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Introduction: accurate measurement with ohmmeters
An accurate measurement requires precision and reliable connections to the device under test. Amperis manufactures precision instruments that measure with great accuracy. If the physical measurement setup involves or introduces a compromised transmission of the measurement signal, the test will give erroneous and unreliable results.
Accuracy in resistance measurement can be significantly affected by the resistance of the test lead. For example, test leads of 18µ in diameter have a resistance of .006 ohms per foot per lead, or .012 ohms per foot per pair of leads. In addition, the temperature coefficient of copper wire is .393% per °C. When measuring resistances in the milli-ohm or micro-ohm range, the errors can be significant if they are not compensated. Simple two-wire measuring devices try to compensate for the resistance of the leads by "shorting" the leads together and forcing the measurement to zero. This is a cumbersome method and it does not take into account the lead resistance or the temperature coefficient of the test lead, nor any other connection (contact) resistance associated with the device under test.
The Kelvin method uses four wires to carry out the resistance measurement. In low resistance measurements, the recommended method is to force the current through two of the leads while measuring the resulting voltage with the other two. Since the voltmeter draws insignificant amounts of current, the possible errors created by any resistance or change in resistance, either in the current-forcing leads or in the voltmeter test leads, are imperceptible. Amperis instruments compute the resulting resistance by dividing voltage by current, thereby obtaining accurate resistance measurements.
The Amperis PDRM10A Kelvin Test Leads model shown below is used for low resistance measurement. Each RG58 test lead is 8 feet long and is terminated with a double banana connector to link to the instrument. The termination at the device under test (DUT) is made of gold-plated brass.
The HTP-100 "Pistol Grip" probe model is designed for tough applications where metal components and surfaces need to be tested for "bonding". These probes are moulded in impact-resistant polycarbonate. The pins are made of hardened stainless steel and rotate when they are pressed onto the DUT. The leads are 8.5 feet long each and are terminated with a colour-coded spade connector. For "Intrinsically Safe" applications, they are coated with an antistatic material.
On the right are shown the probes to be used in resistance measurements that are more delicate than those carried out with the "Pistol Grip" model. The leads are colour-coded to make it easier to connect them to the instrument and the probe tips are marked to indicate the voltage sensed by the pins. This ensures an error-free measurement for the operator. Two versions are offered: the SSP-10 model shown below, with replaceable pins for various severe uses. Amperis will terminate its test leads for use with any manufacturer of four-wire ohmmeters. Contact us stating your model number and we will work with you to supply you with the highest quality test leads available.
Example applications for Kelvin test leads include:
- Material testing (surface and bulk resistivity)
- Contact resistance and resistivity
- Switch and contact brake resistance
- Busbar and cable joints
- Aircraft frame bonds
- Weld integrity
- Transformer and winding resistance
- Rail and pipe joints
- Cable resistance
- Lighting conductor and antenna bonds
- Bonding tests on crane power installations
*Amperis strives for continuous improvement. Please check the specifications with Amperis before placing your order.
| PDRM10A | 7 Feet Each Clamp, gold plated |
| HTP-100 | 8 Feet Each, Hardened stainless steel spring pins |
| HTP-101 | HTP-100 with Intrinsic Safety |
| SSP-10 | Kelvin probe with replaceable spring pins for severe applications |
| BCP-10 | Kelvin probe with beryllium copper spring pins |
| BCP-10 Pins |
KTL-100 supplied with the Amperis R1L-B series meter. HTP-100 supplied with the Amperis R1L-E series meter.
THE BCP-10 PROBE WITH INTERCHANGEABLE PINS
The spherical tips do not damage surfaces.
Choose the Amperis BCP-10 probe series for less complex applications, where the versatility of the probe tip is required. Select from the wide variety of tips shown below.
- Barrel: Nickel/silver, gold plated
- Spring: Stainless steel, gold plated
- Plunger: Hardened Beryllium Copper, gold plated over nickel
- Receptacle: Nickel/Silver, double gold plated