Mass Flow Meter vs Weighbridge

What's the problem? Any hint in your explanation? The WB gives readings you can't explain.

How do you charge for the gas?

Get both calibrated and report back.

Thank you for your reply.

As mentioned on my original post, the difference on the reading varies. From 150kg to 1700kg on a 20000kg~25000kg loading.

The mfm is factory calibrated and the wb, as what they say is calibrated also.

What they want now is to bring the readings closer.

Will appreciate any recommendations, solutions or steps we can do for us to resolve this.

A flowmeter measures gas flow, a weighbridge measures weight. Gas flow and gas weight depend on a number of different conditions such as pressure and temperature, and the end weight calculation needs to take all these into account to give you an accurate value for the weight.

It is likely that one or more of these conditions is not being measured the same way resulting in the differences between the two different measurement techniques.

You should start by looking at the manuals and application notes for both measurement schemes to find out exactly how both techniques calculate the end weight.

Hi sir.

Thank you for your advice.

Very much appreciated.

Assuming correct calibration etc, the weighbridge will be the more accurate mass.

In Australia LPG is Propane, but in many parts of the world it can also contain butane and i-Butane. As your meter is measuring volume the reading has to be corrected for density. Density will vary widely depending on temperature. It will also vary with pressure, but not by that much.

Between 0 - 50degC the density of propane will vary from about 520 - 460 kg/m^3.

Butane will vary from about 600 - 549 kg/m^3.

You can see from these numbers that depending on temperature, pressure and composition of the LPG the measurements could vary quite considerably.

Thank you for your response.

Our instrument is coriolis mass flow meter.

It measures mass, density and volume flow of gases and liquids.

Also, the parameters were set correctly.

I am now contacting our principal but while waiting for their response, I am researching on what factors could affect the reading of the mfm.

Thank you sharing your knowledge.

One presumes that is why the Original Poster is measuring mass and not volume.

A Coriolis meter measures mass flow directly, the SG and composition of the fluid make no difference. That's not to say it can never give a faulty reading.

Coriolis mfm's are very sensitive to external vibration,as well as the orientation of the meter itself.

Cavitation of the liquid can also cause variations in the measurement.

Be certain of the proper mounting parameters as per the installation manual,especially flange alignment and vibration isolation.

The Weigh scales should be calibrated by a factory representative,and weights will be placed at each corner separately,then together to insure proper weight distribution to the load cells and accuracy of the scales.

It should not matter the exact position of the load on the scales,but this is easy to check but moving the load slightly and checking for a variation of the reading.

Dirty mechanical components and wear or weld cracking of the components below the weight plate can cause variations in the reading.

The weighbridge is the less trusted measurement:

• The vehicle's tare weight will vary between arrival and departure because of changes in the weight of fuel carried in it while it is on site.
• The vehicle's operator's weight will vary between arrival and departure because of changes in the materials eaten, drunk and expelled while the operative is on site.
• There may well be changes in the weight measured as a result of things other than the payload being delivered or removed from the site in or on the vehicle.

Without eliminating these variables the <...We...they...> will have a difficult job correlating the two measurements.

There will also be a weight variable were the vehicle operative be in the vehicle on one weighing and out of it on the other.

Or a different driver.

But any of the suggestions you made are small deviations for a weighbridge which is likely to have a 10kg or 25kg accuracy.

I cannot see the tanker driver ‘accidentally’ taking or leaving 1.7tonnes of equipment. 50 - 100kg I can accept.

I expect the answer is the liquid fraction of the gas, so the OP should be taking into account the volume of the tanker, and the pressures before and after loading.

Both readings have their uses. The mass of gas delivered through the meter will be used for the sales invoice. The weighbridge value will be used by the Department of Transport to check the vehicle was not overweight.

There should be allowance made for further condensing of gas as it cools after pumping.

<...Both readings have their uses...>

Quite. That's why the pursuit of identical readings from each source by the Original Poster's <...they...> is pure folly.

I would say you're getting some gas in with the liquid.......

"LPG - Propane Density and Specific Gravity. LPG – propane – gas density is 1.55 times heavier than air at 1.898 kg/m³ vs 1.225 kg/m³ for air (both 15°C at sea level). LPG – propane – liquid density is about half that of water at 495 kg/m³ (25°C) vs 1,000 kg/m³ (4°C) for water.Jan 18, 2019"....

https://www.elgas.com.au/blog/453-the-science-a-properties-of-lpg

https://www.engineeringtoolbox.com/propane-C3H8-density-specific-weight-temperature-pressure-d_2033.html

In principle, if mass flow is measured, it makes no difference if there are gas bubbles. But it might be worth asking to the meter supplier whether this has any effect on accuracy.

Well, it depends...

..."Thermal flow meters provide high rangeability (10:1 to 100:1) if they are operated in constant-temperature-difference mode. On the other hand, if heat input is constant, the ability to detect very small temperature differences is limited, and both precision and rangeability drop off. At normal flows, measurement errors are usually in the 1-2% full-scale range."...

...it could also be a maintenance issue...

https://www.omega.com/technical-learning/thermal-mass-flow-working-principle-theory-and-design.html

OK thanks, interesting

Hello,

If the difference is not consistent, then it's probably NOT the instruments. From what you describe, the maximum accuracy error is +/- 60kg.

If the error was always 50kg per 1000kg (1,000 for 20,000 and 1250 for 25,000) then I'd look at instruments.

For the difference to be so variable, then it's probably something else.

Do you refuel trucks between tare and loaded weighing? Loaded fuel is the major difference (you would typically only observe high wb output for this situation).

Do you switch prime movers between trailers (drop off empty and hook up full) to decrease turnaround times? Trailer tare is the difference (you would observe high and low wb output for this type of situation). Especially the case if trailers have dual and triple rear axle configuration.

Coriolis Dirty Secrets

Coriolis is not a 2 phase (simultaneous liquid and gas) flow meter. The presence of gas in a primary liquid line will induce a measurement error.

Pulsating flows can induce Coriolis measurement error.

Coriolis zero shifts. Coriolis requires a periodic zeroing procedure with full pipe at zero flow.

Vibration in Coriolis is less an error factor than in earlier eras, but can still be a error factor.

Temperature shifts in ambient temperature can induce measurement error in load cells.

Hi. In general a weighbridge should be more accurate than a mass flowmeter since it isn't related to the type of material, it's easier to check the weighbridge accuracy, probably it is under legal metrology laws and so it should be periodically calibrate.

So, after a check on the weighbridgewith the standard tests and/or a cleaning of the pit of the truck scale, try to correlate the readings with the environmental conditions (temperature , humidity) for MFL and weather conditions (rain, wind, full sun etc) for weighbridge. In ths way you should find if the differences you find are due to a specific problem. Just two examples for the weighbridge : if it is in a pit, in summer you could have a dilatation of the platform and it is against the external walls of the pit and you get errors, or when it rains if the load cells cables are damaged you can get a drift .

Ciao