laminar flow element

[darcyracing]

I would like some opinions on the use of laminar flow elements pros and cons

Thanks for the replies

Phil

[thomasvaught-1]

I have used them for years at Ford Research as calibration devices.

They must be kept clean.

You must have a flow curve from Merium Instruments to accurately use them.

At one time I had a shop about 50 miles from me that did test cell deconstruction work. They would have laminar devices on occasion. I was paying $1.00/cfm for the units vs the normal cost (many times higher).

I still like the Orifice plate deal as they are much more robust in terms of dirt/contamination.

You just need several orifices vs one large laminar device.

Tom V.

ps LAMINAR DEVICES MUST BE CORRECTED FOR TEMPERATURE AND PRESSURE CONDITIONS TO BE ACCURATE. THE MERIUM MANUAL GIVES ALL THE CORRECTIONS REQUIRED

A orifice bench being a Ratiometric bench does not need this typically. A ratiometric bench compares the test part with the orifice and gives a percentage of flow difference.

[Tony]

I agree completely with Tom.

An orifice plate is simple, consistent, robust, and reliable. No corrections are needed if exactly the same air flows first through the test piece and then through the measurement orifice. It also has the virtue of costing almost nothing.

From what I have read, laminar flow elements will suffer over time from dirt and contamination buildup if used constantly.
Long term drift is not an ideal feature of any measuring element, but it may be acceptable as a reference standard that is used infrequently, and regularly checked against other flow standards.

[Malvin]

Hi to Tom & Tony & Phil I have never seen one of these laminar flow elements could some one put up a pic or a link and are they more acrurate than a orifice plate?? Thanks Malvin

[Tony]

mod edit: made it a link



Edited By bruce on 1217327521

[Malvin]

Thanks Tony for the link Malvin

[jsa]

Phil,

I am doing an LFE style bench, ever so slowly.

Tom and Tony make some good points, but I disagree to varying extents with some points.

Tom has automatically assumed that you would use a Merriam brand LFE and hence you should use Merriam flow curves.

LFE flow curves are dependant on where the DP reading ports are positioned in the LFE assembly. Merriams are positioned so that entrance and exit losses are included along with the laminar flow pressure losses. This means linear plus quadratic losses are added and require a non liner calibration curve.

Other brands of LFE are different. In particular home brand could be configured so that the DP reading ports only see laminar flow pressure losses.

Dirt contamination can be an issue particularly with the very compact laminar passages used in the likes of Merriam devices. These passages are fine enough to be choked by dust, changing entrance and laminar DP values, hence the drift.

Home brand can be made from robust material with larger passages. Larger passages requires more passages to maintain laminar flow, so the size goes up at a multiple to just the larger passage size. Larger passages are not so effected by dust.

I am not convinced sharp edge orifices wont go dull over time due to dust and the like going through the bench.

Actually home brand LFE has cost me about the $1/cfm to have laser cut from stainless, cheaper than commercial by a long way.

LFE do need to be temp and barro-metric pressure corrected. No big deal with electronics for bench reading. I have purchased a Flowperformace FPLab with internal barro sensor and external temp sensor. This flow computer puts the data to a spreadsheet which crunches the maths. If your using manometers then orifice is easier.

Orifi may be ratiometric, but if you choose to test at a pressure other than the calibrated one, then corrections still need to be applied. Someone kindly posted the data from the big blue brand manual recently.

The biggest issue with orifi benches has been sealing the unused orifi that are not being used for a particular range and sealing around any mechanisms used for changing ranges. Orifi can also be fooled by the proximity of other parts of a bench. Bruce would appear to have these licked in his flow bench plans that are available for sale here.

The biggest advanatge of orifi is that they are cheap.

The fluid mechanics stuff I've learned to design an LFE has been well worth the effort, a lot of it is useful for the porting side of things once the bench is up and going.

If you want a bench up in a hurry then get Bruce's plans and away you go.

The big advantage of an LFE is that it is immune to bench hardware proximity issues and is linear if designed so. Also no mechanisms required for range changes because the linearity allow a wide flow range on just one size device.

Malvin,
Accuracy of the either device is going to come down too a host of issues influenced by the bench design and build quality.

Many people use a test piece to check bench accuracy and repeatability before and after a session. This can equally apply to orifice, pitot, and LFE designs.

[bruce]

John, Any pictures of your LFE?

[jsa]

Bruce,

I don't have photos of it. Maybe a design screen capture will suffice.

[jsa]

and close up