The impact of water depth on power supply and communications.

bluefox0919

The impact of water depth on power supply and communications. [Copy link]

 

The salesperson went to discuss a case. They wanted to place the equipment 15 meters under the sea water and transmit the data to the receiver on the surface through RS485 communication. The equipment on the surface and underwater will be connected through a cable (the cable contains: power, ground, A, B).
What I want to ask is:
1. In addition to being resistant to seawater corrosion, what other characteristics does this cable require (other than twisted wire and shielding)?
2. The underwater equipment requires DC 12V. If DC12V is transmitted from the surface... will it still be 12V when it reaches the underwater equipment? (I understand that it should provide a voltage greater than 12V, but I have no idea how much greater it should be. I want to ask if there is a way to calculate it? Or do I need to provide additional conditions?)
3. In RS485 communication, does A and B only need 120R at the terminal? Or does it require additional design to ensure that there are no problems with data transmission when used underwater?

I have never thought of putting the equipment 15 meters under the sea water...so I have no idea about this. I would like to ask everyone to help me, is it possible to execute this case.

If you need any other information to help you make a decision, please let me know. Thank you.

maychang

2. Underwater equipment requires DC 12V. If DC12V is transmitted from the surface, will it still be 12V when it reaches the underwater equipment? (I understand that a voltage greater than 12V should be provided, but I have no idea how much greater it should be. Is there a way to calculate it? Or do I need to provide additional conditions?)

Some voltage will definitely be dropped on the wire, so the voltage across the load must be lower than the power supply voltage.

How much lower the voltage across the load is than the power supply voltage depends on the load current consumption and the wire resistance. Ohm's law can be used to calculate it.

maychang

1. In addition to seawater corrosion resistance, what other characteristics does this cable require (other than twisted wire and shielding)?

What is more important is the sealing of underwater equipment. Cables must always be passed inside the equipment.

The mechanical strength of the cable depends on your specific requirements.

maychang

3. For RS485 communication, does it only need 120R for A and B at the terminal? Or does it require additional design to ensure data transmission without problems when used underwater?

15 meters is not very far. Whether there will be problems depends on how high the transmission rate (baud rate) you require. The lower the baud rate, the smaller the chance of problems.

qwqwqw2088

15 meters below the sea

The receiver's enclosure protection level must be IP68

dcexpert

RS485 requires a matching resistor at each end, but the distance is only 15 meters, so one less resistor will not have much impact.

dcexpert

The power supply voltage depends on the internal resistance of the wire and the load power, and the voltage drop on the wire is calculated.

bluefox0919

qwqwqw2088 posted on 2020-2-3 18:27 The housing protection level of the receiver 15 meters under the sea must be IP68

Understood, will pay special attention to the waterproof shell to meet the requirements. Thanks

bluefox0919

dcexpert posted on 2020-2-3 20:00 RS485 requires a matching resistor at each end, but the distance is only 15 meters, so one less resistor will not have much impact.

Well, I know it's OK on the surface, I thought there would be other things to consider underwater, thanks

bluefox0919

maychang posted on 2020-2-3 18:17 2. Underwater equipment requires DC 12V. If DC12V is transmitted from the surface... will it still be 12V when it reaches the underwater equipment? (It should be provided with a voltage greater than 12V, but...

Well, I thought we need to consider other external factors underwater (such as water pressure), so the algorithm is actually the same. Now I understand. Thank you.

chunyang

The cables laid in shallow seawater mainly require high strength, including resistance to tensile stress, bending stress, repeated twisting stress, etc., as well as resistance to salt corrosion and ultraviolet radiation. At the same time, the influence of water pressure must be considered, and the sealing process, especially the sealing process of the joints, is very particular. As for the voltage problem, the terminal voltage is calculated using Ohm's law based on the power consumption of the underwater equipment and the DC resistance of the cable, and it can be satisfied. Due to the shielding of seawater, electromagnetic interference will not be serious, so the circuit design requirements are not high. There is no problem of point-to-point branching. The distance of 15 meters is very short for 485, so conventional circuits can meet the requirements.

led2015

Is seawater very corrosive? The various test data are higher than the requirements of the general environment.