Time for another monthly newsletter. This time round we will take a look at some instrumentation devices and we will start with the Electro Magnetic Flow Meter. But just before we get into that, some great news! You can now get your hands on the Manual for Electricians Volume 1 where you will find some very useful information contained in a nice soft cover book that you can use as a quick reference when needed. Be sure to check it out at http://lulu.com/spotlight/SparkyHelp
Enough of that, let’s get into the first instrument.
The flow meter is a very handy piece of instrumentation and is used where it is required to either just measure the flow of the liquid, or use this measurement to control the operation of the system. The principle of operation is simply using the magnetic field which is generated by two poles placed into the device, and then measuring the EMF generated by two Electrodes that are placed perpendicular to the coils into the device. This is put in very simple terms and we will have a look at a little more detail below. What is important here is that you need to ensure you install the correct device for the liquid you are going to measure as the conductivity of the liquid is crucial to the operation of the flow meter
When we apply a voltage across the coils, we will create a magnetic field which is induced into the liquid flowing through the device. This is why the liquid must be electrically conductive. If we now go the basics of the Law of Induction, we will see that the magnetic field will induce a voltage into the liquid that is directly proportional to the velocity of flow, Induction and Tube Diameter.
We could get into the different equations that determine the flow, but for us it is important to understand how it works and not really how to design and build the unit? What happens in the device when we have flow is that this induced voltage is picked up by the electrodes and this is then sent to a signal convertor that will provide us with a signal we can work with. Generally, we will use an analogue signal, 4 – 20mA, which we will then scale on the PLC to give us the correct measurement.
There are some important things to remember when you install these devices and you must follow the manufacturer’s installation notes carefully. You will note that the device may only be positioned in a specific position; the external flange of the unit must be earthed etc. These things are important as they will impact on the operation of the device.
So, where would we use this device as a control method? Let’s say you have a piece of equipment that must maintain a flow of 120L/s to achieve optimum operation. You will now require the flow meter and also a modulating valve. Based on the flow measured, you will either open or close the modulating valve until you maintain the correct flow. Make sense? One very important thing to remember though! Never place your modulating valve on the suction side of a pump. You always go to the delivery line as you are able to open or close the valve without affecting the pump. Should you do this on the suction side, your motor will draw excessive current and you will also run the risk of running the pump dry which will cause severe damage to it.
Well there we go a very simplistic explanation of the flow meter. I hope that you find this useful. Until next month, stay safe and always consider safety first when you are out there working! Also, feel free to share this information with your friends. You might also want to direct them to the website.