Digital Panel Meter for Process and Ratiometric Applications P/N L31402P1

Process & Ratiometric Digital Panel Meter Frequently Asked Questions

Why does ratiometric mode specifically require using the meter's own excitation output, rather than any external power source?

Ratiometric operation works by using the same excitation voltage that powers the sensor as the reference for the meter's own analog-to-digital converter — this specific pairing is what lets excitation variations cancel out mathematically; an independently-sourced external supply would break that reference relationship and reintroduce excitation-variation error.

Does the same ratiometric follower principle apply equally to a rotary potentiometer measuring angle and a linear potentiometer measuring displacement?

Yes — the underlying measurement technique is identical; the difference is purely in how the meter is scaled (degrees or radians for rotary displacement, versus inches or mm for linear displacement) rather than any difference in the electrical measurement method itself.

Why does peak capture run at up to 60 readings per second while the visible display updates only 3.5 times per second?

A genuinely fast event (such as the instant of failure in destructive testing) needs internal sampling at up to 60 readings/second to be caught reliably, but a display updating that fast would be illegible to a human operator — the meter captures peaks internally at full speed while keeping the visible display at a comfortably readable rate.

Can the isolated 10 Vdc excitation output really power four strain gauges simultaneously without exceeding its rating?

This is documented as a specifically supported configuration for the destructive-testing peak-capture application — the same excitation-sharing arrangement used for multi-load-cell weighing platforms applies here, meaning four gauges can share the excitation output as long as their combined current draw stays within the output's rated capacity.

Does custom curve linearization need to be redone if the tank or sensor it was calibrated against is later modified or replaced?

Since the 180-point linearization is based on calibration data reflecting the actual physical setup it was measured against (such as a specific tank's real geometry), any meaningful change to that physical setup could introduce enough deviation that the linearization should be re-verified, even if the replacement component is nominally the same type.

Why does span tempco use "% of reading" while zero tempco is expressed as "counts per degree" rather than the same units?

These describe different error mechanisms: span tempco (0.003% of reading/°C) scales with the actual signal level, reflecting drift in the meter's gain, while zero tempco (0.1 count/°C) is a fixed baseline offset drift independent of signal level — expressing each in the units natural to its own mechanism is what allows both to be combined correctly into an overall accuracy budget.

Does the "rate from successive readings" feature require the underlying process signal itself to already be linear?

No — since rate is calculated by comparing already-linearized readings against each other, the raw input signal feeding the meter can be nonlinear; the custom curve linearization step (if used) corrects the raw signal first, and the rate calculation simply compares those already-corrected readings.

Why does the two-wire transmitter powering scheme specifically use a 10-ohm resistor to convert the 4-20 mA current into a voltage?

A 10-ohm resistor at 4-20 mA produces a proportional 40-200 mV voltage drop — a value low enough to stay within the meter's sensitive millivolt input ranges while still being large enough to measure accurately, making it a practical middle ground for converting the loop current into a voltage the meter's front end can directly scale.

Does the meter's overvoltage/overcurrent protection differ meaningfully between the ratiometric potentiometer application and a standard absolute DC voltage measurement?

The documented protection specs (max applied voltage and overcurrent multipliers) are tied to the selected range rather than to whether the meter is operating in absolute or ratiometric mode — so a potentiometer follower application using the 20.000V range, for example, carries the same 600 Vac maximum applied voltage rating as an absolute measurement on that same range.

Does the 1 GΩ input impedance on the 200.00 mV and 2.000V ranges matter for a typical ratiometric potentiometer application?

Yes, though less critically than for absolute voltage measurement — a very high input impedance ensures the meter draws negligible current from whatever it's measuring, avoiding a "loading" effect that would otherwise slightly disturb the potentiometer's own output; for the lowest, most sensitive ranges this matters most, since a lower-impedance input could meaningfully affect readings from a high-resistance potentiometer element.

NAMUR NE43 & 4-20mA Fault Signaling Questions From the Field

What is NAMUR NE43, and what problem was it specifically created to solve?

Documented industry background specifically describes NE43 as a recommendation created to standardize how transmitters indicate device failure using the 4-20 mA signal itself, so that a control system can distinguish "the transmitter is broken" from "the process variable is at its normal minimum or maximum" without needing separate digital communication — before this standardization, that distinction wasn't handled consistently across manufacturers.

What specific current levels does NAMUR NE43 define as indicating a genuine transmitter fault, versus a valid but extreme reading?

Documented specification defines the normal valid measurement range as roughly 3.8 to 20.5 mA (with 3.8-4 mA and 20-20.5 mA representing saturation at the edges of calibrated range), while readings below 3.6 mA or above 21 mA are specifically defined as a fault condition — a real device failure, not just an extreme process reading.

Why is there specifically a small gap (3.6 to 3.8 mA) between the fault threshold and the start of the valid saturation range?

Documented explanation specifically ties this gap to the needs of two-wire, loop-powered transmitters — since these devices draw their own operating power from the same loop current, a very low current level immediately below 3.8 mA could affect the transmitter's own operation, so the gap provides margin before declaring an outright hardware fault at 3.6 mA.

Why is "upscale" fault mode (21 mA) generally recommended specifically for 2-wire transmitters rather than "downscale" (3.6 mA)?

Documented guidance specifically explains that loop-powered (2-wire) transmitters require a minimum current to operate their own internal electronics — setting the fault indication to a very low downscale current risks starving the transmitter's own power needs, so upscale mode (21 mA) is the documented safer default specifically for 2-wire devices, while 4-wire (separately powered) transmitters can use either mode without this concern.

Does a NAMUR NE43-compliant fault signal need to persist for some minimum time before a control system treats it as a genuine fault?

Yes — documented guidance specifically recommends the fault signal be present for at least 4 seconds and a minimum of 2 signal scanning cycles before being interpreted as a genuine sensor fault, specifically to avoid false alarms from brief, transient signal excursions that aren't actually indicating a real device failure.

Is trying to force a transmitter's output below 4 mA (such as to 0 mA) an acceptable way to simulate or test a fault condition?

No — documented guidance specifically warns against this, noting that driving the signal below 4 mA isn't the correct method for zero calibration or fault testing; the correct approach is using the transmitter's own self-calibration routine or setting zero at approximately 3.75 mA within the documented NAMUR framework, rather than pushing the signal to an out-of-specification value.

Do all instrument manufacturers implement NAMUR NE43's exact current thresholds consistently, or is there variation worth checking?

Documented sources specifically warn that not all manufacturers follow the NE43 recommendation to the letter — some self-declared "NAMUR compliant" transmitters use deviating threshold values, meaning confirming the actual fault-signal thresholds in a specific transmitter's documentation is worth doing rather than assuming universal adherence to the textbook 3.6/21.0 mA values.

Can a receiving instrument like a panel meter or PLC actually distinguish and act on the different NAMUR NE43 signal zones, or does it just see a single continuous current value?

This depends entirely on configuration — documented guidance specifically notes that receiving devices can interpret the defined current ranges and zones (valid, saturation, fault) only if they've been properly configured to recognize those specific thresholds; without that configuration, a receiving instrument would simply treat the entire signal as a continuous analog value without any special fault-zone recognition.