My measurement results look wrong — how to interpret Pulsenics plots

This article helps interpret what Pulsenics plots are showing when results look unexpected. It covers the four most commonly reported visual issues across Bode, Nyquist, time domain, and cell voltage plots.

Probe model scope: Plot behaviour and thresholds described here are based on the Probe V4 200–1200 A and are also applicable to the Probe V4 100. They are indicative for the Probe V3 and Probe 20/40, which have different operating limits. If the client is on a V3 or Probe 20/40, escalate for confirmation on anything threshold-specific.

Agent guidance — when a client shares a plot screenshot

When a client shares a plot, look for these things in order:

What plot type is it? Bode magnitude, Bode phase, Nyquist, time domain DC trace, or CMU cell voltage
Which frequency range or time window is affected? All frequencies, low frequencies only, a specific band, or a single point
What does the DC trace show? Is DC current and voltage stable during the measurement?
Is the issue consistent across runs, or only on one run?

Then use the sections below to match what you see to a known pattern.

Agent triage — ask these first

"Which plot are you looking at — impedance magnitude, phase, Nyquist, the DC current/voltage trace, or cell voltage channels?"
- If unsure: Ask — "Can you describe what the axes show, or paste the dashboard link so I can look directly?"
"Is the issue happening across all frequencies, or only in a specific part of the spectrum?"
- If unsure: Ask — "Does the data look wrong everywhere, or only at the low end (below ~100 Hz) or high end of the sweep?"
"Is the DC current and voltage stable during the measurement, or fluctuating?"
- If unsure: Ask — "On the time domain plot or DC trace, does the DC current hold steady or does it jump around during the run?"
"Does the issue appear on every run, or only sometimes?"
- If only sometimes: Likely intermittent — check connectivity and DC stability first before investigating data quality

Issue 1 — Negative DC voltage or current reading

What it looks like: DC voltage shown as negative (e.g. -2.1 V) in the dashboard, debug messages, or time domain trace. DC current may also read negative when a positive value is expected.

What it almost always means: The DUT+ and DUT- connections are swapped — reversed polarity on the DUT terminals.

By Pulsenics convention, current flowing out of the DUT+ terminal is positive. Current flowing into DUT+ is negative. If the client is in a discharging/fuel cell configuration, negative current is expected and correct. If they are in a charging/electrolysis configuration and seeing negative current, the connections are likely reversed.

What to check:

Confirm DUT+ cable connects to the positive terminal of the stack
Confirm DUT- cable connects to the negative terminal of the stack
Confirm the configuration matches the intended mode — charging/electrolysis vs discharging/fuel cell (see Probe V4 user manual Sections 3.8.1 and 3.8.2)
Do not swap connections while the Probe is powered on or DC current is flowing

Escalate if: Polarity appears correct but DC voltage is still reading negative, or if a "DUT Under Voltage Detected" fault appears alongside the negative reading.

Issue 2 — Data missing at specific frequencies

What it looks like: The Bode or Nyquist plot is missing data points at certain frequencies — gaps in the curve, or a frequency range that appears to have fewer points than expected.

Known causes:

Output Over Current Detected at those frequencies — the Probe could not drive the requested perturbation at those frequencies and skipped them. Most common at low frequencies or near the power supply resonant region (~700 Hz). See: Why am I seeing "Output Over Current Detected"?
V4 frequency skipping — on V4 Probes, the system may automatically skip frequencies where the power supply presents lower impedance (e.g. near ~700 Hz). This is by design and not a fault.
Measurement interrupted mid-run — if the experiment was stopped or restarted during sampling, some frequency ranges may not have completed. Check Probe Messages for incomplete sampling sequences.
Experiment settings — the frequency range or number of points may simply not cover those frequencies. Confirm what range and point density is configured in the experiment settings.

What to ask:

"In the Probe Messages panel, did all frequency ranges show both Sampling Started and Sampling Completed, or are some missing a completion message?"
"Did any error messages appear in the Probe Messages around the time of the missing data?"

