ORD5700.pdf

cheers

From:	Phil Hobbs
Subject:	Re: What gasket material should I use?
Date:	Wed, 29 Dec 2004 11:31:43 -0500

Philip wrote:
> Hi
>
> I've designed a flow cell made up of 2 pieces of plexiglas with fluid
> flowing in between. The top and bottom pieces are separated by a gasket
> and pressed together by fasteners. The thickness of the gasket therefore
> defines the depth of the flow channel. I would like to vary the
> thickness from 0.5 mm to 2mm (after compression) by using gaskets of
> different thickness. When fluid is flowing through the flow cell,
> depending on the flow velocity, the internal pressure can vary from 0 to
> 5 atmosphere. Right now, I'm sourcing for a suitable gasket material.
> Apart from the main criterion that it mustn't leak, it is also
> imperative that its thickness be relatively constant after a certain
> compressive stress. This requirement is necessary as the fasteners are
> hand tightened and so compression forces can vary by quite alot.
> Finally, the material should resists acids and alkalies. What would you
> recommend?
>
> Thanks.

If you put Belleville washers on the screws, you can have nearly constant
clamping force over 20%-80% deflection. Alternatively, you can allow the
flat surfaces to contact each other after some specific deflection.

Cheers,

Phil Hobbs

From:	David Deuchar
Subject:	Re: What gasket material should I use?
Date:	Wed, 29 Dec 2004 17:02:07 GMT

"Phil Hobbs" wrote in message
news:41D2DBEF.9020700@us.ibm.com...
> Philip wrote:
>> Hi
>>
>> I've designed a flow cell made up of 2 pieces of plexiglas with fluid
>> flowing in between. The top and bottom pieces are separated by a gasket
>> and pressed together by fasteners. The thickness of the gasket therefore
>> defines the depth of the flow channel. I would like to vary the thickness
>> from 0.5 mm to 2mm (after compression) by using gaskets of different
>> thickness. When fluid is flowing through the flow cell, depending on the
>> flow velocity, the internal pressure can vary from 0 to 5 atmosphere.
>> Right now, I'm sourcing for a suitable gasket material. Apart from the
>> main criterion that it mustn't leak, it is also imperative that its
>> thickness be relatively constant after a certain compressive stress. This
>> requirement is necessary as the fasteners are hand tightened and so
>> compression forces can vary by quite alot. Finally, the material should
>> resists acids and alkalies. What would you recommend?
>>
>> Thanks.
>
> If you put Belleville washers on the screws, you can have nearly constant
> clamping force over 20%-80% deflection. Alternatively, you can allow the
> flat surfaces to contact each other after some specific deflection.
>
> Cheers,
>
> Phil Hobbs

I would go with the use of an edge design that clamped hard onto a stiff
spacer and use a low modulus gasket for sealing.

From:	Mark Thorson
Subject:	Re: What gasket material should I use?
Date:	Wed, 29 Dec 2004 20:22:44 GMT

Philip wrote:

> Apart from the main criterion that it mustn't leak, it is
> also imperative that its thickness be relatively constant
> after a certain compressive stress.

That rules out elastomers -- all elastomers undergo
cold flow.

> This requirement is necessary as the fasteners are
> hand tightened and so compression forces can vary
> by quite alot. Finally, the material should resists acids
> and alkalies. What would you recommend?

Lead sheet. It's soft enough to be used as a gasket.
Easy to cut. Chemically resistant. For less than $300,
you can by a brand-new rolling mill (made in India)
and make your own sheet stock.

(As others have said, redesigning the joint is the
right way to solve the problem -- don't ask one
material to do two things: be a gasket and establish
a reliable, constant face-to-face distance.)

From:	Philip
Subject:	Re: What gasket material should I use?
Date:	Sat, 01 Jan 2005 03:53:12 -0800

Philip wrote:
> Hi
>
> I've designed a flow cell made up of 2 pieces of plexiglas with fluid
> flowing in between. The top and bottom pieces are separated by a gasket
> and pressed together by fasteners. The thickness of the gasket therefore
> defines the depth of the flow channel. I would like to vary the
> thickness from 0.5 mm to 2mm (after compression) by using gaskets of
> different thickness. When fluid is flowing through the flow cell,
> depending on the flow velocity, the internal pressure can vary from 0 to
> 5 atmosphere. Right now, I'm sourcing for a suitable gasket material.
> Apart from the main criterion that it mustn't leak, it is also
> imperative that its thickness be relatively constant after a certain
> compressive stress. This requirement is necessary as the fasteners are
> hand tightened and so compression forces can vary by quite alot.
> Finally, the material should resists acids and alkalies. What would you
> recommend?
>
> Thanks.

