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
暴露部 Exposure Section
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
通信ケーブル Communication cable
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)
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
The observation is performed by two groups: the "operator," who checks the vessel's position, operates the data recording software, and records the data in a field book, and the "field operator," who drops the probe into the sea from the stern or other exposed part of the vessel. This section describes the observation procedure using the XCTD system (Tsurumi Seiki TS MK-150N data converter, LM-3A hand launcher), which is widely used in Japan.
Before the vessel arrives at the observation point, the observation preparations are made. After turning on the data converter, the operator starts the dedicated software on the data recording PC and completes the software observation settings. The field operator moves with the probe to the work location in the exposure area and connects the hand launcher to the data converter with the communication cable.
Until the observation point is reached, the operator monitors the distance between the vessel and the observation point, and the field operator stays where he is. Each probe type has an upper speed limit for the vessel speed during the observation, so the operator instructs the vessel bridge to slow down below that speed. If the ship's speed exceeds the speed limit, the nichrome wire wound around the canister of the probe case will run out before the probe reaches the observation depth, resulting in the nichrome wire breaking and forcing the observation to end ( Fig. 3 ).
One to two minutes before passing the observation point, the operator instructs the field operator to connect the probe to the launcher. The field operator follows the instructions and sets the probe (Fig. 5). Once the probe is set, the battery inside the probe will begin to be consumed. If the probe is set too early, it may run out of battery power during the observation, making it impossible to take measurements.
Fig. 5 Setting the probe on the hand launcher
コンタクトピン Contact Pin
保護キャップ Protective caps
篏合レバー Fitting Lever
リリースピン Release pin
a) Name of each part of launcher and probe b) Insert the probe into the launcher with the lever pushed up. c) Pull down the lever to engage the probe's electrode with the launcher's contact pin.
The operator confirms on the software that the probe is connected and waits for the right moment to pass the observation point. As soon as they arrive at the observation point, the operator instructs the field operator to drop the probe, and the field operator immediately drops the probe toward the sea surface ( Fig. 6 ).
Fig. 6 Dropping the probe
a) Remove the cap from the case. b) Pull out the release pin. c)Tilt the launcher downwards and the probe will slide out from the case.
When the probe lands on the water, data recording starts automatically, and as the probe descends, the vertical distribution of water temperature and conductivity is drawn on the software. The field operator holds the launcher in place, taking care to orient the signal transmission wire so that it does not touch the hull of the vessel and break. The operator monitors the data and waits for the observation depth to be reached. Once the observation depth is reached or the probe has reached the seafloor, the operator instructs the field crew to cut the wire and remove the probe. The field crew follows the instructions, cuts the wire, and puts away the launcher and communication cable. The operator records the time, ship's position, depth, and other information at the beginning and end of the observation in a logbook.
The flow of the XCTD observation workflow, divided between operators and field workers, is organized as shown in Fig. 7.
Fig. 7 Flow of XCTD observation
到着前 Before Arrival
到着直前 Just before arrival
観測点 Observation Point
観測終了 End of observation
船上局(データコンバータ・データ収録PC)の準備 Preparation of onboard station (data converter and data recording PC)
観測点までの距離を監視 Monitor distance to observation point
プローブセットを指示 Instructs to set probe
プローブ投下を指示 Instructs to drop probe
計測データを監視 Monitors measurement data
観測終了の確認 Confirmation of observation completion
データ処理 Data processing
ランチャとプローブの準備 Prepare launcher and probe
現場にて待機 Standby on site
プローブセット Set probe
プローブ投下 Drop probe
ランチャの保持 Launcher holding
ワイヤ切断 Cutting wire
ランチャの片付け Clean up launcher