A simple, inexpensive and reliable tank level measurement system can be implemented by connecting the the differential sensor MXP5100DP to the DrDAQ. By measuring liquid level by pneumatic means, no electrical connection to the tank is necessary and the level of any liquid, water, petrol, oil, diesel etc. can be safely measured.
This system is based on a well known principle of Physics. If a pipe is immersed in liquid, the pressure at the bottom end of the pipe will be equal to the weight of the liquid column above it. If we push air into the pipe, the pressure of the air in equilibrium (e.g. when air starts to come out from the bottom of the pipe) is proportional to the height of the liquid column. If we use a sensor to measure the air pressure, we can measure the height of the liquid column and hence the content of the tank.
As described below, the system is built with high precision, but very inexpensive standard, quick push/release, 6 mm diameter compressed air fittings easily available at any shop for garage equipment spares. 6 mm quality silicone flexible pipe is readily available and is very inexpensive. For the tank pipe, a section of 6 mm rigid compressed air pipe is used. This pipe can also be used for the other connections, if the plant does not require sharp bends.
The required low pressure air is provided by a small, panel mounted rubber or plastic pump. This item is commonly used on lawn mowers and can be obtained as a spare. To operate the system, this pump is pushed several times until the reading on the PC (on the Picolog panel) reaches a maximum: the PC now shows the exact liquid column pressure and the Picolog math channels can be used to provide a direct readout in litres or gallons or in any possible unit.
Referring to the diagram, air from the pump reaches the non-return valve, whose function is to allow pressure accumulation from one or more strokes. The "T" junction sends the air to the tank. The sensor is connected to the other side of the junction and monitors system air pressure. The sensor's output feeds to one of the (ext.) channels of the DrDAQ through the Buffer board and is displayed on Picolog. C40, R40 & D40 reduce noise pick-up and stabilize readings. Reading reduces after some time because of small leaks in the system, but this is not important, as air pressure must be restablished afresh every time a new reading is required.
In Part 2, next, the simple system mathematics will be described.