Measured value deviation at room temperature
At room temperature, the most important specifications are zero point and end value deviation, as well as non-linearity and hysteresis (usually combined in one value as NLH). These deviations are summarized – not added – in the measured value deviation at room temperature (also called accuracy @25°C at Trafag).
If the measured value of the pressure transmitter in the control system is set to zero in an application before the actual operation, only the NLH is de facto relevant for the functionality of the machine. This is because zeroing eliminates the zero point error – and thus often a large part of the final value deviation – for the specific application. This is the case, for example, when a pump pressure is monitored and all measured values are set to 0 in the control system when starting up – provided that the pump pressure is always 0 bar when the system is started.
Only the NLH is decisive: for machine control by pressure changes
If the pressure change alone is required for the machine’s control system, then the NLH alone is decisive. In hydraulic control systems (or pumps), an action should be triggered at a certain deviation from the set point; for example, to switch the pump back on or to open a valve to keep the applied pressure within a closely monitored range. Thus, the effective pressure at the desired state – whether it is actually 10.0 bar or 9.98 bar, for example – is of secondary importance since most machines are individually adjusted to certain basic parameters during
commissioning. Therefore, only the precision – that is, basically the NLH including reproducibility – plays a role. Absolute correctness is of secondary importance.
In most other cases, where not only precision but also accuracy is required, both zero and full scale deviations as well as NLH are relevant. In this way, the indication of the measured value deviation at room temperature comes into effect.
Temperature influences measured value deviation: offset and hysteresis effects
Temperature-induced errors are similar: If the transmitter is set to zero after reaching a static operating temperature in the control system, or – which is much more frequent – if it is only a matter of controlling deviations from a set point under the same conditions, for example during commissioning, the temperature error of the transmitter is of secondary importance. The measured value deviation of the pressure transmitter due to the influence of temperature is also divided into offset and hysteresis effects. For non-static temperature conditions in an application, at least the hysteresis effects are always relevant. Whether or not the temperature effects of the pressure measurement have to be compensated must be taken into account when selecting the product. This is because the compensation of temperature influences in a pressure transmitter means additional production costs, which are reflected in the product price.