LT DIN Rail Analog Transmitters with Serial Data Communication and Analog outputs for Resistance in Ohms

Resistance-in-Ohms DIN Rail Transmitter Frequently Asked Questions

Why does excitation current decrease so dramatically from 5 mA on the R0/R1 ranges down to 80 nA on the R7 range?

Documented range table specifically shows excitation current dropping as full-scale resistance increases — since the voltage developed across a resistor for a given current increases with resistance, the much higher resistance values on ranges like R6 and R7 would produce excessive voltage (and excessive power dissipation) if excited with the same current used on the low-ohm ranges; the documented reduction to nanoamp-level excitation on the highest ranges keeps the developed voltage within the transmitter's measurable and safe range.

Why does the transmitter operate ratiometrically with respect to its own excitation current rather than needing a fixed, perfectly stable excitation source?

Documented specification specifically states the applied excitation current is sensed by the transmitter, which operates in ratiometric mode and automatically compensates for any changes in excitation — this means the resistance calculation is based on the actual ratio between sensed voltage and sensed excitation current at the moment of measurement, rather than assuming the excitation source is perfectly constant, which removes excitation drift as a significant error source.

Why does the 2 ohm range (R0) get a special 0.1 milliohm resolution specifically called out for contact resistance measurements?

Documented framing specifically ties this resolution to contact resistance testing — genuine electrical contact resistance in production testing is typically a very small fraction of an ohm, so a range offering 0.1 milliohm resolution is documented as specifically suited to resolving meaningful differences between good and marginal contacts, which a coarser-resolution range wouldn't be able to distinguish.

Does the QA passband relay mode's deviation band apply symmetrically above and below the setpoint, or can it be set asymmetrically?

Documented example specifically describes a deviation limit "set up around both sides of a setpoint," illustrated with a single 50 mΩ figure applied to both sides in that particular example — the documented description doesn't detail whether asymmetric upper and lower limits are separately configurable, so the specific example given represents a symmetric passband around the setpoint.

In passband mode, does the relay open when the reading is acceptable, or when it's out of tolerance?

Documented description specifically states the relay closes (or opens) when the reading falls within the deviation band, and opens (or closes) when the reading falls outside that band — this indicates the relay's specific active/inactive state relative to "in-band" versus "out-of-band" readings is itself a configurable choice (closes on pass or closes on fail), rather than being fixed to only one particular behavior.

Does choosing a factory-fixed range (R0, R6, or R7) versus a jumper-selectable range (R1-R5) affect the transmitter's documented accuracy?

No — the documented accuracy figure (±0.01% of reading ±2 counts) is listed once for all ranges in the table rather than varying between fixed and jumper-selectable ranges; the distinction between factory-fixed and jumper-selectable is specifically about how that particular range is configured into the transmitter, not about a different accuracy specification applying to one group versus the other.

Does the 3-wire lead compensation technique require the two excitation leads to be electrically identical to work correctly?

Yes — documented description specifically qualifies the 3-wire technique's effectiveness with the condition "if the two excitation leads are identical," meaning the subtraction-based compensation specifically relies on both excitation leads having matching resistance and matching response to ambient temperature changes; leads with genuinely different characteristics would reduce how completely this technique cancels lead resistance error.

Can the same physical transmitter be field-reconfigured across different resistance ranges, or does changing range require different hardware?

Documented note specifically states all ranges are factory calibrated and user selectable, and describes signal conditioner boards and ranges as changeable in the field via jumper settings, with calibration factors stored in EEPROM on the board — this points to configuration-level flexibility across the jumper-selectable ranges (R1-R5) on shared hardware, while the factory-fixed ranges (R0, R6, R7) are documented as set at the factory rather than user-jumpered.

Does the analog output's documented 16-bit resolution (0.0015% of span) mean the transmitter can resolve resistance changes finer than the resistance range's own listed resolution?

Not necessarily beyond what the input stage actually measures — the analog output's 16-bit resolution describes how finely the output signal itself can be divided across its span, but the output is documented as tracking a reading whose actual precision is set by the input measurement stage; the output resolution figure describes the output conversion's own granularity, not an independent improvement on the underlying resistance measurement's documented resolution.

