Flow sensor

The sensor flow is used for accurate measurement of flow rates. To explain this principle, we take a closer look at the George Fischer Miniflow sensor type FG 02. This type of flow sensor is used for small flow rates in plastic piping systems or flexible tubing.

Materials

Housing	PVDF
Rotor	PVDF
Shaft	PCTFE
Orifices	Teflon PTFE
Seal	Viton®
Power supply	12 V DC (+12 V / -4 V), max. 15 mA
Output signal	Square-wave signal 5 V (TTL)
Precision	<= 1.5 % (see diagram measuring range)
Reproducability	Better than 0.25%
Max. operating pressure	See pressure-temperature diagram
Measuring range	0.1-12 litres/min, depending on orifice
Ambient temperature range	0 to + 40°C
Temp, range of flow medium	See pressure-temperature
Viscosity range (kinematic)	0.2 to 20 cSt
Dimensions	See outline drawing
Weight	115 g
Pipe connections	¼" BSP male thread
Electrical	3-pole cable plug, IP 65,
connections	splash-proof
Max. cable	Shielded: 100m
Length	unshielded: 30m

The operational principle
The free-running rotor of the miniflow sensor is driven by the fluid flow. The rotational speed of the rotor is proportional to the flow rate. The magnets, built within the rotor, actuate an electronic switch in the sensor, creating a squarewave output voltage signal proportional to the flow rate.

The sensor flow is used often in the chemical and pharmaceutical industries. The sensor is also often used for analytical purposes. The sensor is suitable for liquids like water, or other neutral or aggressive liquids that do not contain solid parts and that don't cristallize within the sensor.

The ideal position of the FG 02 miniflow sensor is horizontal. Allthough the sensor can also operate in any other position the lower part of the measuring range can have a greater linearity error. When installing, the orifice is always on the upstream side. The sensor should be installed far away from pumps. It should never be installed directly at downstream of valves, fittings etc. since this most likely will generate false results. During measurements the entire cross-section of the pipeline should be completely filled with liquid. Note that the sensor flow can only be used for liquid flows and not for gas flows.

Flow controllers

The function of a flow controller is to control the liquid flow. Practically this means it measures and adjusts the flow in a tube. It is a device connected with for example two transmitters that are placed in different tubes before the tubes come together. The transmitter has a rotating device that starts rotating by the liquid flow within the tube and generates an electric signal. This signal is send to the flow controller and the flow controller can tell by the signal what the flow speed is within the tube.
The flow controller has a standard flow capacity programmed for the two tubes and with the signals coming from the transmitter, the flow controller can calculate the flow speed in a tube. When the speed is differs from the programmed speed, the flow controller can adjust this by closing the tube a bit using a valve, narrowing the volume that can flow through the tube. Or the flow controller can open the tube more, expanding the opening for a faster liquid flow. The values the flow controller has to maintain can be configured by the user, for optimum results.

Batch controllers

The batch controller is used when processing a certain amount of liquid in a certain period. The low flow sensor gives the batch controller a signal about the amount of liquid flow, being processed. The batch controller then calculates the amount being processed and closes the valve when the proper amount of liquid has been reached. After the batch has been completed, the batch controller starts over again.
The relay output signals the batch status. The batch controller receives a signal from the flow sensor about the flow speed in the tube. When the configured numbers have been reached, the batch controller closes a valve to stop the liquid flow.

Overload protections

The pump can get seriously damaged when it is operating to hard, or not hard enough. That's why it is important to monitor the pump and it's functioning. Temperature sensors are a common way of measuring the functioning of a pump. To monitor the pump, the temperature sensor is used to measure the temperature of the pump. When measuring the temperature, the load of the pump can be calculated and with this information the monitor can decide whether to stop the pump or not.

The thermo-coupling is the part of a temperature sensor that measures the temperature. When two different materials are being connected an electrical impuls will occur when the temperature between both elements are different. The principle of the temperature sensor is equal to this principle. The coupling is kept at a known temperature and when it contacts another material this temperature is used as a reference, thus making it possible to calculate the temperature of the other material.

Pressure sensors

The pressure sensors measure the pressure of the flow within a tube. The pressure of the liquid within a pump can be measured in different ways.

Ceramic
The pressure-difference creates a change in the distance between two ceramic plates, which have a golden layer on it. The condensator is dubbel executed and are connected with eachother. The measured capacity between the two plates is proportional with the pressure-difference.

Metal
The pressure difference causes a change in the function optimized measure element. Because of the almost motionless measuring principle from the measure element as the metal plates, the pressure can be measured very precisely.

Dry running protection

The dry running protection device is a device that monitors the load of the pump. The dry running protectino device does this by measuring the power the pump uses to function properly. The monitor is continuesly measuring the power used by the pump. There are no sensors needed, since the monitor is connected directly with the motorcable. The monitor measures the phaseshift between the voltage and the ampere. When these measurements changes, it means that the load of the pump is changing.
The monitor is programmable and can be configured by the user. The pump can give an error at the following events. When the load of the pump is to little or when the load is to high. So most of the times, the pump is programmed to react at either situation.