Most people believe the least demanding part of making a measurement is installing the sensor, and sometimes it is. But in modern, complex manufacturing processes, the expense of installing the device can be greater than the cost of the sensor itself, and the process can be daunting and time consuming. So let's look at three key installation issues: service access, mounting and protection, and signal connections.
Service Access
You'd think ensuring access to the sensor to allow servicing would be obvious, but that isn't always the case. I recall a new furnace installation in a steel processing plant, where the control thermocouples were installed in the floor of the furnace. The service access points (there were multiple sensors) were 20 ft. above the sub-basement floor, in a confined space between rotating machinery. Replacing a sensor when the furnace was operating was a safety hazard. It was even difficult when the furnace was on a Hot Hold.
The service technicians finally installed a second array of thermocouples in the furnace sidewall, at plant floor level, accessible 100% of the time. To account for the new sensor measuring locations, the staff had to re-tune the control system. Unfortunately, the cost of the new installation was more than three times that of the original setup.
So think hard about problems uncovered during the installation process. Always consult with the people responsible for servicing the device. The time to ensure access is when the sensor is installed the first time. In doing so, you can keep the cost of the project down and avoid delays and lost measurements.
Mounting and Protection
Most modern sensors and signal transmitters mount in the field, close to the measuring point. Many contain electronics and electrical components that must be protected from temperature, vibration, and humidity extremes. Modern vision systems, optical scanning devices, infrared thermometers, and other devices used to inspect specific areas of a process or product need clean lenses or protective windows to operate correctly.
This part of the installation process is also where you ensure the service parameters of your hardware and software are correctly set, (e.g., water flow rates or purge air pressure and flow rates) and functioning. Check that the water-cooling has real flow and the strainers are not blocked.
Similarly, you need to test air supplies for "liquid air." It's incredible how much water and oil can condense in instrument air-quality systems if they're not installed and maintained properly. You may have to add additional filters and driers if your air is inadequate.
All these factors can contribute to measurement uncertainty and must be considered during the mounting process.
Signal Connections
This last element can present some of the most difficult problems of the installation. Many sensors still use low-level (e.g., millivolt, or microvolt) analog signals. Some sensors, such as thermocouples, are wired to local multiplexed digital DAQ concentrators or separate analog transmitters that boost the signal level and even convert them to currents rather than small voltages. The connection between the sensor and the DAQ device is susceptible to ambient noise and interference.
To avoid these issues, you must check the signals and connections to minimize or eliminate these problems. For instance, if output analog signals are isolated from local ground (earth), the chance for ground loop interference is reduced. Fortunately, most digital systems have all but eliminated these types of problems.
In Closing
The sensor's installation is the 10th step of the process I first described in my Sensors article "A Twelve-Step Sensor Selection Checklist". The challenge posed by this phase is often underestimated. The truth is that doing the job correctly the first time will save you time and money that you can ill afford to lose.