The Cargo-Safe transmitters are extremely powerful and will not be attenuated by thin or less dense enclosures ( like a rock-group in a tent ). The sound passes straight through the thin fabric or glass reinforced plastic yacht. What is needed is a much less powerful transmitter like our Cabin-Safe transmitter. This will work with the Cargo-Safe receiver or with the much cheaper Cabin-Safe receiver. The Cabin-Safe system was made for much smaller enclosures.
When the original Cargo-Safes were being developed around 1996 we where given excellent advice by the late Captain Neville Hall of British Marine. He explained and showed us in great detail how cargo hold surveys where conducted and it was his idea to the name OHV and to suggest instead of having an absolute value we modified the receiver to enable a percentage of OHV to be attributed to any leak.
Now and then it is necessary to suspend the transmitter from the centre of the hold. This can be done by using the 3 chord suspension harness to clip onto the Cargo-Safe transmitter and hang from a piece of string, cord or rope. The top of the rope can be secured by a bar of some kind straddling the open hatch. The hatch only needs to be open 25mm, 1 inch to also allow the microphone to be lowered in to take an open hatch value.
The leather cases are made from English cow hides and use a process called "splitting". This divides the hide in two which halves the thickness of the leather; the two halves are then coated with a vinyl plastic to make the resultant leather supple, very smooth and weatherproof. They also offer protection from dust, grease and debris
The Cargo-Safe transmitters is extremely powerful and will not be attenuated by thin or less dense enclosures ( like a rock-group in a tent ). You can't get away from the sound. What is needed is a much less powerful transmitter like our Cabin-Safe transmitter. This will work with the Cargo-Safe receiver or with the much cheaper but less sophisticated Cabin-Safe receiver. The Cabin-Safe system was made for GPR yachts etc where water ingress in cabins is a protracted and expensive problem to solve.
This trick of coating the joints in grease is counter-productive. It immediately alerts you to the fact that there is something to hide. It also can be frustrating if it messes up your microphone. A good idea is to put a small bit ( 2" or 50mm square ) of ladies thin, say 15 denier, tights or stocking over the end of the microphone with a piece of tape or an elastic band to hold it in place. The sound easily goes through, but oil, grease or water is caught in the fine mesh. This way the Cargo-Safe microphone is protected. Plastic or cling film does not work as it stops the ultrasound.
1. If the hatch is long and not so wide then the distance to the access hatch from the centre of the hold will be excessive. The best way to treat this is to move the transmitter to one quarter of the way along the hold and so effectively dividing the hold into two. The receiver should then be used to test that half of the hatch-cover and then the transmitter should be moved to a quarter of the way from the other end of the hold and the other half of the hatchcover tested*.
2. If the access hatch does not adequately open up into the hold and the ultrasound has a difficult route to the transmitter then excessive attenuation will mean that a reduced apparent OHV value. A solution is to take the OVH value from the entrance to the hold which will most likely be at the bottom of the access ladder. Alternatively if there is a possibility of opening the hatch covers you only need 25mm or an inch to lower the microphone down to below the hatch cover to get a very accurate OHV reading.
*The ultrasound attenuates roughly by the inverse square law so doubling the distance reduces the signal by four times which is a reduction in the signal by 12dB. So theoretically the if you had a hold twice as long as it is wide and you put the transmitter in the centre AT THE TOP OF THE HOLD i.e. if it was full of cargo, then the OHV taken at the end will be 12db down on the OHV taken at the other side but if the transmitter is placed at the bottom of the hold the ratio is less so instead of 2 to 1 it may be 1.5 to 1 and the attenuation effect will be less, say 3.5dB which isn't so bad
The effect of varying distances from the transmitter on the OHV setting is therefore less with the transmitter placed at the bottom of the hold than at the top. However it is better to move the transmitter after taking half the readings as explained in 1.
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