Producing modern lithium-ion battery cells requires extremely precise process conditions. In particular, humidity must be reduced to an absolute minimum during critical production steps because even the slightest moisture ingress can compromise the electrochemical stability of the cells. Dew points below -60°C are now considered standard in dry zone production. The precise and reliable measurement of these extreme conditions is technically demanding yet essential for quality assurance.
Mini-environments as an energy-efficient alternative
Instead of extensively drying entire cleanrooms, increasingly more manufacturers are turning to mini-environments. These are compact, enclosed clean and dry air zones in which low-particle, dry air is delivered directly to the point of use. Air is typically supplied actively from above while operating personnel remain outside the critical zone. This minimises the introduction of moisture and particles without compromising ease of operation.
Cleanroom specialist Weiss Klimatechnik designs production-adjacent, locally defined process zones that ensure a consistently dry and stable atmosphere. In addition, users benefit from improved efficiency because of the considerably lower energy consumption compared with conventional methods.
Sensors under extreme conditions
Dew point sensors are the central component of humidity monitoring in mini-environments. Their task: to measure humidity precisely and continuously at low ppm levels. However, in practice, many sensors approach their limits under these conditions – whether because of drift, slow response times, or failures outside the specified measuring range.
To assess the performance of different sensor types under real dry room conditions, Weiss Klimatechnik conducted a six-month long-term test using nine sensors from various manufacturers. The aim was to assess the suitability of the sensors for use in extremely dry mini-environments. The test revealed that the choice of sensor has a direct impact on process reliability.
Response behaviour and dead times
The response time of dew point sensors is a key factor in process reliability. In the event of sudden moisture ingress – for example, caused by leaks, operating errors, or material changes – the sensor must be able to respond immediately and provide reliable readings. The long-term test conducted by Weiss Klimatechnik shows that the differences between the tested sensors are considerable: T90 times (time until a 90% change in value) range from less than one second to more than six minutes.
Interestingly, no consistent correlation was identified between the measuring principle and response time. Nine sensors representing four different technologies – polymer, aluminium oxide, phosphorus pentoxide, and electrolytic-resistive – were tested. While some polymer-based sensors responded very quickly, others within the same technology group showed considerable delays. Sensors based on aluminium oxide or phosphorus pentoxide also produced inconsistent results in terms of response time.
The test suggests that the algorithms used – for example, for auto-calibration or signal processing – have a greater influence on sensor performance than the physical measurement principle itself.
Practical implications
In practice, it is not enough to rely on the technical specifications of a sensor. Instead, actual long-term stability, behaviour in extremely dry air, and responsiveness to moisture events under real conditions must be taken into account.
Sensor technology should be considered an integral part of the overall system. The choice of a specific sensor type should be based on validated performance data and take into consideration the combination of hardware, software, and calibration strategy. Sensors with fast response times and low drift enable the early detection of moisture ingress and thereby ensure the stability of processes in the dry room.