Digital Multimeter Calibration

Purpose

The purpose of this procedure is to provide instruction for the calibration of digital multimeters (DMM).

Scope

All multimeters (multi-range instruments used to measure voltage, current, and resistance) are to be verified in calibration no less than once per year.

Procedure

Preliminary Instructions and Notes

Read this entire procedure before beginning the calibration.
Calibration shall be performed in an environment that conforms to Manufacturer Specifications.
The digital multimeter will hereafter be referred to as the Instrument Under Test (IUT).
Verify that the IUT is clean.
Visually examine the IUT for any condition that could cause errors in the calibration.
If any of the requirements cannot be met, refer to the applicable manufacturer manual.
If a malfunction occurs or a defect is observed while calibration is in progress, the calibration shall be discontinued and necessary corrective action taken; if corrective action affects a measurement function previously calibrated, the function shall be recalibrated before the remainder of the procedure implemented.

Reference Material

Applicable Manufacturers Manual or Brochures.

Specifications

The specifications of the IUT are determined by the applicable manufacturer's documentation. If the manufacturer's documentation is not available, then the specifications identified in this procedure are used.

Equipment Required

The Standards listed below should be selected on the basis of their higher accuracy level when compared to the unit under test. Equivalent Standards must be equal to or better than the Minimum-Use-Specification.

Minimum-Use-Specifications for Standards listed are 1/4 the accuracy required by the IUT.

Rotek Model 2500 Calibrator or equivalent
Diode Test Fixture
Test Leads
Continuity Test Fixture
Capacitance Test Fixture
Thermometer
Hydrometer

Set-Up

Turn all power on (calibrator and the unit to be calibrated). Allow the instruments to stabilize for approximately 5 minutes. Conduct the tests in an ambient temperature of 25 ± 5C and a relative humidity of less than 80%.

Note: If the IUT has an automatic ranging feature, then the function being checked (i.e. voltage, current, or resistance) must be verified in the variable range mode as well.

Battery Replacement

Replace the battery in the IUT and make sure the battery contacts are not dirty.

Detailed Procedure

Display and Switch Test

Turn the IUT on and verify that all LCD segments are working and not dim. Verify that the selector switch(s) is/are working properly. Make sure the Low Battery indicator is not showing.

Resistance Check

Connect the calibrator between the V/ohm and common input terminals of the IUT. Use the following chart to verify the resistance values.

Resistance Measurements

Step	Range	Input	Display
1	200 ohm	Short	00.0 to 00.5
2	2 k ohm	Short	0.000 to 0.001
3	2 k ohm	1 k ohm	.998 to 1.002
4	20 k ohm	10 k ohm	9,98 to 10.02
5	200 k ohm	100 k ohm	99.8 to 100.2
6	2000 k ohm	1 M ohm	998 to 1002
7	2000 M ohm	Open	0.10 to 00.0

Continuity Test

Select the continuity check function on the calibrator.
Connect the test leads to the V/ohm and common terminals of the IUT.
Momentarily short the test leads together and observe that the tone sounds.
Connect the test leads to the continuity test fixture between the 100 ohm test points. No tone should be heard indicating non-terminating connection.
Connect the test leads to the continuity test fixture between the 50 ohm set up. The test tone should indicate continuity between the test points.

DC Voltage Test

Use the calibrator to supply the correct DC voltage listed below.
Set the calibrator for a zero volt input.
Connect the calibrator output to the V/ohm and common input terminals of the IUT.
Note: Connect the ground/common/low side of the Rotek 2500 to the common on the IUT.

With reference to the table below, select the IUT voltage range and set the calibrator output to the corresponding IUT input voltage. Test per the following chart and verify that the display falls within the limits given.

DC Voltage Measurements

Step	Range	Input	Display
1	200mV	190mV	189.7 to 190.3
2	200mV	-190mV	-189.7 to -190.3
3	2V	0.0V	-0.001 to 0.001
4	2V	1.9V	1.897 to 1.903
5	20V	19V	18.97 to 19.03
6	200V	100V	99.84 to 100.16
7	1000V	400V	399.3 to 400.6

AC Voltage Test

Use the calibrator to supply the correct AC voltage in the following test:

Set the calibrator to 60 Hz.
Connect the calibrator output to the V/ohm and common input terminals of the IUT. Connect the ground/common/low side of the calibrator to common on the IUT.

With reference to the table below, select the IUT voltage range given in step 1 and set the calibrator output to the corresponding IUT input voltage. Verify that the display reading is within the limits shown.

