Re: Looking for an understanding of an aqua meter
Here's what I've learnt from pulling apart a couple of AquaMeter speedos.<br /><br />As the boat speed increases the water pressure at the pitot tube increases. The air trapped in the hose connected to the pitot tube, which runs to the speedo, is compressed more as the speed increases and the water pressure increases. <br /><br />The increasing pressure expands a Borden Tube in the instrument. The Borden Tube is a curved slightly flexible metal tube closed at one end and connected to the hose from the pitot tube at the other end. The movement is very small, maybe only a few millimetres or so in the full range.<br /><br />A fixed gear is attached to the Borden Tube which, as the tube expands, rotates another gear mechanism connected to the speedo needle which turns to indicate your speed. <br /><br />The mechanism is very simple and could last a long time with no attention as long as it is protected from dust and corrosion, which its housing does if there are no holes in it.<br /><br />The whole tube system must be airtight or it won't work properly. You need a hard plastic hose that will not expand with air pressure, not a flexible rubber hose.<br /><br />You can test whether it works by fixing a short rubber tube to the speedo and blowing into it through your mouth. I don't seem to be able to get the same response blowing at the pitot tube end with the greater volume of air in the hose. If the needle moves it works. It won't move far, maybe only a few clicks on the dial. Whether it is accurate is different question. You can also use low pressure compressed air to move it through its full range but be careful how you apply it or you'll damage it.<br /><br />If you are removing the glass on an AquaMeter it may just pop out by pressing your thumbs on the glass with your fingers on the bezel, if it is removed from the dash. Otherwise you may be able to push it through the dash. Check carefully first from behind that there are no retaining tabs as some models may have them.
