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Introduction

This document…document is designed to offer some insight into drainage inclination testing equipment, explain some of the logic that is considered inside the WinCan VX application, help readers to understand the output reports and assist with diagnosing problems.

Definitions

Be very careful with these first two words in this list - they are almost the same but have very different meanings.

• Attitude - is the angle of slope of the device at any point during the inclination test.

• Altitude - is the height of the device at any given point during the test measured from a specified datum.

• Gradient - is the slope of the pipe or the inclination test represented as a percentage where 0% is dead flat, -1% is a drop of 1m over 100m length in the direction of flow (i.e. a good drain, a happy drain), +1% is a rise of 1m over 100m length (i.e. an unhappy drain) and -100% is a gradient of 45 degrees or 100m drop in 100m length.

• Delta Height - is the change in height either along the entire length of the pipe, the length of the inclination test or between two measurement points with the test. As with Gradient, negative values refer to a drop in height in the direction of flow and positive values describe a rise in height.

Background

Under ‘normal’ conditions, drains flow by gravity and run downhill in the direction of flow, so the upstream end of a drain is usually at a higher altitude than the downstream end.

The only times that this does not happen are:

• If the pipe is a pumping (or force or rising) main so it has a pump at the upstream end and the downstream end of the pipe is higher than the upstream. It would very unusual to measure the inclination of this type of pipe.

• The pipe has failed or been badly installed so that the downstream end is higher than the upstream end.

The Equipment

Inclination measured by an inclinometer which is a device usually inside a CCTV crawler similar to an accelerometer that can measure very small changes in attitude, which can then be processed into a curve that represents the shape of a pipe in the vertical plane between;

• one specified point and one calculated point, or

• two specified points.

While an inclination test is being carried out, the camera will be sometimes looking up and sometimes looking down and sometime level during the test so we will be recording a mixture of positive and negative values along the length of the test.

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Attitude values.

Measurements

During the inclination test, WinCan VX takes only 2 values from the camera:

• The distance measurement (in metres or feet)

• The attitude (in percent)

Everything that is then printed and reported is calculated from these two columns of data. The data points are read and recorded by WinCan VX every time there is a change in the attitude value, and due to the hyper sensitivity of the inclinometer, this means that a lot of points are recorded.

Having already described the value of 100% attitude as a 45 degree slope, the cameras available on the market are not capable of measuring this value. If you take a CCTV camera in your hand and watch the inclination value on the screen from the device and try to get it level and reading zero, then slowly tip the camera nose up (or down) and you will see that the inclination hits a maximum value of about 23% depending on the manufacturer and will not go any further.

This attitude value of 23% equates to an angle of approximately 13 degrees, so the maximum and minimum gradients that the device can measure are not very far away from a perfectly flat line.

Data Basics

In order for an inclination test to show that a pipe flows nicely downhill, we would expect there to be more negative values in the data points than positive values if the test was done from the upstream end. In other words, the average attitude value over the length of the test will be negative, and the delta height will also be negative.

But, what happens if we test exactly the same pipe with the same camera on the same day from the other end, the downstream end. Now, the majority of the measurement points will be positive, the average attitude value will be positive and the delta height will be positive.

So, we have two tests of the pipe where one says the average gradient is +2% and the other says it is -2% so which one is correct?

Well, they are both correct, bu this is confusing to engineers and operators trying to understand the data, because this idea is utilising an old concept of ‘start’ and ‘finish’ manholes which has now been abandoned by most global CCTV inspection standards. In other words, it relates the data from where the camera was inserted into the pipe (i.e. the upstream or downstream end).

In most drainage standards now we have moved to the more intuitive concept of upstream and downstream manholes instead of start and finish manholes.

WinCan VX uses this concept in all inclination graphs so that the upstream end of the pipe is always on the left side of the graph and the downstream end is always on the right side, regardless of which way the test was done, so that in all cases a happy drain goes downhill on the chart from left to right.

Test Options

An inclination test on a pipe can be carried out any one of 4 styles, where the choice of style that will give the best results is largely dependent on the equipment:

• From the upstream end driving forwards away from the truck to the downstream end (i.e. Downstream CCTV inspection direction and Forwards inclination measurement direction).

• From the upstream end driving backwards towards the truck to the downstream end (i.e. Downstream CCTV inspection direction and Backwards inclination measurement direction).

• From the downstream end driving forwards away from the truck to the upstream end (i.e. Upstream CCTV inspection direction and Forwards inclination measurement direction).

• From the downstream end driving backwards towards the truck to the upstream end (i.e. Upstream CCTV inspection direction and Backwards inclination measurement direction).

Remember that only 2 measurements are recorded from the camera during the test, the attitude and the distance. The most significant decision regarding the accuracy of the distance measurement is usually defined by whether or not the camera has a manual winding cable drum or an auto-winding cable drum.

Why? Because if the cable drum rewind is manual and you try to do a Backwards inclination test, you will be relying on the operator at the back of the CCTV truck to pull the cable extremely smoothly and consistently while also winding the handle, which is extremely difficult when you have a drain that is 100m long, almost impossible. So, in this case, it makes sense to do the test forwards so that the drive of the camera pulls the cable and the distances measurements are smooth and consistent.

But, if you have an auto-winding cable drum, then you can do the test backwards with confidence because the machine will pull the cable at a constant rate and without allowing the cable to become tangled with the camera wheels and without any sudden tugs from the operator that can jolt the camera and give bad attitude data points.

In general, backwards inclination tests give better results than forwards where the equipment allows because there is much less effect on the camera unit from the cable.

It is common to see some bad and extremely high or low data points near to the end of the test that is closest to the truck (so at the beginning of the test if the test direction is forwards or at the end of the test if it is backwards). This is because when the camera is close to the inserting manhole, it is common for the cable to be vertical or sloping up towards the CCTV truck, and this force on the back of the camera can have a serious effect on the attitude values, so for the absolute best results, a low manhole roller should be used for the camera cable so that the cable is always directly behind the camera.

Calibration

The inclinometer in the crawler is extremely sensitive to changes in attitude of the smallest amount. It is a solid state device that outputs an electrical signal that we can interpret and measure and as a direct result of its extreme sensitivity, it is prone to drift.

It is because of this that to get the best and most accurate results from an inclinometer, the equipment must be calibrated regularly by recognised service agents and under extreme cases also on the day of the inclination test.

It is not uncommon for the calibration of inclinometers to be bad, and to consider the consequences - imagine that the inclinometer is under-reading by a value of 3%, so in other words it is reading a value of -3% when it is on a perfectly flat surface (should be zero of course).

Then we do an upstream inclination test on a drain that is perfectly straight and has a gradient of -1% (i.e. it falls at a rate of 1% from the upstream end).

So, we do the inclination test upstream, so we expect the attitude values to be nice and positive at 1% because the camera is travelling in an upwards direction, but the under read of -3% means that we are recording values of -2% all the way up the pipe.

The result is that the graph may be very difficult to interpret and may even look like it is going the wrong way. There is no surprise in this because that is the data that WinCan VX has received from the camera and the data does not lie.

This is a common issue with customer’s inclination reports when they call our support teams and complain the the graph ‘looks wrong’.