ULTRAVIOLET SPECTROSCOPY


BASE enclosureINTRODUCTION

Quantitative Ultraviolet (UV) spectroscopical analysis, provided by the BAGGI® BASE instruments, is used to determine the concentration of an analyte in an aqueous solution or in a gas mixture. In order to be able to do this, the analyte must absorb in the UV region. The instruments are based on a spectroscope that provides high wavelength resolution. The measurement technique relies on the Beer-Lambert law. This one is a relationship that relates the absorption of electromagnetic waves energy to the properties of the material through which the waves are travelling. The material is introduced in a sample cell of specific optical path length. The UV energy is transmitted to the cell via an optical fiber cable, it passes through the cell space and the residual energy is transmitted to the measuring sensor by a second optical fiber. The instruments are available also for outdoors installation (ATEX zone 2). Maintenance is reduced to a minimum due to:

  • Very long life UV lamp
  • Ruggedized components
  • Cell clean-up with instrument air/water, according to the application
  • Self-monitoring
  • Calibration facility
  • No moving parts

 

 

ARCHITECTURE

The implementation of the Analyzer follows the general philosophy of the BASE Instruments Series.

The raw input data from the sensors (UV spectrophotometer) are processed by algorithms provided by BAGGI, running in an embedded computer that is the heart of the system.

The computer, together with the sensors and the power supply, is within an enclosure provided with a protective purge system and a Vortex cooler (connected to the plant instrument air system).

The computer is in charge of:

  • Actuating the UV lamp of the spectrophotometer
  • Reading the electrical signals from the CCD array (related to the intensity of the absorbed UV energy)
  • Calculating the concentration of the analyte(s)
  • Actuating the digital/analog conversion for outputting the calculated values over 4…20 mA signals
  • Actuating the output relays for indicating possible alarms
  • Displaying the measurement output data in a Graphical User Interface (GUI)
  • Storing the measurement archives into non volatile memory (Microsoft Office compatible format)
  • Interfacing the human operator for system configuration and alarm reporting

 

The spectrophotoSpectrophotometermer schema is shown here:



The instrument is composed of an UV lamp and a diode array.
The UV beam, after passing through the measurement cell, reaches a holographic grating disk. This one diverts each wavelength composing the beam onto a specific diode of the array. The voltage emitted by the individual diodes is measured and this information is acquired by the embedded computer through a serial line.

There are no moving parts.

The computer knows the amount of UV energy that has been transmitted by the lamp and is able to draw the absorption spectrum. Finally it calculates the concentration of the components.

 


PRINCIPLE OF OPERATION
As anticipated,Beer Lambert law the measurement is based upon the Beer-Lambert law.

This one is an empirical relationship that relates the absorption of light (or UV waves) to the properties of the material through which the light (UV) is travelling.

The measurement is centered on the wave length where the investigated material has maximum energy absorption.

 

 

 

 

 

 

In the figure, I0 and I1 are the intensity of the incident signal and of the outgoing signal, respectively;

l is the length of the path, c is the substance (e.g. H2S) concentration and α is a constant related to the substance.

α can be written as:

α = εl

where ε is the molar absorptivity of the absorber (H2S).

 

The following relation holds:

 

I1/I0 = 10- εlc

 

The transmission of the signal through the sample is expressed in terms of “absorbance”, which is defined as:

A = -log10(I1/I0)

This implies that the absorbance is linear with the concentration:

 

A = εlc

 

The analyzer establishes the intensity of the signal transmitted by the lamp of the spectrophotometer and measures the intensity of the signal received by a photodiode array.

The signal is transmitted at wavelenghts where the absorbance of the measured substance is maximal.

Then the application software calculates the concentration according to the measured values and the above formulas.


TECHNICAL SPECIFICATIONS

Here follow the standard specifications of the BASE Instrument Series.

 

Power:

- Standard: 90-264 VAC, 47-63 Hz; 6A max

Environment:

- 0° to 40°C (32° to 104°F)

- 0° to 55°C (32° to 131°F) with vortex cooler

Dimensions (without sampling system):

- Skid-mount 500mm H x 400mm W x 250mm D

(19,68” H x 15,74” W x 9,84 D)

Approximate Weight ((without sampling system):

- 15 Kg

Analogue Inputs:

- Four inputs filtered with transient protection

Analogue Outputs:

- Six isolated outputs, 4 – 20 mA

Digital Inputs:

- Six digital inputs

Digital Outputs:

- Eight isolated relay signals

Enclosure Protection:

- IP66

Compliances:

- EN61326, EN61010-1

- ATEX (optional)

II 2 G Ex px II T6

II 3 G Ex pz II T6

 

 

All the specification data are subject to changes without notice

 

 

 

 


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