Technology
Ultrasonic metering for water and heat
What happens inside the measuring tube of an H2O and a DELTA III, why no parts wear out and how flow rate and temperatures turn into energy.
Flow measurement
How does ultrasound measure flow?
Two ultrasonic transducers sit in the measuring tube. They take turns sending short sound pulses, first with the flow and then against it. Reflectors guide the sound along a defined path through the water. Helvatron uses self-cleaning reflectors so that the sound path stays clear.
A pulse travelling with the flow arrives slightly earlier than one travelling against it. The difference between the two transit times is proportional to the mean flow velocity. Multiplied by the cross-section of the measuring tube, this gives the volume flow rate; totalised over time, it gives the volume.
The transit time differences are in the nanosecond range, so a stable measuring section and precise timing in the electronics are essential.
Several measuring channels for large diameters
With large diameters, the flow profile is less uniform. The DELTA III therefore uses two ultrasonic channels from DN50 and four from DN350.
Thermal energy
How do flow rate and temperatures become thermal energy?
A heat meter consists of a flow sensor, a pair of temperature sensors and a calculator. The sensors measure the temperature in the flow and return pipes. The calculator determines the difference Δθ and multiplies it by the volume and a heat coefficient k.
The heat coefficient takes into account that the density and specific heat capacity of water vary with temperature. The values are defined in the EN 1434 standard. This is also why the calculator needs to know whether the flow sensor is installed in the flow or in the return pipe.
Heating and cooling
If the flow is warmer than the return, the system gives off heat. In a chilled-water plant, it is the other way round. The DELTA III recognises the sign of the temperature difference and counts heating and cooling in separate registers. This allows correct billing even for plants that cool in summer and heat in winter.
Why the sensors matter so much
With small temperature differences, as is typical for chilled water, a measurement error of a few hundredths of a kelvin has a large effect on the calculated energy. Heat meters to EN 1434 therefore use paired sensors with matched characteristics. The DELTA III works with Pt500, Pt1000 or Pt100; we recommend Pt1000.
Q = ∫ V̇ · k(θF, θR) · (θF − θR) dt
- Q
- Heating or cooling energy, e.g. in kWh, MWh, GJ
- V̇
- Volume flow rate from the ultrasonic measurement
- k
- Heat coefficient to EN 1434, depending on the temperatures
- θF, θR
- Flow and return temperature, Δθ their difference
A worked example is shown in the DELTA III animation.
Benefits
What are the advantages of ultrasound over mechanical meters?
No moving parts
Nothing rotates in the flow sensor. There is no impeller to wear out or stall at low flow rates.
Wide dynamic range
The H2O achieves Q3/Q1 up to R800 in flanged versions, the DELTA III qp/qi up to R250, so even very small flows are recorded.
No straight inlet or outlet sections
For the H2O and DELTA III from DN15 to DN40.
Diagnostics in the device
Air in the pipe, leaks, incorrect flow direction, sensor faults: the meter reports them via its interface.
Self-cleaning reflectors
The H2O and the DELTA III both use self-cleaning reflectors. In the DELTA III they are metallic.
Digital values
An internal data logger, error codes and a communication interface make the meter a data point in the building services.
Interfaces
Physical interface, protocol, radio technology
Three terms that are often confused. When planning, it pays to keep them distinct.
- M-Bus
- Bus system to EN 13757 with its own physical layer (two-wire line, powered by the master) and its own protocol.
- RS-485
- Only the physical interface: differential transmission on a twisted pair. The protocol defines which data is transmitted.
- Modbus RTU
- The protocol Helvatron meters use over RS-485: the master requests, the slave responds.
- LoRa
- Radio in the 868 MHz band, with devices, gateways, a network server and the application.
- NB-IoT
- Mobile network standard in licensed bands. The meters transmit via an operator’s network, without gateways of your own.