Component Measurement by Fiber Optic Process Photometry

How does process photometry work?

A process photometer is based on the principle that the measured component absorbs light at specific wavelengths. Light from a source is split into the desired wavelengths by optical filters, passes through the sample cell and reaches the detector. Concentration is calculated by comparing the attenuation at the measurement wavelength with that at a reference wavelength insensitive to the component; this compensates for effects such as lamp aging and cell contamination.

In systems using a fiber optic bundle, light is carried from the control cabinet to the measurement cell, and the measurement signal back to the cabinet. The electronics unit can stay outside the hazardous area; only the optical cell and fiber line are on the process side. A gas or liquid sample cell is selected according to the application.

What process photometry measures and where it is used

Photometers measure light-absorbing components in gas and liquid streams:

• Component tracking in gas processing and refinery streams
• Measurement of process gases such as HCl with multi-wavelength photometers
• Component monitoring in liquid streams in chemical processes
• Remote measurement in processes operating in hazardous areas

Where process photometry works well and where it struggles

A fiber optic connection keeps control electronics away from the hazardous area and provides installation flexibility. Multi-wavelength measurement reduces the effect of interfering components. A design with few moving parts keeps maintenance requirements limited. The same instrument can be adapted to several measurement points.

Limitations: the method is suitable only for components that absorb light at specific wavelengths. When sample cell windows become contaminated, the measurement changes; a clean, conditioned sample is required. Reliability of results depends on regular verification with calibration gas or liquid.

Choosing an instrument for process photometry

The sample phase (gas or liquid), target component, fiber optic distance and hazardous area certification are the deciding factors. The measurement cell type and sample system are chosen according to the temperature and pressure conditions of the application. During commissioning, Tetra configures the fiber optic line, sample cell and calibration arrangement according to site hazardous area conditions.

Tetra and process photometers

Tetra supplies, installs, commissions and services process photometers in Türkiye. The product range includes the Galvanic FPA4100 process photometer (fiber optic, ATEX/IECEx Zone 1), configured with a gas or liquid sample cell. Tetra also prepares ATEX analyzer systems that use a multi-wavelength process photometer for HCl measurement.