• RBR
  • weblogger
  • Teledyne O3
  • Opsis SM200
  • opsis DOAS
  • Modules
  • Lephalale

Completing a world first at Cango Caves!

A brief overview of the project.

It takes planning, planning and more planning to successfully complete an installation such as this. When you have completed the planning phase of the project, you do more planning, followed by a few site visits and some more planning. A few kilometres of cable, some emitters and receivers, two eager technicians keen to lose about 8 kilograms in body weight each during an installation, and you have the first ingredient needed to start a project such as this. Have I mentioned planning yet?

C & M were contracted to supply, install and commission a continuous monitoring solution to the Cango Caves for the measurement of CO2, ambient temperature and relative humidity. In addition we had to monitor water temperature at one point approximately 300 meters beyond the fabled Cango 2. Popular belief has it that more people have been on the moon than have been in Cango 2 and beyond. In conjunction with monitoring inside the caves, an additional ambient monitoring system was requested to monitor ambient CO2, wind speed, wind direction, ambient temperature, relative humidity and rainfall.

Due to the high humidity levels and the ever-present threat of corrosion, we decided to make us of the Opsis open-path technology. In following this route we were able to remove the analysis system from the equation by housing it in a location out of reach of humidity and corrosion.

Outside the main entrance to the Cango Caves is a national cultural historical monument commonly known as the tea room. Due to its close proximity to the cave entrance, its sheltered position and relatively constant temperature and humidity, it was identified as the ideal location to house the analysis and data logging equipment. The tea room will in the near future serve as the starting point for tours into the caves. The trained guides will make use of this opportunity to explain the basics of cave chemistry to the visitors via the real-time data displayed on a 32" wall-mounted monitor.

The tea room Dag1_031_small Dag7_020_small

The basics of cave chemistry.

Cango_602_small Cango_585_small

Without going into too much detail, it basically boils down to this.

Rainwater, combines with acidic carbon dioxide from decomposing plant material on the surface. This acidic mixture flows through a fracture zone in the limestone rock.

The limestone, or calcium carbonate, combines with water and carbon dioxide to form calcium bicarbonate, which dissolves in water. This is what flows out into a cave system. Calcium bicarbonate reverts back to calcium carbonate by giving off carbon dioxide to the atmosphere. The calcium carbonate crystallises and forms the magnificent formations and structures found inside the caves.

Background monitoring.

In order to determine the background CO2 as well as meteorological data for the area, we installed the Teledyne-API model 360-E CO2 analyser along with the RM Young range of wind speed/direction and temperature/humidity sensors. In addition, an RM Young rain gauge was also installed.

The meteorological sensors, with the exception of the rain gauge, were mounted on a telescopic mast on an overhang above the caves (see image below). The rain gauge was installed on the roof of the visitor centre, which was probably the most obstruction-free area around.

Installing the meteorological sensors on an overhang above the caves. Dag5_002_small

C & M goes underground with the Opsis LD500.

The Opsis LD500 system used in this installation consisted of:

  • The LD500 analyser;

  • Five sets of LD500 CEM emitters and receivers (one set for each of the measuring points);

  • Four Opsis beam splitters;

  • Optical fibre cable (OF) connected to each emitter;

  • Communication fibre cable (CF) connected to each receiver.

After a thorough initial site visit, we decided that the LD500 CEM system would be the ideal solution for several reasons:

  • Other than the Opsis System 300 ambient monitoring systems based on the use of a xenon lamp, the LD500 system would give off no visible light to the human eye, and generate no heat at the emitter. This was an extremely important point since the continuous presence of light and heat would increase conditions suitable for the growth of moss on the cave structures and formations as well as the equipment.

  • The LD500 CEM system was chosen over the LD500 AQM system purely based on practicality since some of the areas we had to crawl through limited the size of equipment we could take in. Due to the sensitivity of the entire cave system, we had to take extreme care with our movements so as to avoid bumping into the cave formations. The simple action of touching an active formation could literally stop it from growing any further.

