Ultrasonics uncovered

There are about 25,000 CSO structures in the UK. Many have been identified for improvement and are monitored by ultrasonic level sensors. Dr Gurch Chana, who has spent three years researching the subject, and David Walker of Radcom Technologies explain some of the challenges involved

Ultrasonic level measurement in CSOs begins when a high-frequency pulse - typically 40-125KHz - is transmitted in the direction of the fluid surface through a column of air between the crystal and the fluid.

As the characteristic impedance between medium 1 (air) and medium 2 (water) is substantially different, a transmitted pulse of width Tw enters medium 1 and most of the pulse energy is reflected at the boundary of medium 1 and medium 2. The reflected pulse returns back to the crystal at time Tt after the outgoing pulse has decayed. Since Tt is the time for the round trip of distance 2L then Tt = 2L/c. Where L is the distance from the crystal and c is the sound velocity in medium 1. The time Tt is measured accurately using a precision clock. Using this information and the above equation together with site data, we are able to compute the fluid depth of medium 2. (See Fig 1.)

The speed of sound in air varies depending on atmospheric conditions; the most important factor is the temperature. Humidity has little effect on the speed of sound, nor does air pressure per se. (Pressure has no effect at all in an ideal gas approximation. This is because pressure and density both contribute to sound velocity equally, and in an ideal gas the two effects cancel out, leaving only the effect of temperature.) Sound usually travels more slowly with greater altitude, due to reduced temperature. An approximate speed of sound in 0% humidity (dry) air, in meters per second, at temperatures near 0°C, can be calculated from:
cair = (331.5 + (0.6 x ø)) ms-1
Where ø is the temperature in degrees Celsius (°C), not Kelvins.

Temperature compensation is important and must be considered at the first design stage of any ultrasonic level device. With built in microprocessor technology this calculation is performed in the sensor electronics before processing the data.

All ultrasonic level sensors utilising a single crystal for transmission and receiving pulses are limited in a minimum distance typically known as the blinding distance or dead band. This occurs due to the oscillation of the transmitted pulse and the time it takes for the oscillation amplitude to decay below a minimum threshold before the same crystal

can be used in reverse. It is important to build in flexibility to enable this blinding distance to be varied depending upon site conditions. (See Fig 2.) The three major challenges posed by site conditions are availability of power, installation and data collection:

Power
Most CSOs have no power source for driving any instrumentation. Also, any instrumentation used in a sewer network must be Atex-approved for installations where potential explosive atmospheres may occur. These limitations pose significant challenges on power consumption and battery life. Loop-powered level sensors further reduce battery life due to the high power consumption and time required for the electronics circuits to stabilise.

Problems associated with loop-powered sensors using analogue outputs i.e. 4-20mA. (See Fig 3.)
  • Loop power has to be supplied for many seconds to allow voltages to settle resulting in low battery life (typically less than two years)
  • Long sample rates (15 minutes) have to be used to conserve battery
It is possible to overcome this obstacle by using low-power electronics with fast-stabilising circuitry and serial communications between the sensor and data logger.

Installation
Installation needs to be simple and quick to minimise time that engineers spend working inside a confined space, and to minimise traffic disruption.

We can learn a great deal from Microsoft's drive for plug-and-play technology. Costs and delays are more often associated with hard wiring sensors to data loggers.
Advances in radio communications mean sensors can use radio signals to talk to a remote data logger, obviating the need for hard wiring and laying expensive cable ducts.

It is important to ensure the ultrasonic sensor is mounted vertically and the target is not obstructed by other objects such as step irons or cabling already in the CSO chamber.

Thermal currents between the ultrasonic sensor and the level surface will affect the average sound velocity. The sensor should be installed to minimise airflow past the sensor path.

Data collection using SMS and radio
Manual data collection from CSO instruments is costly due to site access and confined space entry requirements, so most CSO monitoring systems comprise a sensor connected to a data logger and a modem.

These devices struggle when the modem antenna is located within the CSO chamber as the chamber acts like a Faraday cage, preventing the sms/gsm signal being transmitted. This is usually rectified in one of two ways.

The first is where the logger is externally mounted in a weatherproof housing. This means having to lay a cable duct between the sensor mounted in the CSO chamber and the externally located data logger.

The second option is to mount everything inside the CSO chamber addressing Atex requirements and create a small duct leading to an antenna mounted outside. The antenna is then usually mounted in the road surface and lightly covered by a road sealant. Both of these options require some form of civil engineering work and add time to the overall installation.

Radcom has overcome this issue by developing a low-powered radio transmitter which allows both the SMS/modem and data logger to be located outside the CSO chamber. Sensor data is communicated from the CSO via radio primarily, and then the data package is rerouted via SMS. This method means there is no need for drilling and laying conduits, which have been the largest costs associated with CSO installations.



An ideal ultrasonic CSO monitoring system includes
  • In-built temperature compensation
  • Variable blinding distance
  • Five-year Battery Life
  • Combined radio and SMS to overcome SMS signal loss in CSO chambers and to reduce installation time
  • Low cost

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