Duration: 09.2011 – 02.2012
Company: Hydrometer GmbH (Diehl Metering)
Title: Development of a water-meter-test-bench for very small flow
In times of rising costs of energy and dwindling resources, the economical use of natural resources gets more and more important. A considerable resource is water.
For this reason, the requirements of water meter in regard to accuracy, reliability and measuring of very small flow rates are rising. Thus the requirement on test benches for qualification and calibration of water meters are rising the same way.
This abstract describes the work, which was done in the course of an internship at by Hydrometer GmbH. The task of this internship was to design a minimal flow rate test bench. In this development work, it was the aim to work out different concepts for the examination of water meters for very smal flow. These concepts had to be tested based on a prototype test bench.
For this purpose a list of requirements has been defined. Due to this list different technologies (including the established technology of Hydrometer) have been investigated. During this investigation it was the aim to find technologies which have advantages regarding the special physical effects while the testing of minimal flow rates.
The acquired technologies have to fulfill the functions of a water meter test bench. With the useful technologies, different concepts for this test bench were proposed. The concepts had been compared overlooking qualification, effort of realization and economical aspects.
One successful concept which was tested in a real test bench during the internship is illustrated in the next graphic.
One main problem was to control and to measure a comparativley wide flow range from a very small minimum flow rate to the required maximum flow
The idea was to keep the test bench as simple as possible. For this reason the measurement standard (a balance) and the volume flow meter for the control of a constant volume flow have been combined. The mass flow through the water meter is calculated as the time derivative of the mass in the balance. This mass flow can be converted into the volume flow using the density of water. With the calculated volume flow a volume flow controller has been designed.
At first the theoretical basics for the topics “temperature-dependent density of water”, “temperature of water in the test bench” and “measuring of evaporation in the balance-system” which are important for the measuring accuracy of the test bench had been worked out.
For the implementation of appropriate algorithms to compensate the error caused by temperature-dependent variation of the density of water, a mathematical model of the test bench has been designed. With this model it is possible to estimate the error due to the temperature changes and to create a compensation algorithm for this physical effect. The following graphic shows a quantitative thermal model of the test bench.
Besides the temperature depending density of water, the change of humidity in the scale had to be taken into account to measure the correct volume of water which has flowed through the water meters.
VL2 ... Volume of Air after the test
VL1 ... Volume of Air before the test
f2 ... Absolute humidity after the test
f1 ... Absolute humidity before the test
L2 ... Water in air after the test
L1 ... Water in air before the test
W2 ... Scale value after the test
W1 ... Scale value before the test
Wp ... Mass flown through the mater meters
Wa ... Water in air which had blown out of the system
With the mass balance equation, the mass and in combination with the density of water, the volume of water which has flowed through the water meter can be calculated.
[Mathies, N.: “Messunsicherheit einer gravimetrischen Kalt- und Warmwasser-Normalmessanlage für große Volumenströme“, Dissertation, 2005]
In the practical part of this work, a selection of the rated technologies was implemented into an existing test bench. This test bench has been extended with a computer based connection which is controlled by LabVIEW.
The LabVIEW program calculates the flow rate through a water meter. The entire developed compensation algorithms are integrated in this program.
In the last step experiments had to be done to validate the different technologies.