For decades the density gauge on a dredger was a caesium-137 transmission gauge. The choice was not enthusiasm for radioactivity — it was that nothing else read a DN800 abrasive slurry line reliably. That constraint has changed, and the cost side of the ledger has grown.
Why a source was the default
Dredge service is genuinely hostile. The line is large. The slurry carries sand, gravel, shell and the occasional cobble. Flow velocity swings from near-standstill to well above design during a cut. The vessel pitches and rolls, and the whole assembly is in salt air. A gamma gauge sidesteps most of this by never touching the process — source on one side, detector on the other, pipe wall in between. That immunity is real, and it is why the technology persisted.
What the source costs across a vessel's life
The purchase price is the smallest part:
- Licensing in the vessel's flag state and in every jurisdiction it works in. A dredger that moves between countries carries a compliance obligation that a pressure transmitter does not.
- Transport. Moving a sealed source across a border is a regulated shipment with its own paperwork, and it is a recurring theme in project delays.
- Periodic leak testing, source inventory, and a designated radiation protection officer.
- Decommissioning. Cs-137 has a 30-year half-life, so the source does not become harmless within the vessel's life. Disposal has to be arranged and paid for, and the cost is incurred at the least convenient moment — scrapping or sale.
- Crew and port acceptance. Some operators and some ports simply decline sources, which narrows where the vessel can work.
None of this appears on a datasheet, and all of it appears on a total-cost sheet.
What replaces it
| Principle | Fit for dredge service |
|---|---|
| Ultrasonic acoustic impedance | Covers DN50–DN1000; sensor face flush with the bore so nothing obstructs gravel or shell; tolerates entrained air and reports a gas index alongside density |
| Ultrasonic attenuation (clamp-on) | No tapping needed, attractive for retrofit, but bounded to DN500 and needs low bubble content — often ruled out by dredge air ingress |
| Differential pressure | Simple and cheap, but a moving vessel and a variable-velocity line make the hydrostatic assumption unreliable |
| Tuning fork | Immersed element in a gravel stream; erosion of the fork is the limiting factor |
| Exempt-source gamma (Na-22) | Retains the non-contact benefit with a much lighter regulatory footprint; a middle path where an external mount is mandatory |
What to check before committing
- Installation position. A vertical rising section beats a horizontal run, because at low cut velocities coarse material stratifies and any meter then reads a layer rather than the line.
- Air ingress behaviour. Ask how the instrument behaves during air draw, and whether it tells you it is happening. An instrument that reports a gas index lets you discount the affected period instead of arguing about it afterwards.
- Vibration and shock rating, and the enclosure rating for salt air on an open deck.
- Cable run from sensor to control room, which on a large dredger can be long enough to matter.
- Calibration on the actual spoil. A calibration made on clean sand does not transfer to clay-bearing spoil.