Accuracy of Calibration

Accuracy of Calibration

With the sale of each MiniRes there is a resistor calibration harness. For quality control, the MiniRes calibration can be checked with this resistor harness before and after each critical project.

The calibration harness for the ultra MiniRes and GPS-Res has two leads from the transmitter and four leads out the receiver side. Those four leads are color coded. Black is common, Red is 19.00 Ohms, Yellow is 1.90 Ohms and blue is19 milliOhms. Those values can be traced back to National Bureau of Standards references. and R Instruments purchased a 19.00 ohm reference resistor with a certified accuracy of 0.01%. That resistor has been measured in a Racal Dana model 6001 digital multimeter. That meter is periodically checked against a set of standards. That set of standards is periodically checked by Vishay Intertechnology Inc. whose primary standard has been certified by NIST Test No. 811/262153-99. The above certified resistor is then used by L and R Instruments to verify the accuracy of its digital multimeter. That multimeter is then used to trim the value of the main resistor in the calibration harness.

The type of main resistor used in the calibration harness has been selected for temperature stability and is specified at 15 PPM or less per degree Celsius. They have a resistance of 19.1 ohms. The trim resistors typically have a resistance of about 3.65 Kohms and therefore temperature changes have little effect on the overall accuracy of the calibration harness. The trim resistors are military specification metal foil with a temperature coefficient of 50 or 100 PPM per degree C. After the calibration harnesses are completed, they are allowed to age and then again checked for accuracy.

The ULTRA MiniRes calibration harness has two leads on the current (transmitter) side and four leads on the potential (receiver) side. It contains three resistance values; 1.900, 19.000 and 0.19000. All three ranges of the ULTRA MiniRes can be tested. On the potential side, the black is common and goes to the black binding post. The red plugs into the red binding post for use of the 19.000 ohm resistance. The yellow plugs into the red potential binding post for use of the 1.9000 ohm resistance and the blue plugs into the red potential binding post to use the 0.19000 calibration resistor. This calibration harness “loads” the transmitter and must not be connected in reverse.

In the MiniRes receiver, accuracy with ambient temperature changes has been optimized by the careful selection of quality components and temperature compensating circuits.

Moisture can have an adverse effect on the accuracy of the MiniRes. Considerable effort has been invested in making the MiniRes as water-resistant as practical. However, if the operator suspects that moisture has entered the instrument, he should use the calibration harness to check the accuracy of the instrument.

A full and complete check of the calibration should performed before beginning a survey. The results of that check should be documented in detail and timed and dated.

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