ABB Advance Optima Module Uras 14

Advance Optima Module Uras 14

Physical principles

Measurement

principle

• NDIR Technique

(Non-Dispersive InfraRed Analysis)

• The measurement effect is based on resonance absorption of gas-specific

vibration-rotation bands of gas molecules with differing atoms in the median

infrared spectrum at wavelengths between 2.8 and 8 µm.

• The individual gases to be measured are identified by their specific absorption

bands. Each gas has such an absorption spectrum (fingerprint).

Exceptions:- Monoatomic gases, such as inert gases- Symmetrical gases, such as N2. O2 and H2- These types of gases cannot be measured with this method.

The relationship between measured infrared emission absorption and the sample

component is based on the LAMBERT-BEER law:

A = (I0  - I1) / I0 = 1 - e -ε(λ)⋅ρ⋅l

where

A

I0

I1

ε(λ)

ρ

l

= Absorption

= Emission entering the cell

= Emission leaving the cell

= Sample component extinction coefficient

= Sample component density

= Sample cell length

The relationship between test component density ρ and its volumetric

concentration c is

ρ = ρ0 ⋅ c ⋅ p/p0 ⋅ T0/T

where

ρ0

p0

T0

= Pure gas density

= Pressure

= Temperature

under standard conditions (1013 hPa, 0°C).

The second equation shows that the sample component's volumetric concentration

depends on the sample cell pressure and temperature.

The first equation finds a non-linear relationship between absorption and volumetric

concentration

Physical principles, continued

IR emission

Choppers

Sample cell

Infrared detector

• Generated by broad-band emitter

• Emitted as a beam package alternately in the form of a sample and reference

beam through the sample and reference chambers of the sample cell and is

partially absorbed by the sample component molecules

• Counterphase modulation by means of a motorized chopper wheel

• Both modulated beam packages appear alternately at the infrared detector

• Created by applicable regulation of the sample and reference beam balance

• Depending on the application, the sample chamber receives a sample, zero-point

or end-point gas flow so that a part of the infrared radiation is absorbed in a

concentration-dependent manner.

• The emission passes unhindered if the reference chamber is filled with a gas that

does not absorb infrared (N2).

• A two-part transmission detector with front and rear chambers filled with the gas

components to be measured; selectivity is determined by the infrared detector.

The two chambers are separated by an infrared-transparent window. Additionally,

the two chambers are separated by a stressed metal membrane with

counterelectrodes. This unit is known as the diaphragm capacitor.

•It reacts in the following manner in the presence of the sample component:

• IR radiation is weakened in the sample cell's sample chamber and enters the

receiver's front chamber.

• The equilibrium between the sample and reference beams initially established

by calibration and the aperture is now disturbed.

• There is an energy difference (temperature change) in the form of reduced

pressure in the front chamber.

• This pressure reduction is transformed into a capacitance change in the

membrane capacitor by deflecting the metal diaphragm.

• Since the diaphragm capacitor is connected to a high-impedance DC voltage, a

corresponding periodic AC signal is generated.

Determination of influence values

Associated gas

effects

Pressure

Flow rate

Temperature

The sample gas is a mixture of the sample component and associated gas

components. If the infrared absorption bands of one or more associated gas

components overlap the sample component's bands, the test results will be

affected.

The influence of interfering gas components is termed cross sensitivity or carrier

gas dependence.

Cross sensitivity is determined by connecting an inert gas (e.g. N2) which is mixed

with the interfering gas components (corresponding to the test gas).

The influence acts on the zero-point measurement value indication.

Carrier gas dependence, which is rarely observed, occurs when the physical

properties of the sample gas differ markedly from those of the test gas. This

interference changes the slope of the device's characteristic curve. This curve is

corrected at the end-point.

The Uras 14 has the following methods available for interference correction:

• Interference filter

• Filter cells

• Internal electronic cross-sensitivity correction

• Internal electronic carrier gas correction

According to the gas laws, the sample cell's volumetric concentration depends on

the pressure in the sample cell and is thus dependent on the process gas and air

pressure. This effect acts on the end-point and amounts to approx. 1% of the

measurement value per 1% of pressure change (therefore, per 10 hPa).

An internal pressure sensor reduces this effect to 0.2%.

The flow rate affects pressure in the sample cell and the module's T90 times.

The flow rate should  be between 20 and 100 liters/hour.

Temperature has a markedly different effect on all optical components in the beam

path. This effect is reduced by:

• Temperature compensation

A temperature sensor in the first infrared detector's preamplifier measures the

temperature in the module.

This signal is used for electronic correction.