Last week I have received a message with a question. How to override current loop transmitters? Of course, this is not a standard situation to solve, however when the vessel is in the middle of the ocean has no access to spare parts. This knowledge is essential to everyone who is working on engines equipped with electronics. Where You can find the current loop? Basically in all industrial installations. They are used with PID controllers, SCADA systems, and PLCs.
Case study – Current loop transmitters
As I already mentioned, I received the message. A question from one of my readers! To be accurate a Chief Engineer, who has an issue with electronics on the engine. Everyone who sails, know that sometimes we have problem with spare parts. He found a faulty pressure transmitter, which is critically important to operate the MAN ME engine -> Starting Air pressure. Without starting air pressure engine is not allowed to start. More of that, if Your starting pressure is ok, but your transmitter isn’t working, You will be not allowed to start an engine. This was an exact problem in this case.
In that case, when spare parts are not available, You have 2 possibilities. 1st this what an experienced engineer has done to survive – You look for the same transmitter at any other non-essential equipment. This was a great step, I have also done similar things in the past. Of course, this is not the most comfortable situation, but at least machinery is still operational. But in this situation, we still need to found any other non-essential pressure transmitter. What if there is no other transmitter and no spare part available?
Let’s figure it out!
2nd a little bit more complicated method is to simulate this signal. The idea which I am presenting is an emergency operation. You shouldn’t use it without the reason! If spare parts are available, You always should replace them. Ok, I think we understand each other now so time to go back to our solution.
So imagine that You are in the situation of a Chief engineer, You are onboard. Starting Air Pressure sensor does not work. You check the manometer, pressure is in range. The short deduction, Mainboard, transmitter, cables, or amplifier. You take a multimeter and You are going to check the voltage at the transmitter. If it’s correct, You already know that problem is with the sensor itself. Clearstory, You are going to the stock control software, You check the spare part availability…
But the spare parts are not available. You check stores, also nothing. You start to check orders, and You clearly see that 2 months ago 3 spare sensors were ordered, but You are still waiting for delivery. This sounds like sci-fi for people who never work onboard. Believe me, 2-3 months is still a very reasonable time for spare part delivery! Ok, You know Your situation. Now You have to find a solution. That’s simple, You can make a bridge and the problem will be solved. But the current loop is not a simple switch. To explain how to solve that situation You have to understand how transmitters are working in the current loop.
What is a current loop and how it works?
If You will be sure how it works, You will be able to found the correct solution from this. The current loop is also called 4 to 20mA standard. The current loop is a very popular sensor signaling standard. Current loops are perfectly matching for data transmission. All the components inside the loop, drop voltage due to the signaling current flowing through them.
As You can see in the drawing above, current loop has 4 main components.
- Power supply – Power supplies for 2-wire transmitters must always be DC. Since changing current flow represents the parameter being measured, AC cannot be used. Common power supply voltages are 24 VDC and 12 VDC.
- Transmitter – The transmitter is the heart of the current loop. It changes physical measures such as temperature, pressure, or humidity into an electrical signal.
- Receiver resistor – Receiver is a precision resistor that has well-defined resistance characteristics. The most common resistance used is 250Ω. Depending on different applications, resistances of 100Ω to 750Ω may be used.
- Wire – Sending current through a wire produces voltage drops proportional to the wire length. The voltage drop can be calculated using Ohm’s law.
Where You can find current loop? Basically in all industrial installations. They are used with PID controllers, SCADA systems and PLC’s.
Ok, I understand the basics of current loop transmitters, but I still don’t know how to simulate this signal…
Now we start with the main point which is transmitter simulation! If You replace the transmitter with a current loop generator, You can simulate the right signal in the system. Signal generators to order You can find here. By the current loop generator, we can simulate the correct value. For example, we can adjust the current in the full range between 4-20mA. So in case when we talk about starting air, we can adjust the value of current, which is applicable to start the engine. With that operation, we have to remember to monitor parameter value by analog meters at the local position!
In the diagram, You can see how properly connect the signal generator to the current loop 4-20mA for simulation of the signal. When You are connected, You just have to add the correct value of current and voltage at the signal generator. In that way, You will simulate the correct reading from a transmitter. The reading value at the Receiver resistor will be correct, and You can start your engine.
The solution is simple and I recommend this equipment for all vessels. Doesn’t matter if You are a mechanical or electrical engineer, if You don’t have this equipment on board, You should order it. This equipment could be very helpful, and in some cases can save You really a lot of problems. At some vessels, there is available also a laboratory power supply. With them, You can also generate a correct signal, but they are not so portable.
Signal generator can be also replaced with correct resistor, however this method requires calculation or set of different size resistors. It is possible to arrange, however it is not the easiest way to proceed, however it is good to know additional option.
Conclusion about current loop transmitters.
Current loops are very popular, You can meet them in almost every installation onboard. Everyone who sail knows that sometimes is problem to get a spare parts. This is the main reason why, I highly recommend being equipped with a simple signal generator which I have presented to You above. That is good If You have correct equipment already onboard. I would highly reccomend to try to do this operation one time for training. You never know when You need it in real life.
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