With so many different measuring instruments such as cable locators, GPR and metal detectors, how do I know which measuring instrument to use?
There are several factors to consider when assessing which tool to use:
What is the shape of the pipe?
Is the material of the pipe magnetically conductive?
Is it possible to insert a probe inside?
For specific instructions, refer to the attached flow chart.
How does the MPL work (theory of MPL)?
When electric current flows through an underground cable or metal pipe, it creates a concentric magnetic field in the surrounding area. We can measure the position and the depth (and current index) of a buried object from above ground by detecting this magnetic field with a receiver.
There are two methods for applying the electric current signal from the transmitter: direct mode, and indirect mode. When a conductive pipe or cable is exposed above ground, current can be applied directly using a connector clip. When no part of the pipe is exposed, a magnetic field can be induced indirectly using electromagnetic induction.
What is the proper way to use direct, external and indirect modes?
When using direct method, directly clip the connecting cable to the gas or water pipe and apply AC current to it. This method has the highest measurement accuracy. Whenever it is possible to connect directly to a pipe, this is the preferred method.
The external coil method is used for the measurement of telecom cables and power lines surrounded by a protective sheath pipe. If it is impossible to directly clip onto the line or pipe using the connecting cable, clamp the sheath pipe using the external coil.
When neither direct nor external coil methods are possible, use the indirect method. This is most often when no portion of the pipe is exposed above ground. Measurements using indirect method are not as accurate as with direct or external coil method.
MPL has various frequencies, but what are the pros and cons of each frequency?
The MPL transmitter can transmit at four frequencies: 80kHz, 38kHz, 9.5kHz, and 512kHz. In addition, the receiver has two modes which do not require a transmitter. These modes are used for detecting radio waves and power cables of 50Hz or 60Hz .
This high frequency makes it possible to get over capacitance of the insulating coupling of the gas pipe, however this can easily result in secondary induction on the other utilities surrounding the intended pipe or cable. In addition, arrival range of the signal is shorter in comparison with low frequency.
As this is the most versatile (basic) frequency, we recommend first starting with this frequency for taking measurements. This frequency cannot to send signals across the coupling of the insulator, making it more difficult to detect steel gas pipes.
This frequency is best suited for detecting communication lines, as it is able to transmit at longer distances. However, similar to the 38kHz frequency, it is more difficult to detect steel gas pipes as it lacks the power to penetrate the coupling of the insulator.
This low frequency signal has little risk of secondary induction on cables or pipes buried near the target pipe or cable. Because it is low frequency, the object pipe or cable should have a grounding terminal and use a lower resistance cable.
Radio, 50Hz, 60Hz
This operation is possible using only the receiver. No transmitter is required. Radio mode uses the natural magnetic field to detect radio interference. Power cable mode detects frequencies of 50/60Hz
Because these methods do not use the transmitter, measurements are not as accurate as compared with the methods mentioned above. Whenever possible, it is recommended to use both transmitter and receiver when taking measurements.
What is the difference between "peak" and "null" modes?
Peak mode can detect the center of the target pipe or cable so that its measurements is more accurate than null mode.
With null mode, an arrow indicates the direction of buried objects so that it is possible to search a route more quickly. In addition, when searching around metal structures such as fences or guardrails which adversely affect the measurement, null mode can minimize measurement interference.
It is recommended to use null mode when searching routes, and then to use peak mode for more precise measurements.
What should I be careful of when using indirect method?
Install Transmitter of which the long side of Transmitter goes directly for the assumed-buried pipe direction
Keep distance more than 10m/3ft between Transmitter and Receiver. This is because Receiver might directly receive the signal from Transmitter not the signal from buried pipe.
Do not put Transmitter on the metal structures such as manholes.The output of Transmitter is always adjusted automatically, and the output goes up on the metal structure. It consumes battery so much. More over, the signal is intercepted by a metal structure and can not send a signal to the object structure.
What should I do if I can't detect anything even using direct method?
First try changing the transmitting frequency.
Increase the area of the electrical circuit when grounding. Keep distance of a point to drive the ground horizontally for measurement line.
Check if there are any metal structures such as guardrails or fences nearby? If there are, use null mode for the taking measurements.