General Overview

Flow Measurement

“I do not see to many badly manufactured flow meters. I know a few badly calibrated flow meters, but I have seen loads and loads of badly applied flow meters in the world”

Market

Ultrasonic: 420 Mln

Thermal Ind: 138 Mln

Corioles: 460 Mln

Vortex: 320 Mln

Yesse Yoder: Total worldwide flow market is 5.5 Bln U$ in 2014

All in U$

Now guess how big the Scientific Thermal market is

World market Thermal Mass flow controllers

Flow Measurement Technologies

Metering of Gas Flow is difficult because…

Gas is compressible. The volume of a given quantity of molecules of gas is strongly depended of the pressure and temperature.

99% of the applications are not related to volume, but to the number of molecules. Examples:

Chemical: the molecules do the work!Flame: the molecules do the work!Pneumatics: the molecules to the work!Respiration: the molecules do the work!

We want to measure the number of molecules, not the distance between the molecules. We want to measure Mass!

Metering of Gas Flow

Flow, as we use it, can be expressed in:

- Volume (f.i. l/min)- Standardized or Normalized flow (f.i nlpm or slpm)- Real Mass flow (f.i. Gr/min or Kg/hr)

Volume and Real Mass are easy, but what about Standardized or Normalized?

Standardized or Normalized means “Referred to predefined conditions”. In case of gas it means predefined Pressure and Temperature, because those are variables that relate Mass to Volume.

The relation between Volume, Pressure and Temperature for gasses is defined in the Law of Boyle/Gay-Lussac

Metering of Gas Flow

The Law of Boyle-Gay/Lussac

constantT

V P

Example:

I have a balloon at surface conditions filled with air. The volume of the balloon is 2 liter. I take the balloon under water to a depth of 10 meter where it is 10°C. What will be the new volume?

Surface conditions: 30°C and 1000 mBara

Pressure at 10 meter under water: 1 Barg

2

22

1

11

T

V P

T

V P

Where:

P = Absolute Pressure (Units should be same on

both side of the equation)

V = Volume (Units should be same on both side of the equation)

T = Temperature (Units should always be in degrees K (=273.15 + °C)

Metering of Gas Flow

The Law of Boyle-Gay/Lussac

Example:

I have a balloon at surface conditions filled with air. The volume of the balloon is 2 liter. I take the balloon under water to a depth of 10 meter where it is 10°C. What will be the new volume?

Surface conditions: 30°C and 1000 mBara

Pressure at 10 meter under water: 1 Barg

2

22

1

11

T

V P

T

V P

CC

10 273.15

V2??bar 1bara 1

30 273.15

liter 2 bara 1

V2= 0.934 liter

Metering of Gas Flow

The Law of Boyle-Gay/Lussac

With this law operating conditions can be converted into normal or standard conditions

T

V Pconstant

n

x

x

nxn P

P

T

TVV **

Vn, Tn, Pn volumetric flow rate, temperature and pressure under reference conditions

Vx, Tx, Px volumetric flow rate, temperature and pressure under operating conditions

Metering of Gas Flow

So what are the pre-defined reference conditions for Standardized or Normalized volume or flow?

Standardized: 70°F (21.1°C or 20°C) and 1013.25 mBaraNormalized: 0°C and 1013.25 mBara

However not everybody uses this!! Natural Gas Industry: 15°C and 1013.25 mBaraOther manufactures: 20°C and 1013.25 mBara

Standardized and Normalized are often reversed!

Normal or Standard Conditions

See: http://en.wikipedia.org/wiki/Standard_cubic_feet_per_minute

Standard cubic feet per minute (SCFM) is the volumetric flow rate of a gas corrected to "standardized" conditions of temperature and pressure. However, great care must be taken, as the "standard" conditions vary between definitions and should therefore always be checked. Worldwide, the "standard" condition for pressure is variously defined as an absolute pressure of 101,325 pascals, 1.0 bar (i.e., 100,000 pascals), 14.73 psia, or 14.696 psia and the "standard" temperature is variously defined as 68°F, 60°F, 0°C, 15°C, 20°C, or 25°C. There is, in fact, no universally accepted set of standard conditions. (See Standard conditions for temperature and pressure).

The temperature variation is the most important. In Europe, the standard temperature is most commonly defined as 0°C, but not always. In the United States, the standard temperature is most commonly defined as 60°F or 70°F, but again, not always. A variation in standard temperature can result in a significant volumetric variation for the same mass flow rate. For example, a mass flow rate of 1,000 kg/h of air at 1 atmosphere of absolute pressure is 455 SCFM when defined at 0°C (32°F) but 481 SCFM when defined at 60°F (16°C).

