High Temperature Humidity Measurement
The measurement of humidity at relatively higher temperatures (more than 100°C) is important in a large number of industrial applications ranging from drying process chambers to industrial-scale large baking ovens. In industrial bakery ovens for instance, humidity measurement is an important parameter to monitor as it can impact the quality and consistency of the baked goods. Subsequently, humidity measurement in bakery ovens can be used to save energy by optimizing the oven's operation based on the moisture content of the air.
The high process temperature is critical for the selection of the humidity sensor. Not too many sensors can measure sufficiently correct at high temperatures. The next generation of high temperature humidity sensor “Standalone (High Temperature) Humidity Transmitter Module SHTM” developed by Angst and Pfister Sensors and Power has a very high-accuracy (±2 %Vol H2O) and a reliable, long lifetime with low drift.
SHTM is primarily developed for different types of ovens & furnaces and to be able to work under the high temperatures. The measurement is done in-situ in the oven/furnace, which is an advantage over other systems, where a gas sample needs to be extracted and analyzed, thereby causing disturbances in the oven/furnace. In addition, the working temperature of the module can be adjusted between 100°C and 325°C, and the sensor can be calibrated on-site with the help of an easy-to-use menu.
The measurement principle of the sensing element in SHTM is based on the oxygen displacement compared to dry air with a content of 20.95 %V oxygen (ambient air oxygen content). With the increase of humidity in the atmosphere, the oxygen concentration will reduce to 0% in case of complete humidity saturation and for high temperatures of more than 100°C. The transmitter generates an output signal of 4 to 20 mA corresponding to 0 to 100 %Vol H2O for the temperatures above 100°C. The sensor head for SHTM can be configured between 300 and 2500 mm in the steps of 100 mm. The sensor head and the control box are connected with a high temperature sustainable cable having a length of 2500mm, where other cable lengths are possible on request. The sensor and the construction materials used for the sensor probe (metal parts, wires, and connectors) are temperature resistant above 300°C and also resistant to high pressures of up to 20 bar. The sensor element is also equipped with a dust and particle protection filter in order to assure robustness and stability against aggressive medium.
The maximum operating temperature of the sensor head (i.e. the front part of the sensor tube) is 325°C. The calibration temperature setting includes a tolerance of ±50°C, in which the sensor head performance is in accordance with the specification values. The sensor head output can be calibrated /adjusted in an easy-to-use menu. The output curve is calibrated/adjusted in either the full-scale or the zero point, or both (based on application requirements and the wishes of the end user). The SHTM sensor head can be mounted on a flange (can be ordered as an accessory). The mounting flange offered by Angst and Pfister Sensors and Power provides mechanical support and stability to the sensor head along with a 2° angle tilt which helps in protecting the sensor, so that the condensing water does not flow back towards the sensor head.
