4 questions to answer to ensure you're using the correct humidity sensor

So it is with capacitive humidity sensors. Because of the difficulties in humidity measurement (thanks to the fact that the sensor must be in direct contact with the moisture it's measuring) humidity sensors are difficult to choose. A lot depends on the environment they will be used in.

The truth is, there is a proper time and place for most modern humidity sensors; the trick is knowing which time and place require what type of sensor. In this week’s blog, Jim Tennermann breaks down some of the advantages and disadvantages of capacitive humidity sensors, identifying applications where they work well, and where they don’t.

 

 

1. Do you require a fundamental, or secondary measurement?
2. Do you require (and do you have the resources for) a sophisticated device or a simple one?
3. What sort of contaminants are in your environment?
4. What are the operating parameters of your environment? (Air pressure, temperature, etc.)

Fundamental vs. Secondary Measurement

Fundamental measurement devices are those which depend on some intrinsic physical phenomenon to provide consistent, high-performance measurement. In the world of humidity, chilled mirror hygrometers achieve this by controlling the temperature of a surface so that condensation remains in an equilibrium state on that surface. The temperature of the surface is measured, yielding a very accurate dew point measurement expressed in the International System of Units.
 

By contrast, the capacitive humidity sensor is a secondary device. It relies on a dielectric material and measures how that material changes as a function of relative humidity. However, the dielectric material has a couple disadvantages. First, it can be “fooled” into responding to substances other than water vapor; second, it can drift over time for several reasons.

So, why doesn’t everyone use a chilled mirror hygrometer to measure humidity? For starters, they are two to three orders of magnitude more expensive than capacitive sensor-based devices. They are heavy, complex, sensitive to flow rates, and need regular maintenance. The tradeoff between the two technologies is that users of capacitive sensors sacrifice some performance for price, simplicity, and ease of use.

Capacitive humidity sensors are simple. They consist of two plates sandwiching a dielectric (insulating) material. Each plate has a “leg” for an electrical connection. The sensor is attached to an appropriate device for measuring capacitance. What could be easier? Well, it’s not really that easy. First of all, the plates have to be permeable to water vapor or the response time of the sensor would be far too slow. The plates are typically very thin layers of metal, often sputtered onto a base material. This requires sophisticated equipment and process knowledge.
 

Some sensors are manufactured in cleanrooms to obtain uniformity and consistency. This is not simple. Furthermore, the actual change in the sensor’s capacitance is quite small over the humidity range of 10 - 90%. Electronic circuits for measuring the sensor have to be designed carefully to measure this small change, and stray capacitance has to be eliminated in all wiring that connects the sensor to the electronics. These are not issues for users, but knowledge of these details might help users understand the differences in price and performance between instruments that appear to be the same.

All humidity sensors must be in contact with the gas they measure. Anything in the gas that “disagrees” with the sensor can alter the sensor’s performance. For example, small oil droplets in an aerosol cleaning agent can coat the sensor, forming a barrier that limits water vapor permeability. Dust can accumulate on the sensor with a similar effect.

The most difficult contaminants are chemicals that interfere with or change the nature of the dielectric material. These contaminants can be sneaky; when sensors are exposed to them, they create measurement error. In some cases, when the sensor is removed from service for calibration, the contaminants may "outgas" and the measurement error disappears. Other contaminants may cause permanent damage to the dielectric.
 

Sensors from different manufacturers may react differently to environments because of differences in dielectric material or sensor design. All sensors have strengths and weaknesses when it comes to contamination, but sorting them out is nearly impossible without direct testing. This is the nature of capacitive humidity sensors. Unfortunately, every humidity measurement technology is also subject to degradation due to contamination. To mitigate these risks, analysing and understanding the environment you are measuring is absolutely key.

An advantage of capacitive humidity sensors is that they can withstand both high and low temperatures. They can measure in saturated conditions and “dry as bone” conditions. They outperform most technologies when it comes to extremes. Furthermore, there are a few clever tricks that can extend sensor performance.

For example, when measuring warm and moist conditions, condensation may form on the sensor and corrupt the measurement. This can be avoided by heating the sensor prior to exposing it to warm and moist conditions. However, having said this, we need to add that you cannot use a torch or a similar heating device to heat the sensor. This will damage or destroy your sensor. Rather, look for a humidity instrument that has its own built-in heating mechanism to purge condensation.

In conclusion, capacitive humidity sensors may not be a universal solution for humidity measurement, but they’re pretty darn close. They are suitable for the vast majority of humidity measurement application. For more information, here is a "Technology Description" paper on Vaisala humidity measurement devices.