DLRO10HD and DLRO10HDX digital low resistance ohmmeters

Resistance measurements are dependent on temperature. If the original data was read at one temperature, but later tests are conducted at other temperatures, this temperature data is required to determine the suitability of the measurements. All materials do not react to temperature to the same degree. Aluminum, steel, copper, and graphite have specific temperature coefficients that will affect the degree of changes that can occur with varying temperatures at the measurement site. 

Low resistance measurements rely on you conducting the tests within the operating temperature range of the instrument (you must be aware of field conditions). When you see out-of-tolerance measurements, one of the first steps is to check the instrument’s reading with a suitable calibration shunt. 

The resistance of all pure metals increases with rising temperature. The proportional change in resistance for a specific material with a unit change in temperature is called the temperature coefficient of resistance for that material. Temperature coefficients are expressed as the relative increase in resistance for a one degree increase in temperature. While most materials have positive temperature coefficients (resistance increases as temperature rises), carbon graphite materials have negative temperature coefficients (resistance decreases as temperature rises). 

When making a measurement on a specific material, you can calculate the change in resistance due to a change in temperature by multiplying the resistance at the reference temperature by the temperature coefficient of resistance and by the change in temperature: 

• R2 - R1 = (R1 )(a)(T2 – T1 ) 
• R1 = resistance of the conductor at the reference temperature 
• R2 = resistance of the conductor when the measurement is made 
• T1 = reference temperature 
• T2 = temperature at which the measurement is made 
• a = temperature coefficient of resistance for the material being tested 

You should also be aware of the operating and storage temperature specifications of the instrument they are using to ensure that it is suitable for the environment in which it will be used.

The relative humidity of the test specimen should only affect the resistance reading if the material is hygroscopic, in which case more moisture will be absorbed into the sample at higher humidities. This will change the measurement conditions and will affect the achieved result. However, most conductors are non-hygroscopic. Therefore, since instruments are typically designed with an operating range of 0 to 95 % RH, providing that moisture is not condensing on the instrument, a correct reading will be obtained.

Four-wire tests are the most accurate method when measuring circuits below 10 ohms, as this method eliminates errors due to lead and contact resistances. This is the test method associated with low resistance ohmmeters. Four-wire DC measurements use two current and two potential leads. The four-wire DC measurement negates the errors due to the probe lead wire and any contact resistance values in the final reading by moving the connection points of the high-impedance voltage measurement from within the instrument to the actual test piece. This results in much more accurate resistance measurements.

These problems can be overcome relatively easily by making a measurement, then reversing the polarity of the test leads and making a second measurement. The required resistance value is the arithmetic average of the measurements. Some instruments, such as those in the Megger DLRO10 range of digital low resistance ohmmeters, feature automatic current reversal so that the correct result is displayed without operator intervention, even if there is a standing EMF on the circuit under test.