© 2007-2016 WELLOG
All Rights Reserved
QUALITY OF DATA:
Data logging involves collection (acquisition) and storage of data. Accuracy and precision are two measures of the quality of data. In order for data to have value, the data must represent as nearly as possible the actual quantity being measured. In other words, the measurement must have accuracy. The second quality is precision. In a digital system, precision is defined by the number of binary bits used to represent the data. Early digital systems used eight bits to represent a measured value. Since eight bits have 256 possibilities of values, a voltage measurement can be resolved to a precision of one part in 256 or less than ½ percent. Typical acquisition systems may use 10 bits. These systems can resolve data with a precision of one part in 1024 or better than 1/10 of one percent.
UNITS OF MEASUREMENT:
When a measurement is performed, it is expressed in terms of voltage (volts), current (amperes) or resistance (ohms), etc. When measurements are expressed in units of measurement, the measurement must have a calibration reference. For example, if a measurement is a voltage measurement made in units of volts and the amount of voltage measured is 1.00 volts, it is expected that the measurement is valid.
In order for a measuring device or instrument to have validity, it must be calibrated. Calibration standards are maintained for purposes of calibrating measurement devices.
Primary standards were initially maintained by the National Bureau of Standards. Primary standards are used to calibrate secondary standards. Secondary standards are maintained by calibration laboratories. Test equipment used in field laboratories are normally calibrated on a regular basis at calibration facilities having a secondary calibration standard. The National Institute of Standards and Technology (NIST), after 1988, became responsible for measurement standards and traceability of calibration. Visit http://www.nist.gov to learn more about NIST.
Calibration must be verifiable. When a calibration is performed on a piece of test equipment, a calibration label is affixed that states the place, date, person (stamp) and date calibration is due. WELLOG uses calibrated test equipment to calibrate logging tools.
LOGGING TOOL EXAMPLE:
In the mineral logging industry, one form of measurement is Spontaneous Potential (SP).
The calibration is performed using a variable voltage produced using a 1.5 volt battery and potentiometer. The potentiometer can be adjusted to simulate the small (0-500 mv) SP voltage produced in a well or borehole.
The potentiometer is adjusted to 500 millivolts and verified using a calibrated digital voltmeter. The SP voltage is amplified and signal conditioned by a low noise amplifier and applied to a digital logging acquisition computer. The voltage is converted to a value between 0 and 1024. For our purposes, we will give the reading a number of 862. Therefore 862 counts = 500 millivolts.
Calibration constant calculation:
1024 x 500 / 862 = 594
Because the computer has a full range of 1024, the full scale reading would be 594 millivolts.
Calibration constant = 1024/594 = 1.724
In an ideal system 500 millivolts would operate over 1024 counts = 1024/500 = 2.048 = calibration constant.
In a real system, as described, 500 millivolts operates over 862 counts = 862/500 = 1.724 = calibration constant.
500 millivolts = 500 x 1.724 = 862 counts.
400 millivolts = 400 x 1.724 = 689 counts.
300 millivolts = 300 x 1.724 = 517 counts.
200 millivolts = 200 x 1.724 = 345 counts.
100 millivolts = 100 x 1.724 = 172 counts.
In the acquisition system, SP (millivolts) = counts / 1.724