The installation.

The installation team arrived on site on Sunday 11 October, and after a quick site inspection and planning meeting, all was ready to start on Monday 12 October.

Since the normal day to day activities of the caves and tour groups could not be interrupted, the majority of the work inside the caves had to be done after hours. This allowed the team to focus on the installation of the ambient monitoring equipment consisting of the Teledyne-API Model 360E CO2 analyser as well as the range of RM Young wind speed / direction and temperature / relative humidity sensors.

Ambient monitoring installation

Cango_037_small Cango_058_small Cango_066_small Dag1_020_small Cango_721_small

As mentioned in preceding sections, it was decided to house the data logging, data management and analyser systems in the "tea room", a cultural historical monument. This site was chosen for its central location and close proximity to the cave entrance. As can be seen from the images above, the site was constructed underneath the cliff overhang, with its rear wall being the actual rock face. This allows for a relatively constant temperature in the area housing the equipment, thereby negating the need for an air conditioning unit.

The original configuration of the ambient monitoring station consisted of the meteorological mast erected on the cliff overhang above the caves. The inlet manifold was installed in the roof overhang of the "tea room". We immediately noticed that the ambient CO2 readings were being influenced by the caves especially when the wind direction was from the cave entrance to the manifold. This was evident in readings increasing from a baseline of 330 ppm to readings in excess of 600 ppm. With this in mind, it was decided to move the inlet manifold to the meteorological mast so as to prevent this influence from the cave.

Making use of the natural environment, all tubing, power and signal cables were carefully and skilfully hidden in cracks, vegetation, and where possible, underground.

Cave monitoring installation

Cango_381_small

The project called for the monitoring of CO2 in the Van Zyl hall, Botha hall, Drum room, Crystal Palace and Cango 2. The following parameters were required at each of the monitoring sites:

  • Van Zyl hall - CO2, ambient temperature and relative humidity;

  • Botha hall - CO2, ambient temperature and relative humidity;

  • Drum room - CO2, ambient temperature and relative humidity;

  • Crystal Palace - CO2, ambient temperature and relative humidity;

  • Cango 2 - CO2, ambient temperature, relative humidity and water temperature.

As previously mentioned, this installation required a lot of cables. The LD500 measurement system requires an OF cable (optical fibre) to be connected to the emitter, and is used to transmit light generated by an internal laser diode from the analyser to the emitter.

The emitter shines the light to a receiver where it is converted to an electronic signal. This signal is sent back to the LD500 analyser via a CF cable (communication fibre) where the signal is analysed, displaying the measured concentrations

Installed at each emitter is a beam splitting device which "splits" the laser beam along the next OF cable, to be carried to the next emitter.

Each receiver is connected to the LD500 analyser via a CF cable (communications fibre), which passes the above-mentioned signal from each receiver to the analyser via a multi-plexing device. This device allows the analyser to evaluate the signal from each receiver in turn.

Each of the five receiver stands was fitted with an ambient temperature and relative humidity sensor, connected via a signal cable to a PLC device also situated at each receiver stand. The PLC devices were installed in an IP65 rated box to seal out any moisture from the cave environment. Each box was also supplied with a desiccant bag to ensure that no moisture would be present inside these boxes.

The PLC devices were connected to each other, with each PLC programmed to receive data from preceding PLC. A PLC would then transmit its own data, along with the data from a down-line PLC, to the next PLC. The last PLC in the line, installed in the Van Zyl hall, would then communicate all the data to the Opsis DL256 datalogger in the "tea room".

Each emitter and receiver is mounted on a stainless steel stand mounted on a cement block. All the stands, nuts and bolts, emitters and receivers had to be manufactured from stainless steel to prevent corrosion in the humid cave environment.

Installation statistics

Now that we know how the installation was done, we can throw in a few statistics.

