Gas filling stations have traditionally employed contact probes incorporating a resistance temperature detector (RTD) and magnetic mounting devices to monitor gas temperature during filling, but a promising alternative has come to the fore in the form of non-contact infrared temperature sensors.
The infrared sensors’ supplier, Pyrodynamics Inc., in Atlanta, Ga., says they bring numerous benefits.
First, infrared temperature measurement boasts a rapid sensor response time (240 ms). Further, the infrared probes can also measure temperature for nonferrous cylinders.
“Using a magnet for mounting, magnetic probes are inherently only capable of mounting to ferrous metals,” says Blake Sanchez, president and CEO of Pyrodynamics.
“Magnetic probes even have problems mounting to newer carbon-fiber-composite-wrapped cylinders, where even if the inner lining of the cylinder is ferrous, it is not enough or close enough to the surface to make a strong connection with the magnetic mounting.”
Neither can aluminum alloy cylinders, which are non-magnetic, be handled by magnetic sensors. Infrared sensors can also measure the surface temperature of cylinders through plastic wrapping. Painted and icy cylinders can also be measured satisfactorily.
Another advantage of infrared probes is accuracy and precision over a broad temperature band. Accuracy of ±1.0 percent of the reading or ±1.0 degree C, whichever is greater, is claimed, while repeatability is ±0.5 percent of the reading or ±0.5 degree C, whichever is greater. Magnetic RTD probes, however, are calibrated at a specific temperature, and on this account, accuracy varies with temperature and is usually around ±0.8 degree C to ±5.0 degree C.
Magnetic RTD probes also need to be calibrated for specific materials. Multiple cylinder types being employed at a filling site might thus require multiple magnetic probes due to their specific temperature-range accuracies.
Pyrodynamics’s sensors are designed for many methods of communication, from the standard thermocouple type 2 or 4 wire output to Modbus serial communication, which is usually found in most logic control systems such as PLCs. USB communication with computers is also possible.
Pyrodynamics also offers the PyroPen U handheld units with data logging and USB communication capabilities for downloading collected data to a computer (software is included). Such handheld infrared temperature sensors can also be used to determine temperature gradients over the height of the cylinder.
When a gas cylinder is filled and the pressure increases, the cylinder will warm up and the resulting temperature variation of the cylinder surface is monitored for several reasons. First, the target pressure is compensated so the amount of gas filled is corrected at a standard temperature, typically 15 degrees C. Further, the temperature is monitored as the cylinder valves are opened. If the valve is opened correctly, a drop in pressure and temperature is observed before rising again as the cylinder is filled. If the temperature drop is not detected, then filling is stopped, as there could be a problem in opening the valve.
Finally, if multiple cylinders are filled simultaneously, the highest and lowest filling temperatures of the batch are monitored. Filling can be stopped if the difference between these two temperatures is too great. Again, this might be attributed to a valve failure or a cylinder or filling system leakage.
Pyrodynamics manufactures its standard PyroCouple sensor but supplies other sensors depending on the requirements of the application for more specific optics or communication types. For larger quantities of cylinders, PyroBus sensors with RS485 Modbus output can be networked in a daisy chain for direct connection to digital instrumentation.
With magnetic probes, prices generally range from $150 to $250 and sometimes higher depending on build material and capabilities, according to Pyrodynamics. “Generally, our infrared sensors will also outlast any RTD-based sensor in use because of its non-contact nature as well as the calibration lasting much longer,” Sanchez says. “Recalibration of our infrared sensors is simple and costs less than an RTD sensor, where recalibration is actually not possible and RTD type probe must be replaced.”