June, 2009

Questions & Answers re Attachment of Sensors

The purpose of this document is to instruct and advise on how to attach Gobius’ sensors on a tank wall.

Why is the attachment of the sensors so important for the Gobius level measurement?
Gobius is the sole level indicator on the market to measure from the outside of the tank regardless of tank material. The measuring technique is unique and patented. The measuring is done by a shaker in the sensor that creates a short vibration which is transferred to the tank wall. The vibration is measured by an accelerometer in the sensor. To perfect this, the sensor has to be attached correctly;
The sensor has to be firmly and solidly positioned against the tank wall to ensure minimal loss of motive energy in the attachment.

Thus, it is not OK to use e.g. polyurethane sealant or silicon adhesive to attach the sensors.

VHB tape or 2 component glue?
In most cases it is sufficient to use the preattached VHB tape to attach the sensors to the tank wall. On fibre glass tanks, painted tanks and dirty rough surfaces (old tanks) you should use the 2 component glue.

VHB tape
The VHB tape’s advantage is that it is easy to use and it fits most tank materials. There are two things to keep in mind: Firstly, the temperature in the air and on the tank wall needs to be above 59 F (+15 degrees Celsius) when you attach the sensors. If the temperature is lower than this, you may increase it by using a warm fan. Secondly, the tank may not curve in, i.e. be concave, where you plan to attach the sensor. It is true that the VHB tape is “forgiving” to a concave tank wall, but we recommend that you try to move the sensor sideways until you find a surface which is not concave.

If the sensor is attached to a concave surface, or, the VHB tape is not attaching fully, this is shown on the panel by that sensor’s light and the lower light shining simultaneously when starting the system. This error stops you from calibrating the system and using Gobius. I.e. the error must be corrected before use of Gobius.

Gobius is developed to work with 3Ms VHB tape or 2 component glue. Our guarantees are valid when the sensors are attached with VHB tape or 2 component glue. We advise against experimenting with other tapes or glues.

Do not use polyurethane sealant or silicon adhesive to attach the sensors.

2 component glue
The best result is with a 2 component glue, e.g. Epoxy or 3M 8005. With these the attachment is not elastic at all. You can peel off the VHB tape and use 2 component glue in its place. We have pre-attached VHB tape to simplify installation of Gobius.
When the surface is concave, the 2 component glue is very advantageous. The VHB tape has difficulty attaching in the centre of the sensor when the surface is concave. On fibre glass tanks 2 component glue is required. These tanks’ surfaces are rarely smooth and we cannot recommend the use of VHB tape.

The same applies to painted tanks and dirty contaminated tank surfaces, e.g. old aluminum tanks.Please note that Epoxy glue is not applicable to polyether tanks (most rotation casted tanks are made of polyether).

Can sensors be attached to all sides of the tank?
The purpose of attaching the sensors to a vertical wall is to enable measurement of the liquid level in the tank. This implies that the sensors can be put on different sides, i.e. they need not be placed all on the same wall. However, you may not put a sensor on the top side of the tank.

We have performed tests to prove that you can put a sensor on the bottom side of the tank to detect a liquid. 4-5 mm of liquid is enough for detection by Gobius. 

Moving a sensor to a new placement on the tank
You may move a sensor, but in order to do that you need to be very careful not to damage the sensor. The sensor is fragile and can easily break if submitted to much force.

How to do it:

• Try to screw the sensor off the surface.
• If this is not possible, use a knife and carefully bend the sensor from the tank wall. Put the knife between tha tank wall and the sensor where the cable is attached to the sensor.
• Bend carefully, wait a few seconds and continue to bend. Repeat until the sensor comes off.
• When the sensor is off, peel off the old tape and put a new one in its place.
•  When the new VHB tape is attached, press the sensor to its new place on the tank.
• You need to recalibrate the system after moving a sensor. You will find out how in your Gobius instructions. All instructions are available at www.fmmarin.se /Products /Instructions

Water-proof installation of sensors
If you require a water-proof installation of the sensors, you should use 2 component glue together with a soft silicon string.

How to do it:

• Start by peeling off the VHB tapen from the sensors.
• Prepare 2 component glue.
• Put glue generously on each sensor.
• Press the sensors to the tank wall and fixate their placement by aid of tape so that the sensor does not move during the hardening.
• Wait for the glue to harden.
• Remove the fixating tapes.
• Finish by adding a thin string of silicone at the edges of the sensor in order to make it water proof.
• If you also require the control unit to be water-proof, please contact us. We can supply a larger box which we recommend to our customers when they want the control unit to be water-proof.

Baffles inside the tank and connectors
Low metal tanks often have baffles inside the tank to reduce the movement of the contents. These are often traceable via welding marks on the surface of the tank. Sensors should not be placed on these places. The distance required is the same as from a corner, i.e. 2 ¾” (70 mm) from the center of the sensor. This is also the minimum distance to pipe connections.

For questions and enquiries please call us for advice at +46-31 65 01 70 or at support@fmmarin.se

PS. You can read more about the VHB tape at www.3m.com