In countries using the SI metric system of unit, the term "normal cubic metre" (Nm3) is very often used to denote gas volumes at some normalized or standard condition. Again, as noted above, there is no universally accepted set of normalized or standard conditions.

Typical Applications (gases)

Lab. / R&D Engineering Industry

Differential pressure

- + ++

VA meter ++ ++ +

Turbine - - +

Vortex - - ++

Oval Gear - - +

Thermal Mass Flow

++ ++ +

Coriolis - + ++

Variable area principle

Vertical measuring glass with conical internal shape, in which a defined body is made to float by the flow acting from below.

Fs = flow resistance of measuring substance

FG = weight of variable area

FA = buoyancy of variable area

DS = diameter of variable area

DK = inner diameter of cone

5 10 15 20 25 30 35

-200%

-150%

-100%

-50%

0%

50%

100%

150%

200% Error graph Volume

Mass

Flow

Err

or

FS

Inlet pressure Atm    

Outlet pressure Atm    

Ambient pressure 1002 Barg    

Indicated Flow read at top of float

Indicated Flow Actual mass Actual Volume Error FS Volume Error FS Mass

lpm slpm lpm % %

8 8.1015 8.145105 0.338% 0.484%

15 15.2707 15.352890 0.902% 1.176%

22.5 22.8653 22.988370 1.218% 1.628%

30 30.9274 31.093860 3.091% 3.646%

Back pressure controller

Mass or volume?

Air

Inlet pressure 1 Barg    outlet pressure 1 Barg    

Ambient pressure 1002 Barg    

Indicated Flow read at top of float

Indicated Flow Actual mass Actual Volume Error FS Volume Error FS Mass

lpm slpm lpm % %

7.5 11.5752 5.824564 13.584% -5.585%

15 22.612 11.378210 25.373% -12.073%

23 35.3579 17.791860 41.193% -17.360%

30 42.945 21.609640 43.150% -27.968%

0 5 10 15 20 25 30 35 40 45 50

-200%

-150%

-100%

-50%

0%

50%

100%

150%

200% Error graph Volume

Mass

Flow

Err

or

FS

Inlet pressure 7 Barg    

outlet pressure 7 Barg    

Ambient pressure 1002 Barg    

Indicated Flow read at top of float

Indicated Flow Actual mass Actual Volume Error FS Volume Error FS Mass

lpm slpm lpm % %

8 24.1672 3.037917 53.891% -16.540%

15 45.0983 5.669043 100.328% -31.103%

22.5 67.3110 8.461272 149.370% -46.796%30 85.4666 10.743510 184.889% -64.188%

0 10 20 30 40 50 60 70 80 90

-200%

-150%

-100%

-50%

0%

50%

100%

150%

200% Error graph Volume

Mass

Flow

Err

or

FS

Pressure effect on VA meters

Variable area principle

Advantages:

+ Suitable for liquids and gases

+ Cost-effective solution

+ No electricity supply required

+ Visually comprehensible

+ Limit contact option

+ Low pressure loss

+ Simple design

Disadvantages:

– Very pressure- and temperature-dependent

– No Mass or Volume really

– Limited dynamics

– Vertical mounting position

– Only suitable for transparent media

– Limited accuracy

– Pressure shocks

Pressure differential principle

Orifice plates

V-cone

Annubar

Pitot tube

Wing

Etc.

Pressure differential principle

Advantages:

+ Suitable for liquids and gases

+ Simple, robust construction

+ Numerous materials

+ Any from NW10…

+ Tried and tested principle

+ High temperatures

+ No moving parts

+ Any mounting position

Disadvantages:

– Susceptible to wear

– Soiling leads to errors

– Gas measurements with additional measurement of Pabs and Tmedium

– Inlet sections

– Limited dynamics (1:3 to 1:6)

– Gas type-dependent

– Lots of potential leak points

– Need more equipment

Displacement meter

Directly measures the total volume flow. The mechanical movement is transferred directly to a counting mechanism or pulse generator.

This category includes rotary pistons, gear wheels, vanes, ring pistons, bellows.

Displacement meter

Advantages:

+ Suitable for liquids and gases

+ Simple, robust construction

+ High dynamics

+ Independent of interferences in the flow profile

+ Independent of gas type

+ High precision

Disadvantages:

– Susceptible to wear

– Risk of soiling

– Gas measurements with additional measurement of Pabs and Tmedium

– Pulsations

– Pressure shocks lead to overload

– Limited operating pressure and temperature

– Leaks through mechanical tolerances

Turbine flow metering

A rotor with turbine blades, supported on a rotating center axis, supplies a signal that is proportional to the flow rate due to its rotary movements. A minimum flow is required for driving the rotor.

