What is accuracy of a mass flow meter, what is repeatability of a mass
flow meter, and what difference does the difference make?
First let’s define the terms. Accuracy of a mass flow meter is the expression
of error (%) as compared to a primary standard. In practical terms it is
the degree to which the actual output of the flow meter matches the calibration
curve of the flow meter.
Repeatability of a mass flow meter is the amount (%) that any single device
can repeat the value previously delivered at the same conditions. In practical
terms this means that the output seen at a given set of conditions may,
or may not, match the calibration curve of the instrument but it will regularly
be the output seen at that same set of conditions.
A highly accurate instrument requires high repeatability, but a highly
repeatable instrument does not require high accuracy.
So, why does it matter? Increasing accuracy tends to come at an increasing
cost which can be justified if required in the application. High repeatability
with lower accuracy may be a cost effective solution for a given application.
Clearly identifying the requirements of an application will allow the best
instrumentation recommendation to be made.
Frequently Asked Questions (FAQ)
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The accuracy (really inaccuracy) of mass flow instruments is specified
in one of two ways, either accuracy as a percentage of full scale (% FS),
or accuracy as a percentage of reading (% RD).
If an instrument has accuracy specified as % FS then the error will have
a fixed value no matter where the flow is in the flow range. Take, for
example, an instrument calibrated for a flow of 100 ln/min with stated
accuracy 1.0% of FS. At a flow of 100 ln/min (full scale) the error will
be 1% of full scale, or +/- 1 ln/min. As the flow moves way from full scale
the error will still be 1% FS (+/- 1 ln/min), so at a flow of 50 ln/min
that error of +/- 1 ln/min becomes a larger percentage (+/- 2%) of flow.
Going further away from full scale flow further increases the error as
a percentage of flow; at a flow of 10 ln/min the +/- 1 ln/min error is
+/- 10% of the flow.
If, however, an instrument has accuracy specified as % RD then the error
will always be the same percentage of the actual flow. Using the 100 ln/min
instrument again as the example, but this time with a stated accuracy of
1% RD, at 10 ln/min of flow the error is only +/- 1% of the flow, better
by 10 times.
The ranges of thermal mass flow meters for gases are specified in such
units as ln/min, sccm or m3n/h. These units look like volumetric units,
but in truth they are expressions of Mass Flow. What is the story behind
this?
Imagine you have a cylinder of 1 litre, which is closed by means of a moveable
piston of negligible weight. This cylinder contains 1 litre of air at ambient
pressure, approx. 1 bar. The weight of this volume of air at 0°C is 1.293
g, this is the mass. When we move the piston half way to the bottom of
the cylinder, then the contained volume of air is only ½ litre, the pressure
is approx. 2 bar, but the mass is unchanged, 1.293 g; nothing has been
added, or left out.
Following this example, mass flow should actually be expressed in units
of weight such as g/h, mg/s, etc. Most users, however, think and work in
units of volume. No problem, provided conditions are agreed upon, under
which the mass is converted to volume. Following the ‘European’ definition,
a temperature of 0°C and a pressure of 1,013 bar are selected as “normal”
reference conditions, indicated by the underlying letter “n” in the unit
of volume used (mln/min, m3n/h). Alternative, a temperature of 20°C and
a pressure of 1,013 bar are used to refer to “standard” reference conditions,
indicated by the underlying letter “s” in the unit of volume used (mls/min,
m3s/h). Please be aware of this, because if the difference is not considered,
it may lead to an error of 7%!
According to the ‘American’ definition the prefix “s” in sccm, slm or scfh
refers to “standard” conditions 101.325 kPa absolute (14.6959 psia) and
temperature of 0°C (32°F).
Volumetric measuring devices, like variable area meters or turbine flow
meters, are unable to distinguish temperature or pressure changes. Mass
flow measurement would require additional sensors for these parameters
and a flow computer to compensate for the variations in these process conditions.
Thermal mass flow meters are virtually insensitive to variations in temperature
or pressure.
The IP Rating (Ingress Protection Rating) of an instrument consists of the letters IP followed by two digits and an optional letter. As defined in international standard IEC 60529, it classifies the degrees of protection provided against the intrusion of solid objects including body parts like hands and fingers, dust, and accidental contact (the first digit after IP), and water (the second digit after IP) in electrical enclosures.
The National Electrical Manufacturers Association (NEMA) in the United States also publishes protection ratings for enclosures similar to the IP rating system published by the International Electrotechnical Commission (IEC). NEMA however also dictates other product features not addressed by IP codes, such as corrosion resistance, gasket aging, and construction practices.
For this reason while it is possible to map IP Codes to NEMA ratings that satisfy or exceed the IP Code criteria, it is not possible to map NEMA ratings to IP codes, as the IP Code does not mandate the additional requirements.
Turndown ratio is also commonly referred to as rangeability. It indicates the range in which a flow meter or controller can accurately measure the fluid. In other words, it’s simply the high end of a measurement range compared to the low end, expressed in a ratio and is calculated using a simple formula.