Can the Extended board's custom curve linearization be applied to a resistance-based sensor whose output isn't linear with the physical quantity being measured?

Yes — documented capability describes custom curve linearization as a general Extended-board feature using up to 180 user-entered data points to create spline-fit segments, without restricting this to any specific signal type; a resistance-based sensor with a genuinely nonlinear relationship to the physical quantity it's measuring (such as certain thermistor-type sensors) is a documented candidate for this same linearization approach used elsewhere for level or flow signals.

Nanoamp High-Resistance & Insulation Measurement Questions From the Field

Why does measuring very high resistance values specifically require paying attention to surface leakage current, in a way lower-resistance measurements don't?

Documented explanation specifically identifies surface leakage current — current flowing along the outer surface of an insulator rather than through its bulk — as a component of total leakage current that becomes proportionally significant at high resistance levels; at low resistance, the bulk current dominates so completely that surface leakage is negligible by comparison, but at megohm-and-above resistance levels, surface leakage can meaningfully distort the true resistance reading if not addressed.

What is a "guard" terminal or guard technique, and specifically what problem does it solve in high-resistance measurement?

Documented explanation specifically describes the guard terminal as providing a separate current path that diverts surface leakage current away from the actual measurement circuit — by connecting the guard to a point that intercepts surface current before it reaches the measurement electronics, the instrument is documented as then reading only the genuine bulk (volume) resistance of the material being tested, rather than a reading corrupted by surface leakage.

Do high-resistance readings settle to a stable value immediately, or is there a documented practical reason to wait before recording a reading?

Documented guidance specifically notes that due to capacitance and dielectric absorption effects within the material under test, high-resistance readings can take meaningful time to stabilize — practical guidance specifically cites 60 seconds or more as a common wait time used to obtain a genuinely steady insulation resistance reading, rather than recording the initial, still-settling value.

Is there a documented reason cable and connection quality matters more for nanoamp-level measurements than for typical ohmmeter measurements?

Yes — documented guidance specifically recommends using a dedicated shielded measuring cable when measuring high resistance, with the shield itself carrying away leakage current that would otherwise corrupt the reading; at nanoamp signal levels, stray leakage paths through ordinary unshielded cabling or connectors can introduce errors that would be completely insignificant at the milliamp-to-microamp current levels used for low-resistance ranges.

Does a very low ratio between measured resistance and a reference or minimum-acceptable value always indicate the same type of underlying problem?

Documented guidance specifically describes a related diagnostic concept, the Polarization Index — a ratio of resistance readings taken at two different times during a test — where a low ratio (approaching 1.0) is documented as signaling that insulation is likely contaminated with moisture or conductive material, since genuinely sound insulation is documented as showing absorption current that decays normally over the test period rather than being dominated by steady leakage current from the start.

Can environmental contamination affecting a high-resistance measurement be distinguished from genuine, irreversible material degradation?

Documented field practice specifically describes a way to distinguish these: cleaning and thoroughly drying the material under test, then re-testing — if the resistance reading recovers after cleaning and drying, documented guidance attributes the original low reading to environmental contamination (moisture, dirt) rather than permanent damage to the material itself.

Does the specific test voltage used for a high-resistance or insulation measurement matter, or is any convenient voltage adequate?

Documented guidance specifically ties test voltage to the rated voltage of the equipment or material under test, commonly citing test voltages up to roughly twice the rated voltage for equipment rated at or below 500V, with higher-voltage equipment tested nearer to its own rated voltage — using a documented, appropriate test voltage matters specifically because it's meant to genuinely stress the insulation enough to reveal weaknesses that a lower, more casual test voltage wouldn't detect.

Does a documented minimum acceptable resistance value exist independent of what's actually being tested, or does it vary by application?

It varies by application — documented guidance specifically ties minimum acceptable insulation resistance to the voltage rating of the equipment being tested, citing one documented industry rule of thumb of twice the kV rating in megohms (for example, 10 MΩ minimum for a 5 kV-rated panel) — there's no single universal minimum resistance figure documented as applicable across every type of equipment or material.