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GRF18S - How can I measure oil other viscose liquids?

This article describes how the GRF18S can also be used with higher viscosity media such as oils.
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The GRF18S is well-suited for water-based media. When using the GRF18S in higher viscous liquids, a few things must be observed.


There are sensitive areas on its transparent prism. If there is medium in these areas, the sensor switches.

In the case of viscous media, droplets may form after contact with the sensor. If these drops form in the sensitive areas, the sensor does not switch back.

Where the droplets form can be influenced by the installation angle. For highly viscous media, installation from above (vertical) is recommended. This causes the droplets to form at the tip of the prism and not in the sensitive areas.

What else needs to be considered:

  • With adhering media, the sensor does not switch back if the medium sticks to the sensitive areas.
  • The medium must be at least partially optically transparent.
  • Where the sensor thread begins is not defined. The sensitive areas can therefore be located at a different position for each sensor.
  • Tests should be performed with the specific medium and the GRF18S.


The following explains in detail what you need to pay attention to in order to detect non-water-based media.



1. Functional principle

The GRF18S is an optical point level switch. It uses a light emitter and receiver, which are placed behind a Prism (made out of PSU - Polysulfon).


In the animation you can see how the light is emitted from the optical emitter and directed into  the receiver. If a liquid covers the prism the light no longer gets directed into the receiver and gets dispersed into the liquid. If there is no light detected by the receiver the GRF18S switches its output signal. 


The picture on the left (front view) shows the openings of the transmitter and receiver unit

located behind the prism. On the left side the opening of the transmitter and on the right side the opening of the receiver.


The GRF18S is sensitive at precisely these two points (marked by blue circles). If there is medium at these points, the sensor recognizes this and switches.


If the medium touches another part of the prism, the light, which is transmitted from the transmitter to the receiver, is not affected. The sensor does not switch.


Accordingly, there are sensitive and non-sensitive areas on the surface of the prism. All areas of the prism are either sensitive or non-sensitive.

  • Sensitive: These are the areas above the transmitter and receiver openings. Outlined in blue in the picture. If the medium touches these areas, the sensor switches.

  • Non-Sensitive: These are the areas that are not at the transmitter and receiver openings. If the medium touches these areas, the sensor not switches.

2. The challenge with viscous media

The problem with viscous media: Depending on the viscosity, the medium forms droplets or sticks to the prism.



Drops on the prism:


Viscos liquids in the range of oil form drops that remain on the prism. If the drop gets stuck in the sensitive area, the sensor switches - even though only a single drop touches the prism.

The picture on the right show an oil with a cST of 72 mm²/s.

The green light at the process connection indicates the sensor not switches.


Adhesions on the prism:


Very highly viscous or, above all, sticky media can adhere to the sensor. If this happens at one of the sensitive points, this causes the sensor does not switch back after the fill level has dropped.


The picture on the left shows tomato ketchup sticking on the prism.

The orange light at the process connection indicates the sensor switches.

     2.1 Drops on the prism

Viscous media form droplets that stick to the prism of the GRF18S.
If these droplets stick to a sensitive area of the prism, it does not switch back.


In the following video, the sensitive area is at the bottom and top. The drop of silicone oil (1500 cSt) remains exactly in the sensitive area. The orange light indicates that the sensor is switching. You can see that by rotating the sensor by 90°, the drop flows from the sensitive area into the non-sensitive area. The sensor switches back, recognizable by the green light.

The video shows silicon oil with 1500 cSt.

When screwing in the sensor into a tank, it is not possible to predict where the sensitive areas of the prism are located. The thread of the GRF18S is different for each device. This also applies to many tank threads.

The solution: The installation angle.

          2.1.1 The solution for drops on the prism

There are three different installation angles. Horizontal (0°), vertical (90°) and oblique at a 45° angle.

  • To ensure that the GRF18S switches and switches back when it comes into contact with oils and other viscous liquids, it is recommend installing it vertically (90°C angle).

  • If a installation in a horizontal position (0° angle) is required, care should be taken to ensure that the sensitive area is not at the top or bottom.

  • Oblique installation (e.g. 45° angle) is not recommended.

If the sensor is installed horizontally (0°), drops form in the lower area.

If the sensor is installed so that the sensitive area is at the bottom and top, a switch-back delay may occur.

See the video above in chapter 2.1.

In a vertical (90°) installation, drops form at the prism tip. The drops are therefore never in the sensitive area and false signals are avoided.

In an oblique (45°) installation, the drops form in the center of the prism underside. Exactly in the zone of the prism in which they are sensitive areas.

This can lead to incorrect switching signals.


     2.2 Adhesions on the prism

          2.2.1 The solution for adhesions on the prism

If deposits form on the sensitive areas of the prism, the GRF18S cannot be used. It does not switch back after contact with the medium and is therefore not suitable for such liquids.

An good alternative for such liquids is the LFC capacitive level switch or the LFV200/LFV230 vibration fork level switch.


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