Relative humidity (RH) is one of the more difficult readings to interpret accurately, especially when comparing different devices.
Relative humidity (RH) is one of the more difficult readings to interpret accurately, especially when comparing different devices. RH measures the moisture in the air relative to the maximum amount it could hold at a given temperature. Since temperature affects RH, when temperature increases while the absolute humidity stays constant, RH decreases. Temperature is a key factor in the calculations that a Conserv Smart Collection Sensor (SCS) performs to determine RH.
Measuring RH
SCSs measure RH using the physical properties of materials. Most RH SCSs consist of two conductive layers. If you think of an RH SCS as a sandwich, these layers are the "bread" with a moisture-absorbing material in between, the "filling." As the filling absorbs moisture, its electrical properties change (resistance or capacitance), and this change is measured to determine RH.
Accurate RH readings also require temperature data because temperature impacts humidity calculations. The combination of both SCS readings provides a reliable RH value. The combination of the readings from the two SCS components calculates a reading that is close to the truth.
Factors that Affect Accuracy
The accuracy of an RH reading depends on factors like the SCS's accuracy range, repeatability, hysteresis, response time, and annual drift—all of which can be found in vendor documentation. Environmental conditions and SCS movement also affect readings. For details, see the SCS Technical Specifications.
Accuracy Range
An RH SCS's accuracy is typically expressed as "Typical ± 2%, Maximum ± 3%," meaning most readings are within 2% of the actual humidity, but could vary up to 3%. For example, at 50% RH, the SCS might read between 48% and 52%, but could range from 47% to 53%. When comparing SCS with the same ± 3% accuracy, one could show 47% and another 53%, creating a 6% difference, which complicates determining the true value.
Repeatability
Repeatability measures how consistently a SCS provides similar readings under the same conditions. Modern sensors generally have good repeatability. For instance, SCSs have a repeatability of 0.08-0.1% for RH depending on the model, meaning each reading should be within 0.08-0.1% of previous ones when conditions are constant. However, a SCS can have high repeatability but low accuracy, leading to variability between devices or "jitter" in stable conditions.
Hysteresis
Hysteresis refers to a system's dependence on its past state. In practice, as RH changes, a SCS will continue to reflect its previous reading. For instance, moving a SCS from low to high humidity will result in it initially showing lower readings until it fully adjusts. This effect is similar to response time: SCSs should be allowed to stabilize in a new environment before taking measurements for comparison.
Response time
All sensing devices, from mercury thermometers to electronic sensors, require time to provide accurate readings. This time, known as response time, is how long it takes for the SCS to reach equilibrium with its environment. For RH SCSs, this involves the material absorbing or releasing moisture until it matches the surroundings. Therefore, when moving the SCS to a new environment, you should disregard initial results until the SCS stabilizes.
Annual drift
Annual drift is the loss of accuracy that the SCS experiences each year due to wear and tear, expressed as a percentage. For example, the SCS drifts approximately 0.25% per year, depending on the model, accumulating to a noticeable deviation over time. Conserv addresses this by replacing SCSs when drift exceeds acceptable levels as part of a subscription service. For other instruments, drift can be measured against a reference or through manufacturer recalibration.
Placement of SCSs
Even small environmental variations can cause significant differences in readings between devices. To minimize this, devices should be placed close together. Factors like airflow, light, and proximity to people or entrances can affect readings. Temperature differences are particularly impactful, as a 1°C change can lead to a 3-4% difference in RH calculations, depending on the SCS and RH level.
Environment
Environmental factors can damage SCSs. Exposure to solvent vapors or chemicals, especially in labs, can degrade SCS materials. SCSs should not be touched, blasted with air, or contaminated because contamination often leads to accuracy issues. Improper storage, particularly in very high humidity, can also cause temporary or permanent drift.
Resolve Differences in Readings
Comparing readings between devices can be tricky, but the following steps can help you address differences:
- Ensure that the devices that you integrate with SCSs are in good working order, recently calibrated, and properly stored.
- Allow time for devices to acclimate to a new environment to reduce the effects of hysteresis and response time. After readings stabilize, comparisons are more accurate.
- Place devices as close together as possible because small position differences can affect readings.
- Check the temperature readings of each device. In a closed system, there is an inverse relationship between temperature and RH. As temperature declines, RH increases. If you compare two SCSs and one is reading a slightly higher temperature, it should show a lower RH reading (within its accuracy range).
- Check the manufacturer's specifications for the reading accuracy range of both devices. If you compare two devices that are both recently calibrated and see a large difference in the reading, it is possible that both devices are reading within their specifications and the reading accuracy range may be the root cause of the difference.