Standardisation work on heat meters has resulted in a European standard, EN 1434. Furthermore the OIML R 75 has been updated, based on the EN standard. The latest news is that the European Parliament accepted MID, a Measurement Instrument Directive which includes essential requirements for heat meters, early in 2004. The essential requirements do not specify the detailed test requirement, but it is expected that standards such as EN 1434 and the OIML recommendation will be published as possible ways to meet the specifications.
A number of European countries have had national regulations for the measurement of heat energy for district heating for several years. These regulations have often been based on national standards. In recent years, however, a common European standard for heat meters, EN 1434, has been developed, and an increasing number of countries are adopting its common demands on pattern approval tests as national regulations. The objective, through the use of tested and certified meters, is to give security to the customer as well as to the supplier and the producer of district heating.
EMC-testing with the meter mounted in a mobile flow rig
The testing standard EN 1434-4 has been developed on the basis of the collective European experiences of problems, functional and other, that may occur in a heat meter. This therefore makes the testing rather extensive. Heat meters must work, and also supply ‘correct’ measurement results, in the most varying environments and over the course of several years. At the same time the meter is a very important component from an economic point of view for both the producer and the distributor and consumer of heat.
Electrical disturbances applied on a flow sensor
SP – the Swedish National Testing and Research Institute – has performed pattern approval testing of heat meters since the early 1980s, and is accredited for testing according to EN 1434, as well as according to the anticipated MID directive. The MID directive specifies essential requirements for different types of measurement equipment for legal metrology, and will replace the current national regulations in a few years’ time.
TESTING FOR FUNCTIONAL SECURITY
The test programme for heat meters and their sub-assemblies according to the EN 1434 is divided into two sections. The first is testing with ‘Influence Factors’ which includes performance test, dry heat, cold and static deviations in the supply voltage. The second is testing with ‘Disturbances’, including durability, damp heat (cyclic) short time reduction in supply voltage, electrical transients, electromagnetic fields, electrostatic discharge, static magnetic field, electromagnetic field at mains frequency, internal pressure, pressure loss, electromagnetic emission and interruption of supply voltage.
Performance testing of a flow-sensor
FUNCTIONAL AND SERVICE LIFE TESTS
The pattern approval testing includes tests at different temperatures and flow rates. The SP laboratories are therefore equipped with test benches and different test rigs suitable for heat meters of different sizes and designs. Before and after accelerated service life tests, a performance test of the meter is done. In the accelerated life test, the flow sensor is subject to a specified number of on/off flows, or duty with high flow rate at high temperatures. The function and correct indication is also checked after a test with increased internal pressure.
Is there a risk that the meter is sensitive to mobile phones or other electromagnetic sources? Or can the meter itself cause disturbances? These are important questions, not least because of requirements in the EMC directive. Tests are performed in an EMC chamber with the meters under actual working conditions or, for larger meters, simulated conditions.
What happens to the meter during thunder and lightning? Wire-bound disturbances can occur in different ways, but if the meter can cope with short time reduction in supply voltage, electrical transients (burst and surge transients) electrostatic discharge and electromagnetic field at mains frequency, it should work well in most situations. The test programme includes several tests of the function of the meter during or after induced disturbances.
TEMPERATURE AND DAMP TESTING
The meter in its installation is subject to both heat and humidity and perhaps also to low temperatures. The environmental tests include all of these. In order to test the flow sensor under working conditions, it is mounted in special cabinets where the temperature and humidity can be set, with glands for the pipe so that the meter can be operating with flow. The calculator is tested in a similar manner.
VIBRATIONS AND MECHANICAL IMPACT
Electronics can be sensitive to vibrations and mechanical impact, and components can come loose because of careless handling. This testing is not included in EN 1434 but can be important when evaluating some installations. During the vibration test, components are exposed to vibration frequencies of e.g. 10 - 150 Hz. It is advisable to perform the mechanical impact test on temperature sensors and calculators, where the latter is exposed to
ADDITIONAL TESTING AND RESEARCH
What happens if the meter is mounted close to a pipe bend or a half-closed valve? How is the function influenced by the water quality (e.g. the conductivity and magnetite)? What is the dynamic behaviour of the meter when used together with small volume heat exchangers for heat and hot water in houses (residential districts)? These types of test are so far not included in the requirements of the EN standard, but can still be very important when a meter or an installation is to be selected. SP performs research and testing assignment, as well as training, within these and other metrological areas.
METERS FOR COOLING
It has recently been announced that the standard EN 1434, with its amendment A1 (2002) is also applicable for cooling meters using water as the cooling liquid. The test requirements are thus the same as for heat meters, but the test conditions regarding water temperature are of course much less.
Testing of a calculator