Industry Terms Briefly Explained: Part 5
Some engineering terms have been found to be misunderstood or confusing in the industry. In part five of our goal to properly define key phrases in the engineering field, we focus on dispelling misinterpretations of various terms.
Dewpoint Temperature is the temperature at which water vapor condensates. It is the temperature to which air must be cooled at constant pressure and water content to reach saturation. One cannot measure dewpoint temperature with a thermometer; it is an expression of the relative humidity at the actual temperature.
E.g., if a dewpoint meter measures the relative humidity to be 50 %RH at a certain temperature and pressure, it can calculate the corresponding dewpoint temperature. There are dewpoint meters, like chilled mirrors, working on a different principle, but the majority are humidity / temperature meters with a dewpoint calculator on board.
Relative Humidity is the ratio between the actual partial vapor pressure and the saturation vapor pressure above water that is expressed in percentage. For example, if that ratio is 0.5, the relative humidity is 50 %RH. This is a bit of a strange engineering unit because typically, the accuracy of a humidity transmitter is given as e.g. ±2 %RH, which does not mean that the accuracy is ±2% of the measured value. The ±2 %RH is an absolute error and not a relative error. Using the same value as before, it means that the true value of the relative humidity is somewhere between 48 and 52 %RH.
Accuracy is and always has been, a matter of confusion – what does it all contain? The accuracy of an instrument is the deviation from the absolute truth; this figure should include linearity, repeatability, and calibration uncertainty. In spec sheets, relative accuracy should be stated in relation to full scale or to measured value. Just a number of a percentage is not good enough. Furthermore, there is quite a difference between a full scale and a percentage of measured value or reading error.
Most mechanical flowmeters – like turbines, paddle wheels, and Pelton wheels – produce a pulse output and it is easy to understand why. A stationary detector picks up the magnet field of permanent magnets embedded in the rotor. As the rotor spins and the magnet approaches the detector, the signal gets stronger, until it reach its maximum and then declines, having created a pulse. The number of pulses per a certain amount of volume is called the K-factor, which basically determines the performance of the flowmeter.
An external converter reads the pulses and provides a readout, as well as an analogue or digital output. When the converter is attached to the flowmeter as one package, it doesn’t change the basic principle of the measurement. It is still a mechanical flowmeter that creates pulses as an output while the external converter happens to be very nearby. Therefore, considering the flowmeter/converter package a different and better flowmeter than the separated version is based on a misunderstanding of the properties of the flowmeter.
We hope our descriptions help provide a better understanding of frequently misused engineering terms. Stay tuned for our next blog, which will contain more information relating to the devices we distribute, such as the high temperature humidity sensor and the Shaw dew point meter.