A related question. Because the flow thro' device is quite small (60mm x
90mm), I had to restrict the width of the gasket. Is there a good way of
estimating the minimum "no leak" width?

thanks
philip

From:	jbuch
Subject:	Re: What gasket material should I use?
Date:	Sat, 01 Jan 2005 16:33:16 -0600

Philip wrote:
> Philip wrote:
>
>> Hi
>>
>> I've designed a flow cell made up of 2 pieces of plexiglas with fluid
>> flowing in between. The top and bottom pieces are separated by a
>> gasket and pressed together by fasteners. The thickness of the gasket
>> therefore defines the depth of the flow channel. I would like to vary
>> the thickness from 0.5 mm to 2mm (after compression) by using gaskets
>> of different thickness. When fluid is flowing through the flow cell,
>> depending on the flow velocity, the internal pressure can vary from 0
>> to 5 atmosphere. Right now, I'm sourcing for a suitable gasket
>> material. Apart from the main criterion that it mustn't leak, it is
>> also imperative that its thickness be relatively constant after a
>> certain compressive stress. This requirement is necessary as the
>> fasteners are hand tightened and so compression forces can vary by
>> quite alot. Finally, the material should resists acids and alkalies.
>> What would you recommend?
>>
>> Thanks.
>
>
> A related question. Because the flow thro' device is quite small (60mm x
> 90mm), I had to restrict the width of the gasket. Is there a good way of
> estimating the minimum "no leak" width?
>
> thanks
> philip

You would want the contact thickness to be several time the "wavelength"
of the surface roughness features.

In other words, you don't want the surface asperities and gouges widths
to be of the order of the contact thickness.

Further refinements of the idea would be based on the depth/height of
the surface asperities and gouges.

There are certain gouge and asperity geometries that would be
particularly hard to seal, if you apply your mind and think about what
making a surface seal involves.

Jim

--
................................

Keepsake gift for young girls.
Unique and personal one-of-a-kind.
Builds strong minds 12 ways.
Guaranteed satisfaction
- courteous money back
- keep bonus gifts

http://www.alicebook.com

From:	Mark Thorson
Subject:	Re: What gasket material should I use?
Date:	Sat, 01 Jan 2005 18:00:50 GMT

Philip wrote:

> A related question. Because the flow thro' device is quite small
> (60mm x 90mm), I had to restrict the width of the gasket.
> Is there a good way of estimating the minimum "no leak" width?

Add a tracer dye to the working fluid and observe the
penetration into the joint.

From:	Uncle Al
Subject:	Re: What gasket material should I use?
Date:	Sat, 01 Jan 2005 11:47:47 -0800

Mark Thorson wrote:
>
> Philip wrote:
>
> > A related question. Because the flow thro' device is quite small
> > (60mm x 90mm), I had to restrict the width of the gasket.
> > Is there a good way of estimating the minimum "no leak" width?
>
> Add a tracer dye to the working fluid and observe the
> penetration into the joint.

About 100 ppm sodium fluoresceine (uranine) in water (alkaline)
interrogated with a blacklight,

You can buy it by the kilo for tracing underground streams. Defizzed
tonic water about 0.1M in acid glows nicely under blacklight,

10X more acid doesn't get you anything,

--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/qz.pdf

From:	Philip
Subject:	Re: What gasket material should I use?
Date:	Sat, 01 Jan 2005 21:49:17 -0800

Uncle Al wrote:
> Mark Thorson wrote:
>
>>Philip wrote:
>>
>>
>>>A related question. Because the flow thro' device is quite small
>>>(60mm x 90mm), I had to restrict the width of the gasket.
>>>Is there a good way of estimating the minimum "no leak" width?
>>
>>Add a tracer dye to the working fluid and observe the
>>penetration into the joint.
>
>
> About 100 ppm sodium fluoresceine (uranine) in water (alkaline)
> interrogated with a blacklight,
>
>
>
> You can buy it by the kilo for tracing underground streams. Defizzed
> tonic water about 0.1M in acid glows nicely under blacklight,
>
>
>
> 10X more acid doesn't get you anything,
>
>
>

I understand what you guys are saying. But that device hasn't been built
yet. I was hoping there is some kind of standards (or rule of the thumb)
relating internal pressure and gasket width (not thickness). Due to
limited funds, I'm aiming (perhaps unrealistically) to get it right the
first time instead of going back and forth between experimentation and
redesigning.

philip

From:	Mark Thorson
Subject:	Re: What gasket material should I use?
Date:	Sun, 02 Jan 2005 18:45:06 GMT

Philip wrote:

> I'm aiming (perhaps unrealistically) to get it right the
> first time instead of going back and forth between experimentation
> and redesigning.

Overdesign to the limits imposed by other considerations,
such as the size of the stock material you'll be fabricating
the device from.

Normally, you learn the actual limits empirically, not by
working forward from first principles. You've got too
many variables for there to be an empirically-derived
rule of thumb. If it's critical to make it work right the
first time, overdesign it to the extreme limit that does
not require an increment in cost. For example, if you're
cutting it from a 100 mm x 100 mm blank, use the whole
100 mm x 100 mm. If it still fails, do the same thing
at the next larger size of stock material.