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Heat Pumps Technology Guide SEAI 8.1.2. Flow sensors Figure 20: Flow sensor There are various types of flow sensor available, the most commonly used in heat meters are: • Ultrasonic flow sensors, which measure the transit time of the sound in the moving fluid between an upstream and downstream sensor. They are non-invasive and low maintenance. However, they need relatively long unimpeded upstream pipework for accurate measurement. • Fluid oscillatory flow sensors, which fall into two groups – vortex shedding flow sensors and fluidic oscillatory flow sensors. In the former, a buff body in the fluid stream causes the shedding of vortices on alternate sides of the body; the frequency of which is proportional to the fluid velocity. In fluidic oscillatory flow sensors, a jet enters a separate chamber and oscillates between two feedback channels, at a frequency proportionate to the main fluid flow velocity. The choice of flow sensor will depend on the heating medium, the accuracy required, and the cost and practicalities of fitting the flow sensor within the installation. 8.1.3. Temperature sensors Temperature sensors are supplied as matched pairs and are calibrated to minimise the margin of error in measuring the temperature difference between flow and return streams. Temperature sensors are normally of the platinum resistance temperature detector type PT100 or PT500.6 To maintain accuracy, it is important that the temperature sensors are positioned correctly, usually within thermal pockets and that their cables are not altered. 8.1.4. Integrators The heat meter integrator determines the instantaneous heat flow by using the flow rate of the heating fluid, the temperature difference between the flow and return, and the specific heat capacity of the heating fluid. Integrating this over time gives the cumulative heat consumption of the heating circuit (in kWh, MWh or GJ). As well as displaying the heat consumption, most integrators can provide other useful parameters, such as the instantaneous rate of heat use, flow and return temperatures, and the flow rate of the heating fluid. Furthermore, many integrators can store historical data (e.g. 18 months of month-end cumulative heat data), and also output/download their data to other monitoring systems, such as building management systems. Depending on the manufacturer, heat meters can be configured for a number of communication protocols (such as Meter-Bus (M-Bus) and RS 232) by adding modules to the integrator. 6 PT100 means that the RTD has a resistance of 100Ω at 0oC. A PT500 RTD has a resistance of 500Ω at 0oC. Higher resistance RTDs have lower errors for the same cable types, which is important where longer cable lengths are needed. 32PDF Image | Heat Pumps Technology Guide
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