More focus on energy efficiency with higher energy costs – how correct are your measurements?

As energy prices rise, incorrect measurements are a growing problem. No one wants ambiguities, either on invoices or in energy balances, and accurate instruments are necessary for energy efficiency.

– Correct measurement is an art, says Anders Andersson, measurement technician at Fagerberg.

Rising energy prices have led to greater focus on energy efficiency. It is necessary to ensure that we measure correctly, and flow measurement is among the most complex tasks. There are many measurement principles, dimension traps, and assembly problems that might cause trouble.

A heat meter includes flow and temperature measurements, along with calculations. We must summarize the errors in these three parts to find the total error in the measured heat energy. Meters certified according to the measuring instrument directive (MID) are available, but they are only mandatory for households in Sweden, and the maximum number of errors allowed is relatively high.

Errors in the flow measuring

– We often choose an ultrasound or magnetically inductive meter for the large pipe dimensions we usually work with, says Anders Andersson, measurement technician at Fagerberg.

The inductive meter is more forgiving regarding disruptions to the flow profile and better suited for cramped spaces where straight, non-disrupted piping is missing. An ultrasound meter is less sensitive to dirt and fits better in older systems with lower water quality (low conductivity and high magnetite levels). However, given the right conditions, both types perform roughly the same, with maximum errors of just a few tenths of a percent of the measured value (within the meter’s nominal measuring area; see Figure 1).

Other types of meters might have totally different error limits. This applies to meters that are properly installed, in other words, according to the manual and good practice. Each type of meter has its unique installation requirement. A bad installation might lead to enormous errors, says Anders.

Errors when measuring temperatures

– We deliver all our heat meters with Pt-100 sensors of the finest accuracy classes, says Anders. Errors when measuring temperatures occur in part from the chosen class and in part from what temperature you measure. Fagerberg’s sensors are tested before delivery against a requirement (class) of 1/6 DIN corresponding to ± 0,05 °C at 0 °C. Sensors are produced with a target of 1/10 DIN, meaning that most sensors are a few tenths of a grade better. For different classes and their maximum errors, calculated according to the IEC60751 standard, see Figure 2.

There are obviously requirements for installing temperature meters. To ensure that the right temperature is measured, the sensor must be installed in a representative place. Depth of insertion and isolation are other key factors.

Added to the sensor’s error is the error in the electrical circuit that measures the sensor’s resistance. At Fagerberg, we use energy calculators from the Swiss Integra (previously Aqua Metro). The maximum error of the temperature entries is reported as “better than the requirements of EN1434,” and the entry points have a resolution of 0.005 °C (20 bit). Since the same circuit can measure both inbound and outbound temperatures, the error of the differential temperature measurement might be considered neglectable. Typical errors in the measured absolute temperature correspond to the size of the sensors. Using four wires for the installation improves the result and is highly recommended.