本研究の目的は，ウミガメなど海洋生物の遊泳中の対水速度を計測するためのピトー管型の流速センサを実現することである．そのために，提案するセンサ構造を試作し，遊泳速度に相当する圧力を印加した際の応答を計測しその特性を評価した．
一般的なピトー管を水中の計測に使用した場合，管口から気泡が入りセンサ応答に影響を及ぼすと考える．本研究ではピトー管の管口を薄膜で塞ぐことで気泡の流入を防ぎ，さらに内部を非圧縮性流体で満たすことで周囲の水圧の影響を除外する構造を提案した．また要求仕様として，流れに影響を与えないセンサの長さとしてアオウミガメの体長の5%以下の50 mm，流速の計測域を遊泳速度の2.5倍の5 m/sとした．
実験では，ピトー管の筐体部分は3Dプリンタを使用して製作した．筐体は筒状の管部分と球体部分の2つで構成されており，管の先端部分に動孔，球体の側面部分の1ヵ所に静孔を設計した．管部分は，内径6 mm，外径10 mmの筒状で長さは30 mm，球体部分は直径20 mmであり内部が空洞になるように設計した．筐体に対して，動孔と静孔に厚さ0.2 mmのシリコーンゴムを貼り付け，中に非圧縮性のシリコーンオイルを密閉した．さらにシリコーンゴム上にセンサ素子となるひずみゲージを貼り付けた．
試作したセンサに対して，流速に相当する圧力を動孔に加えた際のセンサの応答を計測した．実験方法として、ピトー管の先端にシリコーンチューブを通じて圧力校正器の圧力の出力ポートをつなげ，圧力を印加した際のひずみゲージの応答を計測した．圧力を0 kPaから9 kPaまでを1 kPa刻みで与えて実験を行い，ひずみゲージは圧力差に対して線形に応答していることが確認した．これらの結果から，本研究で提案するセンサは原理的に水中での流速を計測可能であることが分かった．
The purpose of this research is to realize a Pitot tube type flow sensor to measure the velocity of marine animals, such as a sea turtle during swimming. For this purpose, a prototype of the proposed sensor structure was developed. The response to pressure corresponding to the swimming speed was measured.
When a general pitot tube is used for underwater measurements, air bubbles will enter the tube and affect the sensor response. In this study, we proposed a Pitot tube of which inlets are sealed with a thin film to prevent the inflow of air bubbles and of which interior is filled with incompressible fluid to exclude the influence of ambient water pressure. We designed that the sensor is 50 mm in length (less than 5% of the body length of a green marine turtle) and the measurable range of the flow velocity is 5 m/s (2.5 times the swimming speed).
In the experiment, the housing part of the Pitot tube was fabricated by a 3D printer. The housing consists of two parts, a tubular part and a spherical part. The tubes were designed to be cylindrical with an inner diameter of 6 mm and an outer diameter of 10 mm, with a length of 30 mm and a sphere with a diameter of 20 mm. Silicone rubber with a thickness of 0.2 mm was attached to the inlet holes, and incompressible silicone oil was sealed inside. In addition, a strain gage as a sensor element was attached on the silicone rubber.
The response of the sensor was measured when pressure corresponding to the flow velocity was applied to the inlet. The pressure calibrator was connected to the tip of the pitot tube through a silicone tube, and the response of the strain gage was measured when pressure was applied. It was confirmed that the strain gage responded linearly to the pressure by applying pressure from 0 kPa to 9 kPa with 1 kPa interval. From these results, it is suggested that the proposed sensor is capable of measuring the flow velocity in water in principle.