A heat meter is composed of three parts, a flow sensor, a temperature sensor pair and a calculator.
The calculator is a unit which calculates volumes and energy consumption using the values from the
temperature sensors and the flow sensor.
The most common type of temperature sensor is a resistance thermometer of platinum type Pt 100, Pt
500 or Pt 1000. The sensors measure the temperature difference between the incoming and outgoing
liquid .
The flow sensor is probably the most troublesome assembly of the heat meter. Despite an accuracy
requirement of only 4-10% it is very easy to fall outside these limits. In order to counter these effects as
far as possible, there follows a summary of the various types of flow sensor and their advantages and
disadvantages.
The sizing of meters to match their required duty frequently turns out to have been incorrectly estimated
when the heating plant commences operation. In most cases heat meters that are too large for their eventual duty are specified and accuracy at low load suffers as a result. Whilst this paper will give some
guidance on essentials, it is felt that more information on this topic would be welcomed.
Heat meter accuracies at times of rapidly changing heat demand are unlikely to be high. Whilst at times
of low demand the effect of meter inaccuracy in terms of lost revenue is likely to be small, rapid changes
involving high demands on the network may possibly have important implications in lost revenue if meter
reaction to rapid changes is slow. Research into the subject seems to have been largely neglected so
far.
The most commonly used types of flow sensors have been listed in Annex B and the effect on accuracy
of different types of disturbances for each of the listed types of flow sensors are considered.
There is little information on the effects of flow and flow disturbances on the service life of the flow
sensor, as distinct from its effect on the sensor’s accuracy. To be welcomed, therefore, is the long term
research project on this topic initiated in Germany which should result in useful data.