Digital Flow gauge required

[Thumpick]

I'm trying to find a digital flow gauge that will accurately measure (+/- 1%) a water flow of approx 100 ml/min. The water flows through 1/4 inch plastic pipe work and is low pressure, around 2 psi.

I've searched high and low and can't find anything suitable. Any suggestions would be greatfully appreciated.

 

[zdas04]

Good luck with that one. "Accurately" (I'm going out on a limb here and assume that you are talking about "uncertainty" and "repeatability" since there isn't any other technical term for the media's term "accuracy" that does not actually have a meaning) and "100 mL/min" don't really go together. That flow rate is far too small to be a custody-transfer volume, so there isn't really an economic driver (as in big enough market) for a manufacturer to develop one. You'll be really lucky to find a ±10% (of full range) uncertainty meter with pretty messy repeatability for that volume. I don't know what to tell you.

If you gave us some more information someone might be able to come up with a path to a solution (maybe a metering chamber?).

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. ùGalileo Galilei, Italian Physicist

 

[Compositepro]

Thermal mass flow meters might be suitable. They will input a certain wattage of heat into the flow and measure the temperature rise. These measurements can be made very accurately.

 

[zdas04]

I haven't seen one of those that would fit in a 1/4 inch plastic tube (never been explicitly in that market, but I watch the literature pretty closely). Also every one I've seen has had an uncertainty number greater than 100 mL/min.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. ùGalileo Galilei, Italian Physicist

 

[bimr]

If you have the money, Endress has the meter for you:

http://www.us.endress.com/en/Tailor...roduct-Coriolis-flowmeter-Proline-Promass-83F

The Coriolis flowmeter is digital, 3/8-Inch diameter and 0.05% accuracy. It should work.

 

[mbt22]

David, they do exist. Brooks thermal mass flow meters go down to 100 ml/hr Link

I've had success with their coriolis meters as an alternative to the E+H ones above.

Matt

 

[moltenmetal]

You've picked a particularly difficult region to find a reasonably priced flowmeter for.

Yes you can buy a E+H, ABB, Emerson Micromotion or Bronkhorst coriolis flowmeter which will measure this flow accurately- at a cost of many thousands of dollars. All these brands have a unit with a very small tube or tubes- less than 1/8"- and hence have a zero stability low enough to permit accurate measurements in the range you're looking for with reasonable turndown. The Brooks Quantim units are overpriced but do work also- to imagine their tube, think of a capillary tube about the size of a large paperclip...

At 100 mL/min you might find a liquid thermal unit which will work for pure water. Brooks has an old product, but Parker and Bronkhorst do too. Thermals will still cost thousands, but perhaps less than half the cost of a coriolis.

If you can tolerate a more modest accuracy and not much turndown, an oval gear meter might be able to do this flow for a few hundred bucks. It gives a pulsed (frequency) output, and will be blocked by any tiny particle of debris so a prefilter is essential.

If it's not deionized water, you might find a very small tube magmeter for this service, but turndown will be poor. Usually when we have small water flows like this it's water of sufficiently low conductivity that mags aren't an option.

I'd suggest based on our experience that you stay away from the laminar flow differential pressure units and the straight tube thermal units that some offer. Our experience with both these designs has been poor enough that we won't buy them again.

 

[Thumpick]

Thanks for all the replys. I don't particularly want to spend more than a few hundred. I'm using it on a 'lash up, for research purposes in the lab rather than out on nuclear significant plant.

I would tolerate a +/- 3 % error if I could source one for around £500

Talking about oval gear flow meters, I found

http://www.flowmeters.co.uk/oval_gear_meters.htm

Not sure about cost but will look into it.

Thanks

 

[bimr]

Make sure that it will work on water:

Disadvantage: Oval-gear meters are generally not recommended for water or water-like fluids, because the increased risk of fluid slippage between the gears and chamber walls. Fluid slippage will cause a slight degradation in accuracy, with low-viscosity fluids being more prone to degradation. As viscosity increases, the wall slippage quickly becomes minimal, and the best accuracy is realized. Since the oval-gear meter is really designed for higher-viscosity fluids, it can be argued that running water through them is not a viable application anyway.

http://www.coleparmer.com/TechLibraryArticle/667#anchor7

 

[Thumpick]

Hi bimr,

http://www.ukflowtechnik.com/sites/ukflowtechnik.com/files/catpdf/EM_series_Ovalgear_2012.pdf

According to the data sheet for the oval gear above, it's designed for use on fuel oil, of which Petrol has a lower viscosity. Will get a price.

Cheers

 

[StoneCold]

If you can stand the pulses, these little metering pumps will do the job for you, rather accurately and the prices are not bad.

http://www.fluidmetering.com

 

[moltenmetal]

I'd presumed that the OP actually wanted to measure the flow of an existing stream rather than to generate a metering flow of water of a certain rate. If the latter is required, a metering pump with a rotameter will do the job admirably, but without feedback when something messes with the efficiency of the metering pump. Add a weighscale on the feed tank and you'll have both.

 

[Thumpick]

The water I want at 100 ml/min is currently supplied via a pump (non adjustable) to a constant head (about head hight). It flows out of the constant head through a flow gauge with rotameter (not accurate enough), flow being controlled using a needle valve.

I could in theory I could replace the constant head with an accurate metering pump ??

 

[bimr]

Yes, it is possible.