How to Deploy UHF RFID Readers & Keep EIRP Below Safe Limits
Digitalisation of the process industries is gathering pace. The deployment of wireless technology is both a key driver and enabler of this rapid rate of change. Radiofrequency identification (RFID) systems operating at ultra-high frequencies (UHF) are one such solution finding increased uptake in the sector. More specifically, users are increasingly looking to deploy Gen 2 UHF RFID readers and tags for location tracking and asset identification across plants. However, RFID technology comes with its own challenges.
Using wireless electrical equipment near dangerous substances and explosive atmospheres naturally must be done within a regulatory framework. Standard UHF RFID systems are unlikely to be compliant with the hazardous area certification requirements. Yet a suitable solution is to use pre-certified hazardous area wireless enclosure systems. Provided the wireless device fits into the enclosure and meets some basic limitations such as maximum power dissipation, it can easily be installed in a hazardous area using a pre-certified enclosure. This allows standard industrial equipment to be installed in, Zone 1/Div 1, and Zone 2/Div 2 areas (for gas, vapour, and mist), alongside Zone 21 and Zone 22 (for dust atmospheres).
One such product range is the iWAP Universal Wireless Enclosure System.
Understanding UHF RFID Tag Detection & Maximum EIRP
It is important to note that the maximum RF power that can permissibly be transmitted in a hazardous area is an essential consideration. Standards such as IEC60079-14 stipulate the maximum EIRP that can be transmitted – see table below.
Credit: IEC 60079-14 International Standard
Standard UHF RFID readers struggle to meet these regulatory requirements because they have a relatively high maximum RF power output. Plus, when an antenna is connected, the EIRP dramatically exceeds safe limits, especially for Gas Group IIC and IIB/Group A & B.
A Quick Case Study in Tag Detection
Let’s look at an example to highlight this challenge. A typical directional antenna used for fixed RFID reader applications is in the range of 5dBi to 8dBi. It is also possible to find higher gain antennas, often these are circular polarised and the gain will be expressed as dBic. As the maximum hazardous area radiated power is expressed as EIRP the circular gain needs to be expressed as a linear gain as typically used with an isotropic antenna. An 11dBic antenna gain is equivalent to an 8dBi antenna gain (-3dB) The maximum EIRP is calculated as follows:
Max EIRP = Max Radio Power + Antenna Gain – Cable Losses – Connector Losses
Max EIRP = 33dBm + 11dBic – 1.5dB – 0.5dB
Max EIRP = 33 + 8 – 1.5 – 0.5
Max EIRP = 39dBm which is 7.94W which exceeds the 6W allowed in a IIA area, meaning it is not deployable in any hazardous area
There are two possible ways to remedy this to keep under the safe EIRP limits for the specific gas groups:
- Reduce the max EIRP by using a lower gain antenna or adding in more losses to the RF circuit. This can reduce the maximum power transmitted at the antenna to safe limits, but it would also have an adverse effect on the read range performance. This is because the passive RFID type of tags rely on the readers having a very high receive sensitivity, so any losses in the RF path need to be reduced to a minimum to get the maximum read range possible. In the given example: installing a UHF reader in a IIC hazardous area with a max’ EIRP of 2W would require 6dB of attenuation to keep the power below the IIC limit. In a IIA hazardous area, only 2dB would be required, thus the read range performance would not be affected as much; but it would be in comparison with a safe area installation.
- A preferred approach would be to reduce the maximum power output to the antenna port. The regulatory limit for UHF RFID in most countries is 2W EIRP and all readers have parameters to adjust the max EIRP to consider the antenna gain plus the cable and connector losses. But user settable parameters to limit the power as per the IEC60079-14 are not permitted. The only workaround would be for the manufacturer to make a version of the product that has a lower power output. It is allowed to set power limits in the firmware as users are not able to adjust this.
As you can imagine the above is not likely to be a practical option unless the reader manufacturer sees a benefit in making another product variant to deal with this issue specifically for hazardous area applications. It is not impossible to get round the problem as shown above, but the read range would be a limiting factor in many applications.
The Extronics Approach to UHF RFID Tag Detection in Hazardous Zones
Extronics has partnered with CAEN RFID to develop a version of their Quattro reader with a lower RF power output that allows the maximum conducted power to be set in the factory to suit different application scenarios in Zone 1/21, Zone 2/22 and C1D1, C1D2 hazardous areas. The max power can be set at specific non user settable level to suit each gas group that enables an optimal read range in typical applications. By not using insertion loss to meet the requirements the user can expect to see the same performance in a hazardous area as they would in a safe area.
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