27 Nov 2018

Cells vs Probes for Process Analysis | Animated Guides

Specac have designed fibre optic probe systems since the 1980s. Our first probe patents date from 1991. So why are we such strong advocates for using cells in on-line process applications? It is first important to fully understand the difference between the two.

What is the difference between a cell and a probe?

In a process cell, the optical beam path goes directly across the process flow path. But in a probe, the beam is reflected back on itself.

  1. Cell

    A cell has a fibre optic input on one side of it with an optical system to collimate the beam through the sample. A similar (usually identical) optical system sits on the other side to re-focus the beam onto the output fibre.

  2. Probe

    In contrast, a probe typically has a single optical system that collimates the radiation from the input fibre, passing it across the sample to a mirror. The returning beam is then re-focused by the same optical system onto an output optical fibre.

Why are cells better than probes for process systems?

  1. Larger pathlengths

    For some probe designs, the gap through which the sample flows is only half the size for the equivalent cell (e.g. a 2 mm pathlength gives a 2 mm wide flow path for the sample in a cell, but only a 1 mm wide path for a probe). For long pathlengths this may not be important, but for typical NIR pathlengths there is always a concern that the sample being measured in the cell is constantly being replaced and is not “stagnant”.

    Potential fouling areas Chemical process 1) Cell 2) Probe

    A 1 mm gap can easily become fouled in many processes and the system ceases to give a representative measurement of the material properties. Furthermore, in cells the flow properties can be controlled as the complete flow of the pipe has to go through it; e.g. all of the flow can be forced between the windows if desired or a certain amount can be designed to by- pass them. However, in a probe, particularly if inserted in a “T” connection in the pipework, this cannot usually be achieved and it may be difficult to ensure that air bubbles are not trapped between the windows.

  2. Resistance to fouling and easier cleaning

    Fouling of the optical windows is a concern in many processes. Even apparently “clean” processes can give rise to a slow build up of contaminants over a long period of time. Occasionally, a process “upset” may cause deposition of something in the plant and the windows may get coated. When this happens the spectrum from the cell will have an additional absorption from two passes through the contaminating film (one from each window).

    The spectrum from the probe will suffer with double the amount of absorption because the light has to pass through the contaminant four times instead of two. This will clearly cause a faster degradation in the quality of the results from the probe compared to the cell. Many Specac cells have the option of cleaning ports. These allow easy access to the windows for cleaning – either on a regular basis, or in case of an unexpected fouling problem. This is not possible with probes – they have to be removed from the plant in order to be cleaned, which involves a higher workload due to safety considerations.

  3. Better seals

    Most probes are designed to be small in diameter in order to fit into the process through a standard pipe fitting. This inevitably causes compromises in the mechanical design. In particular, the seals that prevent process fluid leaking past the windows have to be extremely small and it is not usually possible to offer a choice of seal types to suit the application. Many probe designs resort to using epoxy adhesives and these will often break down when exposed to the process solution for long periods. In addition, it is not possible to offer options such as back-up seals or tell-tales because of the space restrictions. Specac liquid cells come with a wide range of seals to meet even the highest demands on process integrity.

  4. Stronger structurally

    A small physical size for the probe also limits its mechanical strength – particularly for probes made of glass or quartz. What if someone tries to stand or climb on it after it is installed? – it’s not as unlikely as you might think! Probes can also suffer a significant bending force from high flow rates in the pipe line and need to be able to withstand this along with any vibrations or other forces. A small physical size also limits the options for terminating a rugged armoured fibre cable. It is no good having a rugged process probe or cell but not have proper protection for the long fibre cables. Specac liquid process cells have been designed for the harshest process environment and offer additional IP65-rated end caps for peace-of-mind protection of delicate optical fibres.

  5. More flexible optics

    In many probes the fibre optic cables are permanently fitted during manufacture as they need to run down the inside of the probe to reach the optical system at the head. This can give rise to three problems.

    a) handling and installation of the probe is much more difficult with long fibre cables attached (particularly if they have been properly armoured to protect them),

    b) the length of fibre inside the probe can limit the probe’s maximum temperature capability (the fibre or its protective coating may degrade after exposure to elevated temperatures) and

    c) it requires an in-line coupling to join the probe cables to the main cables running back to the spectrometer. In line couplings have to be water and dirtproof and it is critical that they do not cause any optical degradation such as channel fringing which would degrade the spectral performance.

    Some manufacturers avoid the handling problems by fitting SMA fibre connectors directly on the back of the probe. However, these still suffer from problems b) and c). Another approach is not to use fibre cables within the probe but to use hollow light pipes instead. This avoids all the above problems but, instead, the probe requires a system to purge the light pipes with clean dry air – otherwise they act as very effective long path gas cells and will start to monitor the spectrum of the ambient air rather than the process fluid!

    Specac process cells come with industry standard SMA905 optical connectors and give the user flexibility to connect any type of spectroscopic fibres to it. Should there be an issue with the fibre in the field the process cell can remain installed, only the fibre optic cable has to be replaced.

Types of process flow cell

Vortex Flange-Mounted Phase Process Liquid NIR Flow Cell

  • flange-mounted cell
  • reliable in aggressive settings
  • easily trace-heated
  • choose body/seal materials
  • selection of sizes
  • cleaning port option
  • up to 300ºC temperatures
  • factory-aligned optics

Cascade Pipe-Mounted Process Liquid NIR Flow Cell

  • pipe-mounted cell
  • UV/Vis and NIR
  • fiber interfaces for 300 to 600 µm
  • choose body/seal materials
  • cleaning port option
  • factory-aligned optics

Typhoon Phase Process NIR Gas Flow Cell

  • reliable in aggressive settings
  • analyze gas/vapour in process
  • choose body/seal materials
  • simple and flexible process integration
  • cleaning port option
  • factory-aligned optics

Contact us for a free consultation

Get in touch for a price estimate and consultation with one of our specialists.

With state of the art design and material modelling tools and over 15 years of process application experience to draw from, Specac's range of process flow cells have a proven track record of reliability and robustness to ensure long-term operation in the most demanding industrial environments.

Computer-aided optical design ensures that our process flow cells have the highest levels of optical throughput to ensure the best quality of spectroscopic data.

To learn more about what spectroscopy can do, check out #SpectroscopySolutions for more insights into the applications XRF and FTIR can fit.

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