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Auto Calibration

UnitKal/DX™ - Automatic Sensor Calibration


UnitKal Automatic Sensor Calibration

UnitKal-DX™ - Automatic calibration for gas sensors in mining.

Monthly calibration of gas sensors is mandated by MSHA regulations. However, the implementation of this monthly routine can consume a great deal of time and resources. Calibration events are usually scheduled for a specific date, however, there may be delays encountered, causing missed calibration dates and risking safety violations.

UnitKal-DX™ is a small version of Rel-Tek's AutoKal-DX™ system. AutoCal-DX was developed to calibrate up to 32 sensors at a time, ideal for above ground applications involving a large numbers of sensors concentrated in a particular locale. UnitKal-DX™, however, was developed as an economical utility for mining, where sensors are scattered along a conveyor belt, and where only one or two sensors are within reasonable cal-gas delivery range.

The UnitKal-DX™ panel and the automatic calibration process are controlled by the Millennia-DX® computer that is already on site monitoring the mine for CO, CH4 , O2 , smoke, air velocity, etc.; activating alarms and controls; logging data; displaying status and printing reports.


The UnitKal-DX™ box contains two tanks of calibration gas, one having a up-scale (span) mixture and one having pure (zero) air. Two solenoid valves are controlled by the computer through a DX2202 I/O card which successively admits span and zero gas, precisely timed for a preset duration through a ¼ inch diameter tube connected to the sensor. Sufficient time) is allowed for the sensor outputs to stabilize. The computer then re-scales the sensor setup information stored in the computer memory to capture the new parameters, using a very precise 12-bit (1/4096 or +/-.024%) resolution. If the sensor fails to provide sufficient range, the computer reports the sensor as faulty and records this information for management attention.

The automatic calibration process is over in about four minutes, faster than with manual calibration. The process will be automatically repeated on a monthly basis, yet without the bother and cost of scheduling and deploying manpower again. Sufficient calibration gas is included in the initial charge to handle about 20 calibration events, more than enough to cover a year's time. At the end of the year, just replace the gas tanks, and you're good for another year. Calibration cycles are normally scheduled monthly, although the duration can be set for semi monthly, weekly, or even daily, if needed. A click on "calibrate now utility is also available to check a sensor out of schedule, as following sensor service or replacement.

A gas leak is always possible, although not likely. But, in the event of insufficient cal gas reaching the sensor to complete a calibration cycle, the sensor will fail to span properly. The computer recognizes the problem and aborts the automatic calibration cycle. A "failed calibration" entry is made in the calibration file, alerting management of the need for maintenance.

UnitKal SensorA significant cost benefit of UnitKal-DX™ that may not be obvious at first glance is the life extension of the sensor. Sensing elements tend to loose sensitivity as they age. When the sensor signal drifts beyond normal zero and span adjustment, it is normally replaced, usually at a cost about half the price of the sensor. UnitKal-DX™, however, disregards the normal adjustments at the sensor and relies only on the digitized range. The computer uses 12-bits (4096 digitizing steps) to represent the extreme high/low boundaries of the range. On automatic calibration, the computer assigns a sub-range to whatever movement is still available on an aging sensor. As long as this sub-range contains at least 256 bits, the sensor is deemed OK, and life continues. This sub-range can shrink by a factor of 16 (4096/256) before the computer finally retires the sensor and calls for service. CO sensors as old as 8-10 years can still have sufficient response left to perform adequately, a far cry from the usual 1-2 year replacement cycle for most sensor suppliers. Similarly for methane and oxygen sensors, an aging cell need not signal end of life when UnitKal-DX™ is in use. Methane sensors with a lifespan of 10-20 years is possible, while oxygen sensors, usually with 1-year life at best, can continue in operation for extra years.

The economics of automatic calibration depend on the cost of labor, as well as on other factors such as availability of trained personnel, the likelihood of missing calibrations, the dependability of technical personnel, the availability of supplies (cal gas), instruments (portable meters and tools) and the like.

But safety aspects are also to be considered. UnitKal-DX™ can eliminate exposure of calibration personnel to hazardous conditions, as when visiting sensors in gob areas, in silos and over moving belts and machinery.

UnitKal-DX™ isn't for every mining operation -- only those which utilize a Rel-Tek monitoring system or which are planning to install a Rel-Tek monitoring system. Although UnitKal-DX™ essentially doubles the initial cost of the sensor -- considering the ongoing savings in labor and the sensor life enhancement mentioned above -- the economics can be very favorable.

For more information, contact the factory or your sales representative.