Signaldoppler - Simplified solution or risk?

In planning within cleanroom and plant construction, the demand for signal dopplers instead of separate sensors for control and monitoring has noticeably increased. Technical, logistical, and economic reasons are cited for this. For a qualitative assessment, a risk evaluation is always necessary in GMP and related areas.

Technical Consideration
From a technical perspective, separating the sensors between control and monitoring makes sense but is not strictly necessary. An independent, self-contained system is easier to plan, verify, and correct or replace. Technically, there is no reason for a combined solution with only one sensor and connected signal doppler.

Logistical Consideration
Logistically, a signal doppler solution appears to offer simplified installation and certainly easier handling during commissioning or periodic maintenance. This begins with the cables and hoses for the sensors, which only need to be laid once, and means no adjustment of the air conditioning system to the monitoring system values. It ultimately ends with the seemingly lower calibration effort due to only one sensor for control and monitoring.

Of these arguments, only the technical challenge of aligning both measurement systems is worth considering. For quality-oriented HVAC technicians, this should be an interesting and motivating task. Planning, laying, and commissioning additional cables and hoses is very minor in relation to the overall system. Calibration is governed by the relevant specifications or SOPs, and duplicate calibrations can be avoided at any time.

Economic Consideration
Signal dopplers are often requested to save costs on duplicate sensors in ventilation technology. Similarly, cost reductions are expected in sensor cabling and the tubing of differential pressure sensors. However, a qualitatively equivalent signal doppler costs exactly as much as the actual sensor, and the additional lines or hoses to be laid are usually negligible in the project.

It should be noted that during maintenance, calibration, or sensor failure, the entire system must be shut down (frozen). During this time, work is not permitted. This also means unnecessary waiting times for patients and economic losses for production.

From a purely economic perspective, installing signal dopplers is certainly worth considering. However, the perceived savings are subjective and do not compare to the safety benefits of an almost fault-tolerant alternative solution with two sensors.

Risk-Based Consideration
Under "risk-oriented consideration," a tandem solution with only one sensor for control and monitoring initially appears hardly feasible.

What is the risk, what can happen?
The worst case would be a "gradual sensor error." The sensor "malfunctions," deviates slightly, and the room pressure no longer reflects reality due to this slow sensor drift. If the measurement error increases, for example, upward, the ventilation system will regulate the room pressure downward in the same proportion, while the monitoring system still receives the setpoint, but the actual value is lower.

Example:
- Sensor error: slow drift from 15 Pa to 17 Pa
- System slowly decreases by 2 Pa
- Sensor reading: 15 Pa
- Monitoring: 15 Pa
- In reality, it is only 13 Pa

...it continues...
- Sensor drifts another 2 Pa
- System adjusts again downward by 2 Pa
- Sensor then measures 15 Pa again
- Monitoring shows "correct" 15 Pa
- In reality, we only have 11 Pa
Result: No alarm is triggered!

All scenarios considered during planning are thus invalidated. The set alarm limits (e.g., ± 3 Pa) are completely ineffective in this case. No alarms are triggered, and there is no fault tolerance! If the product or process requires specific values, this solution with signal dopplers is entirely excluded under risk considerations.

Of course, total failures of sensors or electronics are also conceivable. Due to the always active differentiation between control and monitoring when using two sensors, deviations are immediately noticeable, and alarms can be activated independently. Production can potentially continue based on a risk assessment. In the alternative tandem solution, total failures are detected just as reliably, but continued operation until the final fault correction is not possible.

The simplest way to avoid this problem is the classic and proven installation with separate sensors and avoiding signal dopplers. This means ventilation technology and monitoring each have their own sensor. This demonstrates fault tolerance with only minimal increased costs. It allows for internal system feedback and thus increased reliability and control. This is a clear advantage for your patients, for economic calculations, and for your discussions with customers, inspectors, and your peace of mind.