Turndown Ratio = maximum flow / minimum flow
For example, if a given flow meter has a 50:1 turndown ratio the flow meter is capable of accurately measuring down to 1/50th of the maximum flow. So, suppose a flow meter has a full scale rating of 20 l/min the flow meter will measure down to 0.4 l/min of flow.
Keep in mind that the maximum and minimum flow capability of a meter or controller is likely to be a greater span than the measurable and controllable range. For example, a mass flow controller with a 50:1 turndown ratio may have the capability of measuring as high as 25 ln/min or as low as 0.16 ln/min but the turndown ratio will govern the actual measurable range. In this example if the calibrated high flow is 25 ln/min, then the lowest that can be measured is 0.5 ln/min (1/50th of 25). If the application requires that the calibrated minimum flow is 0.1 ln/min, then the maximum flow that can be measured is 5 ln/min (50 times 0.1).
A mental image of this concept may be to picture a set of 100 stairs (the overall minimum and maximum flow of an MFC), and a length of carpet that will only cover 50 stairs (turndown ratio). You can cover (measure) any 50 of the 100 stairs, but you can not stretch the carpet to cover more than 50.
This video provides you with the most important information about the purpose and operation of FlowDDE.
The kinematic viscosity [m^{2}/s] is the ratio between the dynamic viscosity [Pa.s = 1 kg/m·s] and the density of a fluid [kg/m3]. The SI unit of the kinematic viscosity is m^{2}/s. Other units are: 1 St (Stoke) = 1 cm2/s = 10^{−4} m^{2}/s. 1 cSt (centiStoke) = 1 mm^{2}/s = 10^{−6}m^{2}/s. Water at 20 °C has a kinematic viscosity of about 1 cSt.
It is common to identify pipes by inches using NPS or “Nominal Pipe Size“. NPS is often incorrectly called National Pipe Size, due to confusion with national pipe thread (NPT). The metric equivalent is called DN or “diametre nominel“. The metric designations conform to International Standards Organization (ISO) usage and apply to all plumbing, natural gas, heating oil, and miscellaneous piping used in buildings. The use of NPS does not conform to American Standard pipe designations where the term NPS means “National Pipe Thread Straight”. Based on the NPS and schedule of a pipe, the pipe outside diameter (OD) and wall thickness can be obtained from reference tables such as those below, which are based on ASME standards B36.10M and B36.19M.
Diameter Nominal DN (mm) |
Nominal Pipe Size NPS (inches) |
Outside diameter (OD) inches (mm) |
6 | ⅛ | 0.405 in (10.29 mm) |
8 | ¼ | 0.540 in (13.72 mm) |
10 | ⅜ | 0.675 in (17.15 mm) |
15 | ½ | 0.840 in (21.34 mm) |
20 | ¾ | 1.050 in (26.67 mm) |
25 | 1 | 1.315 in (33.40 mm) |
32 | 1¼ | 1.660 in (42.16 mm) |
40 | 1½ | 1.900 in (48.26 mm) |
50 | 2 | 2.375 in (60.33 mm) |
65 | 2½ | 2.875 in (73.02 mm) |
80 | 3 | 3.500 in (88.90 mm) |
100 | 4 | 4.500 in (114.30 mm) |
150 | 6 | 6.625 in (168.27 mm) |
200 | 8 | 8.625 in (219.08 mm) |
250 | 10 | 10.75 in (273.05 mm) |
If you ordered the wrong article, please read the exchange procedure on our website and act as soon as possible.
Mass flow should actually be expressed in units of weight such as g/h, mg/s, etc. Most users, however, think and work in units of volume. No problem, provided conditions are agreed upon, under which the mass is converted to volume.
Our STP or standard temperature and pressure conditions used are stated
here:
Normal conditions (ln/min): a temperature of 0 °C (or
32°F) and a pressure of 1.013 bar (or 14.69 psi) are selected, and these
reference conditions are indicated by the underlying letter “n” in the
unit of volume used. The direct thermal mass flow measurement method is
always based on these reference conditions unless otherwise requested.
Standard conditions (ls/min): here the reference conditions
are based on 20 °C (68°F) instead of 0 °C (32°F). Please be aware of this
difference, because mixing up these reference conditions causes an error
of 7%!
Typical flow unit |
Reference conditions for gas temperature |
Reference conditions for gas pressure |
ml_{n}/min (milliliter normal per minute) |
0 °C / 32 °F |
1.013 bar / 1 atm / 14.69 psi |
l_{n}/min (liter normal per minute) |
0 °C / 32 °F |
1.013 bar / 1 atm / 14.69 psi |
sccm (standard cubic centimeter per minute) |
0 °C / 32 °F |
1.013 bar / 1 atm / 14.69 psi |
slm (standard liter per minute) |
0 °C / 32 °F |
1.013 bar / 1 atm / 14.69 psi |
ml_{s}/min (milliliter standard per minute) |
20 °C / 68 °F |
1.013 bar / 1 atm / 14.69 psi |
l_{s}/min (liter standard per minute) |
20 °C / 68 °F |
1.013 bar / 1 atm / 14.69 psi |
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