Introduction
This note explains some slight but important differences in the design evolution of Access Tubes for Profile Probes, and describes a simple bevelling operation recommended for tubes of older construction.

Access Tube Design Changes
Some minor design changes were made to access tubes at the time of the introduction of the PR2 Profile Probe. This advice applies equally to long and short access tubes (types ATL1 and ATS1). The basic dimensions of the access tube have not changed, and Profile Probes type PR1 and PR2 can be used in any access tube. You can distinguish access tubes made for Profile Probes by the shape of the bottom plug. For the PR1 it was conical with a rounded end. For the PR2 it is pointed, as shown in the picture.






















Installation Warnings!
The PR1 access tubes were designed for installation using a hitting block placed in the open end (the mouth) of the access tube.
The PR2 access tubes are designed for installation using the Insertion Rod (type AT-ROD1) which applies the driving force to the bottom plug of the access tube.
Do not use the PR2 Insertion Rod installation method with tubes of the older PR1 design! You will probably dislodge the bottom plug and destroy the tube. Use only the original hitting block to install this type of access tube.

If you are extracting older access tubes for re-installation in different locations, confirm the type of construction, and use only the insertion method appropriate to that type of tube.
Some of the earliest PR2 access tubes were incorrectly manufactured. They were supplied around January to March 2005, and do not have enough adhesion between the bottom plug and the fibre material of the tube. If you cannot see a whitish area of internal grinding in the 2 cm above the bottom plug (see the location in the picture above) then the tube is suspect and should not be used. Delta-T can supply free replacement tubes. Please contact your distributor.

Access Tube Bevel Upgrade
A further feature of access tubes introduced since mid 2005 is an internal bevel in the entry to the tube. This significantly improves the ease with which the Profile Probe is inserted into the tube and helps to avoid damage to the rings on the Profile Probe, especially if it is not correctly aligned when inserted into the access tube.
The internal bevel is worth applying retrospectively to any access tube that does not already have it. Delta-T can provide a de-burring tool suitable for this operation. It can be used on access tubes whether or not they are already installed in the soil. Please contact your distributor.

Use of the Bevelling Tool
If your access tube does not already have this feature you can quickly create it with the bevelling tool. The desired effect is illustrated in the diagram.









In any of the processes mentioned below, avoid fibre composite dust by mopping it up with a damp cloth at all stages. If there is any possibility of inhalation of dust, use a filter face mask before you start the process.
First of all inspect the open end of the access tube. The top edge of the tube should be flat and in good condition. If it has been abraded or damaged by use, then use a fine toothed flat file to make it flat and smooth.
Apply the bevelling tool (use the pointed end of the tool) to the open end of the access tube. Rotate it carefully by hand for a few rotations to create the desired amount of bevelling. Aim to produce a bevel of not more than half the wall thickness of the tube.
Do not overdo this process! This is particularly important on the older PR1 tubes, because the whole of the installation force is transmitted through only the remaining flat area of the tube wall. Remember to clean and dry the inside of the access tube before using the Profile Probe.

The Profile Probe User Manual recommends that you use a Theta Probe (ML2x or ML2) to do a two-point soil-specific calibration.

You need to measure the moisture content of a wet sample of your soil, using the Theta Probe, and then oven-dry the sample and measure its water content once again.

The process is fully described in the Theta Probe User Manual and is also available in a detailed application note available from Delta-T. This should enable you to decide if the possible gain in accuracy is worth the additional effort of the calibration.

If you do not possess a Theta Probe, please contact your distributor or Delta-T for advice. The User Manual is available by email in electronic Acrobat pdf file format (and may be available for download from the Delta-T website).