summary

 The XCTD (eXpendable CTD) is a drop-type salinity-water temperature-depth meter that measures the vertical distribution of water temperature and salinity by dropping a probe with a sensor from the vessel while free-falling in seawater (Figure 1). The probe is connected to the onboard station via a thin signal transmission wire (nichrome wire), allowing real-time monitoring of measurements on board (Figure 2). When the probe reaches the target depth, the signal transmission wire is disconnected and the probe is dumped into the ocean without being recovered, hence the name "disposable CTD. The nichrome wire is coiled both in the probe that drops into the sea and in the case that remains on the ship's side, and the nichrome wire is ejected in two directions from the drop point: the direction in which the ship moves and the direction in which the probe drops (Figure 3). This enables observations down to depths of 1000-2000 m without breaking, even with very thin wires.

Fig. 1. Schematic diagram of XCTD observation

XCTDプローブ　XCTD probe

Fig. 2. XCTD system configuration

実験室　Laboratory

暴露部　Exposure Section　

モニタ　Monitor

オペレータ　Operator　

データ収集PC　Data Collection PC

データコンバータ　Data Converter

現場作業者　On-site operator

ランチャ　Launcher

通信ケーブル　Communication cable

プローブ　Probe

Fig. 3 Unrolling the signal transmission wire

ハンドランチャ　Hand Launcher

プローブケース(キャニスタ)　Probe case (canister)

信号伝送ワイヤ(ニクロム線)　Signal transmission wire (nichrome wire)

スプール　Spool

プローブ　Probe

 The case and probe are equipped with spools from which the nichrome wire is drawn during observation. The nichrome wire is wound around the spool so that it can be pulled out smoothly even with very weak force. Therefore, no tension is applied to the wire to cause it to break, and the free-fall of the probe is not disturbed.

 Stopping a vessel at an observation point is time-consuming. In contrast, XCTD can be used to obtain high-density spatio-temporal temperature and salinity profiles or to save observation time during a voyage, since it can be used while the ship is running and the observation time is short. It can also be used in stormy weather when large equipment such as CTD water sampling systems cannot be used. However, the accuracy of acquired data is lower than that of wired or self-recording CTD instruments. In particular, XCTD does not have a built-in pressure sensor, and the nature of the depth value differs from other CTD instruments, which determine depth by directly measuring pressure. The depth of XCTD (d) is obtained from the elapsed time (t) from the probe's landing in the water to its fall in the sea using empirical equation (1), which assumes free fall.

d(t) = at + bt²・・・・・・・・（１）

 

 The constants a and b in empirical equation (1) are given for each probe type and are determined empirically based on comparative observations with more accurate CTD instruments. These coefficients are recorded in the RAM inside the probe and are automatically loaded into the data recording PC when the probe is attached to the launcher.

 While XCTD can measure temperature and salinity at each depth, XBT (eXpendable Bathy Thermograph), which only measures water temperature at each depth, is also widely used. The order of development as a product is XBT first.

Fig. 4 Observer holding XCTD launcher