Designed as a complete fitting or plug-in probe.

Turbine flow metering

Advantages:

+ Suitable for liquids and gases

+ Compact design

+ Independent of gas type

+ High precision

Disadvantages:

– Susceptible to wear

– Risk of soiling

– Gas measurements with additional measurement of Pabs and Tmedium

– Pressure shocks lead to overload

– Measurement cannot take place from zero

Vortex meter

Vortices are generated downstream of a defined baffle (Kármán‘s vortex street). The number of vortices (frequency) is proportional to the flow velocity.

Vortex meter

Advantages:

+ Suitable for liquids, gases and steam

+ High dynamics (typically 1:25)

+ high + Low temp suited

+ Calibrate with water only

+ Largely independent of changes in pressure, temperature and viscosity

+ Digital from sensor upwards. High long-term stability possible with non abrassive liquids.

Disadvantages:

– Pulsating or swirl flow influence the ability to measure low flows

– No measurement possible with low flow rates (Re < 5000). (Not suitable really for low pressure gasses)

– Non-linear below Re of 20.000

Coriolis principle

Measuring principle that measures the mass independent of temperature, pressure and medium. A measuring tube is brought into resonant vibration state. Sensors are fitted at the entry and exit of the measuring tube. Without flow the sensors issue signals simultaneously. If there is flow a phase shift occurs between entry and exit. This phase shift is directly proportional to the mass flow.

Coriolis principle

Advantages:

+ Suitable for liquids and high pressure gases

+ Direct mass measurement

+ No “moving” parts

+ High precision

+ Independent of flow profile

Disadvantages:

– Require minimum medium density

– Relatively high purchase price

– To some extent sensitive to vibrations

Thermal mass measurement

Several versions, the main ones are:

• Immersable sensors (Mainly Industrial)

• Capillary flow meters

• CMOS or MEMS Thermal flowmeters

Thermal mass measurement

• Temperature Sensor

• Mass Flow Sensor

• Ta• Tv

• Q, heat carried away by gas stream

Immersable sensors

Thermal mass measurement

Immersable sensors

Features

Direct Gas Mass Flow Reading

Fast Response (1 Sec)

No Moving Parts

Rugged, 316SS Sensors

High Turndown & Rangeability

Negligible Pressure Loss

Convenient Installation

Accuracy 1.5%

Flexible due to modern electronics

Suitable up to 400 C

Specific Considerations

Gas dependent, cal with real gas

Dry gas only

Relative clean gas only

Temperature bandwidth 80 C

Insertion: installation sensitive

Up to 10 Barg

Immersable sensors

Thermal mass measurement

Thermal mass measurementCapillary principle

• Small capillary tube (0.2 till 0.9 mm)

• Platinum windings (PT)

• Due to high current, heating of coils

• Mass flow cools R1 more than R2 (Max flow 1 till 10 sccm)

• PT elements will have difference resistances

• Difference in resistance proportional to mass flow through tube

Thermal mass measurementCapillary principle

• Sensor is placed over a LFE

• LFE= Laminair Flow Element

• Flow is separated in known ratio

• m1 proportional to m2

• Total m = m1 + m2

• Behaviour in principle close to linear

Thermal mass measurementCapillary principle

Examples of capillary sensors

Thermal mass measurementCapillary principle

Examples of LFE elements

Thermal mass measurementCapillary principle

Thermal mass measurementCapillary principle

Advantages

- Wetted materials: 316SS + O-rings (other materials possible, metal seal possible)- Compatible with most gasses- Calibration with air and conversion to other gasses possible- Established, lots of manufacturers.- 1 sccm till 5600 NLPM possible (Mostly used up to 1000 NLPM)- Up to 700 Bar possible

Disadvantages

- Lots of bad manufacturers (Drift, temp comp problems, etc)- Very sensitive for liquid and pollution- Expensive, difficult to manufacture them properly.

Thermal mass measurement

Thermal profile: on a silicone MEMS or CMOS chip heat is introduced into the medium with constant heating output. In the presence of flow, temperature sensors arranged symmetrically before and after the heating system detect a shift in the temperature profile towards the sensor downstream of the heating system. If there is no flow both sensing elements measure the same temperature.

Thermal mass measurementT1, T2

Flow

T2-T1

temperature of medium before

heating

temperature of medium after

heating

Delta-T

T1 T1

Thermal mass measurement

Advantages:

+ Mainly for gases

+ Indirect mass measurement (medium-dependent)

+ No moving parts

+ High precision

+ Independent of flow profile

+ Fast, dynamic measurement

Disadvantages:

– Medium-dependent

– Susceptible to soiling

– Several wetted materials

– Limited to non-corrosive gasses