Distances

As with any air quality monitoring installation, whether it be ambient or stack, the distances between the analysers and the various measuring points is of high importance. This was especially the case in this installation where we were required to have both the optical (OF) and communications (CF) fibres made to exact lengths in Sweden by Opsis.

The tearoom building was the starting point for all the measurements as it was chosen to house the analysers and logging equipment. After an extensive (repeated for accuracy) measurement exercise, we had the following distances to work from:

  • Distance from the tearoom to the Van Zyl Hall = 175 m;

  • Distance from the tearoom to the Botha Hall = 300 m;

  • Distance from the tearoom to the Drum Room = 480 m;

  • Distance from the tearoom to the Crystal Palace = 820 m;

  • Distance from the tearoom to Cango 2 =  1120 m.

Each of the rooms required three cables, being an optical fibre (OF), communications fibre (CF) and signal cable. With the exception of the signal cable, all cables were pre-cut and prepared for the installation. It took 15 separate cables of varying lengths to connect each of the measurement points inside the caves to the equipment.

A total length of 1120 m of optical fibre (OF) cable was used in the following lengths:

  • Tea room to Van Zyl Hall beam splitter = 175 m;

  • Van Zyl Hall beam splitter to Botha Hall beam splitter = 130 m;

  • Botha Hall beam splitter to Drum Room beam splitter = 175 m;

  • Drum Room beam splitter to Crystal Palace beam splitter = 340 m;

  • Crystal palace beam splitter to Cango 2 emitter = 300 m;

Since each of the five receivers had to be connected to the LD500 with a separate, individual CF cable, the CF cable was by far the longest of the installation, 2895 m to be exact. The CF lengths were as follows:

  • Tea room to Van Zyl Hall receiver = 175 m;

  • Tea room to Botha Hall receiver = 300 m;

  • Tea room to Drum room receiver = 480 m;

  • Tea room to Crystal Palace receiver = 820 m;

  • Tea room to Cango 2 Receiver = 1120m.

A total of 1165 m signal cable was used inside the caves to collect temperature (ambient and water) and relative humidity data. An additional 50 m length of signal cable was used to connect the meteorological equipment from the ambient monitoring station to the data logger. That makes it a total length of 1215 m of installed signal cable, which was applied as follows:

  • Tea room to meteorological mast = 50 m;

  • Tea room to Van Zyl Hall = 175 m;

  • Van Zyl Hall to Botha Hall = 130 m;

  • Botha Hall to Drum Room = 190 m;

  • Drum Room to Crystal Palace = 350 m;

  • Crystal Palace to Cango 2 = 320 m.

For the readers who have not been keeping track of the cable lengths, all of the above leaves with a combined length of 5230 m of cable used in this installation.

Getting everything connected

In connecting all the emitters, receivers and ambient monitoring equipment, the first thing we had to always keep in mind was - BE CAREFUL! We had to be mindful of the formations as well as the cables themselves as we couldn't afford for any of the fibres to break. Had this happened, and entire length of fibre would have to be replaced.

Carefully measured and pre-cut lengths of cables were bundled together with the longest cable, the CF cable to Cango 2. Once the bundle was made up we could proceed to route the cables to the various measurement points as one thick cable, thinning out towards Cango 2 as the various strands reached their destinations. As was previously mentioned, we could only perform these activities after hours, sometimes until 2 am. The process of routing the cable bundle took four nights and a team of 14 people to feed the bundle from one measurement point to the next.

Planning was the key to the successful completion of this installation. This proved true when all the cables were pulled in to their pre-determined positions, requiring almost no adjustment in the positioning of the emitters and receivers. The care taken in installing the cables paid dividend when all measurement points went live when the system was switched on.

And finally...

We would like to, once again, express our heartfelt thanks to Mr. Hein Gerstner (manager of the caves) and his team of guides and volunteers for their assistance during the installation. Without them the project would have been a lot more difficult, and taken a lot more time to complete. Have a look at the photo gallery for more images of this installation.