AC Voltage Measurements

Step	Range	Input	Display
1	200mV	100mV	99.0 to 101.0
2	2V	1V	0.990 to 1.010
3	20V	10V	9.90
4	200V	100V	99.0 to 100.1
5	750V	400V	294 to 406

DC Current Test

Use the calibrator to supply the correct DC current listed in the table below:

Set the output of the calibrator to zero mA.
Connect the output of the calibrator to the lowest current terminal (typically 200 mA) and common input terminal on the IUT.
Note: Do not exceed the maximum current rating of the meter. Only test for values the meter is capable of reading.

With reference to the table below, select the IUT current range and set the calibration output to provide the corresponding IUT input current. Verify that the display reading is within the limits shown.

DC Current Measurements (Range 1)

Step	Range	Input	Display
1	200?A	190?A	189.7 to 192.3
2	2mA	1.9mA	1.897 to 1.923
3	20mA	19mA	18.97 to 19.23
4	200mA	190mA	189.7 to 192.3

Disconnect the calibrator and change the current terminal on the IUT to the highest setting (typically 10 A) leaving the common input terminal connected.

Use the following table to verify that the meter is functioning within the specified parameters.

DC Current Measurements (Range 2)

Step	Range	Input	Display
1	10A	1A	0.98 to 1.02
2	10A	5A	4.90 to 5.10
3	10A	9A	8.82 to 9.18

AC Current Test

Use the calibrator to supply the correct AC current listed in the table below:

Use the following table to verify that the meter is functioning within the specified parameters.
Set the output of the calibrator to zero mA.
Connect the output of the calibrator to the low current input terminal (typically 200 mA) and the common terminal of the unit under test.
Note: Do not exceed the maximum current rating of the meter. Only test for values the meter is capable of reading.

With reference to the table below, select the IUT current range and set the calibrator output to provide the corresponding IUT input current. Verify that the display reading is within the limits shown.

AC Current Measurements (Range 1)

Step	Range	Input	Display
1	200?A	190?A	18.27 to 19.28
2	2mA	1.9mA	1.872 to 1.928
3	20mA	19mA	18.72 to 19.28
4	200mA	190mA	187.2 to 192.8

Disconnect the calibrator and change the current terminal on the IUT to the highest setting (typically 10 A) leaving the common input terminal connected.

Use the following table to verify that the meter is functioning within the specified parameters.

AC Current Measurements (Range 2)

Step	Range	Input	Display
1	10A	1A	0.97 to 1.03
2	10A	5A	4.87 to 5.13
3	10A	9A	8.77 to 9.23

Logic Test

Use this procedure to test weather the logic function is indicating the correct state.

Set the IUT to the logic test mode and insert the test leads into the V/ohm and common terminals.
Short the leads together and verify that the meter indicates a low logic state.
Set the calibrator to 0.400 VDC and connect the meter to the output terminals. Verify that the meter indicates a low logic state.
Set the calibrator to 3.500 VDC and verify that the IUT indicates a high logic state.

Frequency Test

Connect an oscilloscope to a function generator to measure AC sine waves.
Set the function generator to output 10 VAC and use the oscilloscope to set the frequencies from the following table.
Connect the IUT leads to the V/ohm and common terminals on the IUT and to the ground and output of the function generator.

Verify the meter is displaying proper results from the table.

Frequency Measurements

Step	Range	Input	Display
1	Hz	60 Hz @ 10V	59.4 to 60.6
2	Hz	1 kHz @ 10V	.990 to 1.010k
3	Hz	1 MHz @ 10V	.990 to 1.010M

Capacitance Test

Set the IUT to measure capacitance, insert the test leads into the V/ohm and common test terminals.

Connect the meter to the capacitance test fixture and use the following table to determine the ability of the IUT.

Capacitance Measurements

Step	Range	Input	Display
1	Capacitance	10 micro F	9.85 to 10.15
2	Capacitance	1 micro F	.985 to 1.015
3	Capacitance	100 micro F	98.5 to 101.5

Diode Test

Set the IUT to the diode check function and connect the test leads to the V/ohm and common terminals.

Connect the test leads to the diode test fixture and follow the steps in the proceeding table to verify diode test operation.

Diode Measurements

Step	Range	Setup	Polarity	Display	Tone
1	Diode Check	Short	N/A	.000 to .001	Yes
2	Diode Check	Si	Forward	.800 to .600	Yes
3	Diode Check	Si	Reverse	1.	No
4	Diode Check	Ge	Forward	.300 to .500	Yes
5	Diode Check	Ge	Reverse	1.	No