Relative Humidity FAQ
Alicat® Scientific’s M‑Series, MC‑Series, and IS‑Max™ Series devices with 10v firmware support an optional relative humidity (RH) sensor that accurately measures changes in water vapor and automatically adjusts flow readings to account for humidity.
In this article, we provide answers to common questions about RH. For more information on how Alicat devices measure and correct for humidity in gas flows, check out our blog on humidity sensing.
What are the RH sensor's specifications?
- Humidity accuracy :
The humidity sensor itself is specified to have an accuracy of ±1.8% RH when operating between 0°C and 50°C and under 90% RH. However, at high humidity levels (90–100% RH), the accuracy may degrade to approximately ±3% RH even within that same temperature range.
In our system, we further recompute relative humidity (RH) using the humidity sensor’s internal temperature sensor and the Alicat’s internal temperature readings. Since both temperature measurements are involved in the final RH calculation, the effective system-level RH accuracy may be slightly worse than the sensor manufacturer’s standalone specification.
- Operating humidity range:
While the sensor is technically capable of functioning across 0–100% RH, Alicat devices typically specify an operational humidity range of 0–95% RH. This is a conservative limit set to reduce the risk of condensation inside the device due to rapid temperature changes, which could impact performance or accuracy.
- Operating temperature range:
Like the humidity range, the sensor itself may support a wider temperature range than the Alicat device it’s integrated with. However, the effective operational temperature is constrained by the Alicat system specifications. For reliable performance, it is recommended to adhere to the temperature limits stated for the Alicat hardware, even if the sensor is rated for broader conditions.
Specifications are subject to change without notice.
Which Alicat flow products offer the RH sensor option?
Devices with 10v firmware may be eligible for retrofit with an RH sensor; however, because the sensor requires physical space that may not be available in all device form factors, feasibility must be evaluated on a case‑by‑case basis. To learn more about the capabilities of a specific device for your process, you can talk to an engineer today.
How do the sensors measure water vapor percentage?
The percentage of water vapor is updated whenever new humidity data is received. If the change is significant, the device may update gas properties—potentially as often as once per second.
Although water vapor content is pressure-dependent, the device assumes that pressure changes come from adding or removing gas with a similar vapor percentage. Therefore, the water vapor percentage remains unchanged until new humidity data is provided.
Does the RH sensor affect my choice of gas and gas mixes on Alicat instruments?
No, the RH sensor itself doesn’t limit your choice of gases or gas mixes on Alicat instruments.
You can still select from the full range of standard gases and gas mixtures supported by Alicat’s COMPOSER™ feature and Gas Select™ list. The RH sensor simply adds an additional layer of measurement: it detects water vapor content in the gas stream and allows the device to correct flow readings for humidity effects.
However, there are considerations:
- The RH sensor is incompatible with certain volatile organic compounds (VOCs) because they can contaminate the sensor’s polymer layer (e.g., fumes from glues, adhesives, coatings).
- The RH sensor is incompatible with corrosive gases.
- If your gas mixture contains these VOCs, periodic bakeout maintenance widely reverses the impact.
How do RH devices handle dynamic/changing humidity levels?
Alicat devices with RH sensors can compensate for changes in water vapor by continuously measuring moisture content and automatically correcting flow readings to reflect dry gas conditions.
This process must be set up by providing the device with humidity data for the COMPOSER™ function to base its adjustments on.
How do I provide humidity data to the Alicat device?
You can supply humidity data to the Alicat device either through an integrated sensor or manually via display, serial commands, or industrial protocols.
The device uses relative humidity (0 – 100%) and, optionally, the temperature at which that humidity is valid. For accurate results, providing the correct temperature is important; otherwise, the device assumes it and will match the internal gas temperature.
What communications work with Alicat RH sensors?
When humidity support is enabled, the Alicat device provides humidity‑related readings through several interfaces:
- ASCII Serial: Relative humidity is added to the data frame by default. You can include all three readings (relative humidity, dew point, and water vapor percentage). This is done during device configuration at the factory.
- Industrial protocols (DeviceNet, EtherCAT, EtherNet/IP, IO‑Link, Modbus RTU, Modbus TCP/IP, PROFIBUS, and PROFINET):
- Other than PROFIBUS, each has at least one data slot that reports relative humidity by default. That slot can be reconfigured by Alicat to show dew point or the percentage of water vapor instead. Some protocols support all three simultaneously.
- PROFIBUS does not support reporting relative humidity, but can employ humidity corrections in the reported flow.
- All protocols support commands to set relative humidity and its corresponding temperature, if the device is configured for user‑supplied humidity.
Humidity readings are also available through analog outputs, and on integrated displays where applicable.
For advanced or custom placements of these readings contact Alicat Engineering.
How does the Alicat device handle dew point calculations?
Dew point is reported in the same units as the gas temperature, but has two built‑in limits:
- It will never show a dew point lower than −30°C, due to high sensitivity at low temperatures.
- It will not exceed the current gas temperature, as this would imply condensation.
Because dew point depends nonlinearly on temperature, its accuracy varies with temperature even if relative humidity accuracy remains constant.