Issue 3 — Noisy or spiky data at low frequencies

What it looks like: The Bode magnitude or Nyquist plot shows scattered, irregular, or spiky data points at the low frequency end of the sweep (typically below ~10 Hz or ~100 Hz). Higher frequency data may look clean.

Known causes:

DC instability during sampling — low frequency EIS is most sensitive to DC current fluctuations. If the power supply or load is not stable at the time of measurement, low frequency data will be noisy or unreliable. Check whether the DC trace shows stable current during the affected frequency range.
System Transient Detected — the Probe detected a large DC change during sampling and interrupted EIS. Check Probe Messages for "System Transient Detected." If present, EIS resumed automatically but the data around the transient may be unreliable.
Power supply in voltage control mode — the power supply or load connected to the S/L terminal should be in current control mode (CC) for the full frequency range to work correctly. Voltage control mode can cause instability at low frequencies.
Insufficient DC filtering — ripple or noise on the DC bus is most visible at low frequencies. Review whether additional DC filtering is in place.
Too many simultaneous frequencies — increases total current demand and can worsen noise at low frequencies. Try reducing simultaneous frequencies.

What to ask:

"Is the DC current stable during the measurement, or does it fluctuate?"
"Is your power supply set to current control (CC) mode?"
"Does the noise appear on every run, or only sometimes?"

Issue 4 — Phase wrapping (phase jumps to +180° or -180°)

What it looks like: On the Bode phase plot, the phase trace suddenly jumps to +180° or -180° at certain frequencies rather than following a smooth curve. Clients may also describe this as:

"The phase looks flipped"
"The phase is inverted"
"Phase is showing positive when it should be negative"
"The phase curve is mirrored"
"Phase is at 180 degrees across the whole spectrum"
"Something looks wrong with the phase"

What it almost always means: The positive and negative leads of the voltage sense cable connected to the CMU are swapped — reversed polarity on the sense wiring. This inverts the measured phase response and is the most common confirmed cause.

What to check first:

Confirm the positive lead (red) of the voltage sense cable is connected to the positive terminal of the cell or cell group being measured
Confirm the negative lead of the voltage sense cable is connected to the negative terminal
Check all CMU channels involved — if multiple channels show the same phase inversion, all leads on those channels may be reversed
Do not swap connections while the Probe is powered on or DC current is flowing

If the client is using bundled voltage sense cables, confirm the labelling on each cable end matches the intended cell polarity. It is easy to connect a bundle in reverse if the cell orientation is not clearly marked.

Other contributing causes (less common):

At very low frequencies where signal-to-noise is poor, phase estimates can become unreliable
Near the power supply resonant region (~700 Hz), phase behaviour can be unexpected due to the power supply opposing the Probe's perturbation

What to ask:

"Can you confirm which way the voltage sense cable leads are connected — positive lead to positive terminal, negative lead to negative terminal?"
"Does the phase look wrong on all channels or only specific ones?"
"At roughly what frequency does the phase jump happen, or is it across the whole spectrum?"

Escalate if: Sense wiring polarity is confirmed correct but phase is still inverted or wrapping, or if the issue persists after correcting the connections.

What to ask when a client shares a plot screenshot

Use these questions to extract the most useful information quickly:

"Which plot type is this — magnitude, phase, Nyquist, DC trace, or cell voltage?"
"At roughly what frequencies does the issue appear?"
"Is the DC current stable during the measurement? Can you check the DC trace?"
"Does this happen on every run or only sometimes?"
"Are there any error messages in the Probe Messages panel around the time of the measurement?"
"Can you paste the dashboard or experiment link so I can look directly?"

Article status

This is a living reference article for interpreting Pulsenics plot results. Additional patterns and confirmed causes will be added as more cases are identified.

Tags: plots, Bode plot, Nyquist plot, phase wrapping, negative DC voltage, missing data, noisy data, low frequency, DC instability, measurement quality, results, impedance, phase, data quality, V4