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Ceramat or SensoGate: Choosing a Retractable Fitting

A retractable fitting lets you take a sensor out of a live process without shutting it down. Knick makes two families of them and they are not interchangeable: the choice comes down to what your process does to a seal. This is how to tell which one your measuring point needs.

What a retractable fitting is for

The sensor sits in the process. Periodically it has to come out, to be cleaned, calibrated or replaced. A retractable fitting moves the sensor out of the process and into a sealed calibration chamber without opening the vessel or the pipeline, so production keeps running and nobody is exposed to what is inside.

Everything then rests on the seal between that chamber and the process, because that seal is what stands between your maintenance engineer and the medium. It is also the part that wears, and it wears at a rate your process decides. That is precisely where Ceramat and SensoGate diverge, and why choosing between them on price is a false economy.

Ceramat: the ceramic seal

Ceramat is an automatic retractable fitting that seals with two exceptionally flat, polished ceramic discs. They separate the calibration chamber from the process by turning against one another, a lock-gate arrangement rather than a sliding seal. The ceramic is harder than steel and highly resistant to chemical, thermal and mechanical attack.

That construction is aimed at the processes that eat conventional seals:

  • Highly corrosive media, where an elastomer is on a countdown from the day it is fitted.
  • Hot and pressurised media.
  • Depositing, abrasive and crust-forming solids. This is the one people underestimate. Solids do not simply wear a seal, they build on the sealing faces so that it no longer closes properly, and a rotating ceramic face shears that build-up off rather than dragging it through.
  • Safety-critical duty involving highly poisonous substances, where a seal that fails is an incident rather than a maintenance item.

The range spans materials and depths, which is the practical part of specifying one:

  • WA150: pneumatic, in carbon-fibre-reinforced PVDF or PEEK, for chemical resistance where a metal body is the wrong answer.
  • WA153 and WA154: pneumatic, in stainless steel, titanium or Hastelloy, with the WA154 offering immersion depth up to 300 mm.
  • WA160: a retractable immersion fitting reaching up to 2000 mm, for measuring points in tanks and basins rather than in line.

SensoGate: modular, and built for hygienic duty

SensoGate is a modular retractable fitting whose natural home is hygienic processes with CIP and SIP. Where Ceramat is about surviving what the medium does to a seal, SensoGate is about cleanability and the sanitary design that pharmaceutical, biotech and food processes require.

Modularity is the practical advantage. The fitting is configured from components rather than bought as a fixed object, so it can be specified to the process connection, the drive arrangement and the control system you actually have rather than the one the catalogue assumes. The range includes:

  • WA131: pneumatic, intended for use with simple, commercially available controllers.
  • WA131MH: manual, with a rotary drive, for hygienic applications where an air supply at the measuring point is not wanted.
  • WA130: pneumatic, specified for use with Uniclean and Unical, which matters if the fitting is going to sit inside an automated cleaning system.

That last one is the tell. If automation is in the plan, the fitting and the cleaning system are one decision, and picking the fitting first constrains what you can automate afterwards.

Choosing between them

The question is not which fitting is better. It is what your process does to a seal.

  • Choose Ceramat where the medium is corrosive, hot, pressurised or toxic, or carries solids that deposit, abrade or form crusts. Anywhere a conventional seal has become a consumable with a predictable replacement interval, the ceramic lock-gate is the answer to the underlying problem rather than to its symptom.
  • Choose SensoGate where the process is hygienic and regularly cleaned and sterilised in place, and the requirement is sanitary construction, cleanability and modular configuration rather than resistance to chemical attack.

Three things worth settling at specification rather than discovering later. Immersion depth varies enormously across the range and is not adjustable afterwards. Body material should be chosen against the medium rather than against the pipework, since these are different problems. And the drive, pneumatic or manual, needs an honest look at whether clean, dry instrument air is genuinely available at that measuring point, because a pneumatic fitting on poor air is a recurring fault.

Where the fitting sits in the measuring point

A measuring point is four decisions: the sensor, the fitting, the cleaning and calibration system, and the transmitter. The fitting is the one most often chosen last and it should not be, because it determines whether the sensor can be serviced safely and how often somebody has to be there.

Get it wrong in the aggressive chemicals direction and you have bought chemical resistance you are not using. Get it wrong in the other direction and the seal becomes a maintenance item in its own right, on a process that will not wait for it. Neither is recoverable by choosing a better sensor afterwards.

What happens inside the calibration chamber

The chamber is not simply a parking space. Once the sensor is retracted and the seal is closed, the chamber is where the sensor is rinsed, cleaned and, on an automated system, calibrated before it goes back. That sequence is why the fitting has service connections on it, and why the media supply is part of the specification rather than an accessory.

Two things follow that are easy to miss on a drawing. The chamber needs a drain that can actually take what comes off the sensor, which on a dirty process is not clean water. And the rinse media has to be compatible with the process, because a residue of the wrong thing going back in with the sensor is a contamination route straight into the vessel.

The specification mistakes we see most

  • Choosing on body material alone. The body and the seal face different problems. A Hastelloy body with a seal the medium destroys is still a fitting you will be servicing.
  • Forgetting the sensor length. Immersion depth, insertion length and the sensor itself have to agree with each other, and the fitting is the least adjustable of the three once installed.
  • Assuming instrument air is available and dry. Pneumatic drives are reliable on good air and a nuisance on bad air.
  • Leaving no room to withdraw. A retractable fitting needs clearance behind it. It is a common and expensive discovery on a congested skid.
  • Specifying the fitting before the cleaning system. If automation is coming later, say so now, because it narrows the sensible choices considerably.

At DP-Flow we supply both families along with the sensors, cleaning systems and transmitters that go with them. If you can tell us what the medium does to a seal, and how deep the sensor has to sit, we can tell you which fitting your measuring point actually wants. That is a more useful conversation than a datasheet comparison.