The coaxial tube for TDR level sensors

TDR (time domain reflectometry) level sensors can be fitted with a coaxial tube. This is a metal tube around the probe of the sensor, which improves the performance of the TDR level sensor.

The coaxial tube is necessary in the following cases:

Tanks from plastic

The signal of TDR level sensors propagates between the inner conductor and the outer conductor. The inner conductor is the sensor's probe. In tanks made from metal the outer conductor is the wall of the tank. If the tank is made from plastic the wall cannot function as an outer conductor because it is electrically non-conducting. Level measurement by TDR level sensor is still possible but a coaxial tube must be used. Here, the signal propagates between the sensor's probe and the wall of the coaxial tube.

Media with a low dielectric constant (DC)

The dielectric constant (DC) of a substance determines how much of the signal is reflected from the surface of the medium to the sensor: The greater the DC of the medium, the stronger the reflected signal. Media with low DC can be measured by TDR level sensors if a coaxial tube is used. The tube reduces the loss of signal energy, thus making even week signals measurable. Examples for media with low DC are lubricants and hydraulic oils.

The TDR level sensors LFP Cubic & LFP Inox require a coaxial tube if the media DC is between 1.8 and 5.

Further benefits of a coaxial tube

Even if the sensor is installed in a metal tank and the media DC is greater than 5 a coaxial tube may be used to improve measurement. General benefits of the coaxial tube are:

• Shields the sensor probe from installations inside the tank (agitator, parts very close to the probe, etc.). The sensor does not measure anything outside the coaxial tube.
• Shields the probe from foam and jets of media
• Increases lateral load capacity, which may be necessary if there are strong media currents
• AutCal and AutoTn can be conducted outside the tank if the tube is installed to the sensor. The tube can even be touched and held by hand during AutCal and AutoTn.

Note 1

The gap between the probe of the LFP Cubic & LFP Inox and the coaxial tube is 5 mm. If the media contains solids, these may build up in the gap between probe and tube. This may cause wrong level readings. In case such media build up may occur, the use of a coaxial tube may not be possible. In that case a non-contact measurement principle like free space radar (LXRH, LXRC) or ultrasound (UP56, UP56 Pure) may be preferred.

Note 2

The coaxial tube can be combined with both the LFP Cubic and the LFP Inox. However, the LFP Inox is designed for hygienic applications, and the coaxial tube does not meet hygienic design requirements. Therefore, the coaxial tube is more frequently combined with the LFP Cubic than with the LFP Inox.