Why the "Zero" Function is Disabled in True Ohms Mode
Applies to: Fluke 8558A/8588A Reference Multimeter
Function: True Ohms (Resistance Measurement) and zeroing
Subject: Understanding self-correcting measurement cycles
Overview
When operating the Fluke 8558A/8588A in True Ohms mode, users will find that the Zero (Relative) function is unavailable or disabled. This is not a hardware limitation or a software bug; rather, it is a deliberate design choice. The True Ohms measurement technique is inherently self-zeroing for thermal offsets, making a manual "Zero" mathematically redundant and potentially detrimental to measurement integrity.
The Technical Reason: Current Reversal
In standard resistance modes, Thermal Electromotive Forces (EMFs)—tiny voltages created by dissimilar metals and temperature gradients in your test leads—can cause significant errors.
How the 8588A True Ohms Mode Operates
Instead of a simple "On/Off" cycle, the meter performs two distinct measurement phases to mathematically cancel out Thermal EMFs ( V emf ):
- Positive Cycle: The meter applies a positive current ( +I ) and measures the voltage:
- Negative Cycle: The meter reverses the current to negative ( -I ) and measures the voltage again:
- The Result: By subtracting the second measurement from the first and dividing by two, the V emf is perfectly cancelled out:
Why Manual "Zeroing" is Not Possible
The "Zero" button on a DMM is typically used to store a static offset (like lead resistance) and subtract it from subsequent readings. However, True Ohms mode is incompatible with this for two reasons:
- Dynamic vs. Static Correction: True Ohms is a dynamic process that solves offsets in real-time during every measurement cycle. Applying a static manual offset on top of a dynamic correction cycle could lead to "double-correction" or masked errors.
- Measurement Philosophy: The Fluke 8558A/8588A Reference Multimeter is designed for primary-level metrology. In this mode, the meter provides a "True" reading based on the physics of the connection. If a user sees a significant offset in True Ohms, it indicates a physical issue with the 4-wire Kelvin connection or lead integrity that should be corrected physically rather than nulled out digitally.
Summary
The Zero function is disabled because True Ohms mode is already zeroing the measurement at the hardware level. By using the current reversal technique, the meter ensures that Thermal EMFs are removed from the final value automatically, providing the highest possible accuracy without the need for manual intervention.