LT DIN Rail Digital Transmitters with Serial Data Communication and Analog Outputs for Pulse to 4-20 mA Converter for Frequency, Rate, or Period Applications

Frequency, Rate & Period DIN Rail Transmitter Frequently Asked Questions

Why does Channel A support frequencies up to 1 MHz while Channel B is limited to 250 kHz?

Documented specification lists these as genuinely different maximum frequencies for the two channels rather than a shared figure — this asymmetry means an application specifically needing to measure a very high-frequency signal (up to 1 MHz) should route that signal to Channel A, while Channel B remains suited to signals within its own documented, lower maximum frequency.

Does square root extraction being "standard" on the Standard main board mean it's always active, or is it a selectable option?

Documented phrasing specifically describes square root extraction as a standard capability of the Standard board, distinguishing it from features requiring the Extended board — this indicates it's available as a configuration option on the Standard board (relevant for signals like differential-pressure-based flow that need square root correction), rather than a feature the Standard board lacks entirely.

Why does external reset of totals require a special three-position screw terminal connector rather than a standard two-wire contact input?

Documented specification specifically calls out a three-position connector for this function, though the page doesn't detail the specific reason for the third terminal — this is presented as the documented physical interface for triggering an external total reset, distinct from the transmitter's other two general-purpose control inputs described elsewhere in its documentation.

Does the A/B-1 (draw) function apply only to material tension between rollers, or can it be used for other ratio-based measurements?

Documented example specifically illustrates A/B-1 with material draw (relative elongation between rollers) as one application, alongside A/B itself being documented separately as flow ratio for ingredient mixing — since A/B-1 is mathematically just A/B with 1 subtracted, it's a general-purpose relative-difference-ratio calculation that the documented example applies to draw specifically, though the underlying math could suit other applications needing a similar relative-ratio figure.

Why does the output update rate formula include "0-2 signal periods" as a variable component rather than being a fixed number?

Documented formula (gate time + 30 ms + 0-2 signal periods) specifically reflects that after the programmed gate time and fixed 30 ms overhead elapse, the transmitter may need to wait for the current signal period to complete before returning a valid reading — since where in its cycle the input signal happens to be when the gate time ends is not fixed, this documented uncertainty range accounts for that variability.

Does custom curve linearization for a nonlinear turbine flow meter need to be re-entered if the meter's own K-factor calibration changes?

The documented custom curve linearization process specifically uses up to 180 user-entered data points reflecting the actual relationship between the transducer's raw output and true flow at the time those points were established — since a changed K-factor would shift that underlying relationship, the documented linearization data would need to be re-derived and re-entered to remain accurate, rather than automatically adjusting to a new calibration on its own.

Can the AxB arithmetic function (documented example: horsepower from force and RPM) be reconfigured for a different multiplicative relationship?

Documented framing presents horsepower as one specific example application of the general AxB multiplication function — since AxB is a general-purpose multiplication of whatever is measured on Channel A by whatever is measured on Channel B, the same documented function supports any other application genuinely needing the product of two rate or frequency inputs, not solely the horsepower example given.

Does peak reading transmission (mentioned under system-level capabilities) apply to both Channel A and Channel B independently, or only to a combined arithmetic result?

Documented system-level description specifically states that peak readings, along with arithmetic combinations of the two rates, can be transmitted via RS232 or RS485 — this is presented as available for the individually scaled and displayed channels as well as for their combined arithmetic result, rather than being limited to only one or the other.

Does selecting a shorter noise filter setting (250 Hz) instead of 1 MHz improve accuracy, or only noise rejection?

Documented specification lists these three filter settings (1 MHz, 30 kHz, 250 Hz) as noise filter options rather than accuracy settings — the correct choice depends on matching the filter bandwidth to the actual signal frequency being measured; selecting too narrow a filter (such as 250 Hz) for a genuinely higher-frequency input signal would filter out the signal itself along with the noise, so the filter setting should be chosen based on the input signal's own documented frequency, not simply toward the narrowest setting for "better" noise rejection.

Does the transmitter's ±2 ppm crystal time base accuracy apply equally to both the frequency measurement and the totalized count in Extended mode?

Documented specification lists the ±2 ppm figure specifically under Time Base Accuracy, describing the fundamental timing reference the transmitter uses to derive frequency, rate, and period readings — since totalization in Extended mode is built from the same underlying pulse-counting and timing mechanism, the same time base accuracy figure applies to both the instantaneous frequency/rate reading and the accumulated total, rather than the total being governed by a separate, undocumented accuracy specification.

Turbine Flow Meter Straight-Run Piping Questions From the Field

Why do turbine flow meters specifically require a documented minimum length of straight pipe upstream and downstream?

Documented guidance specifically explains that turbine meters must be installed to minimize measurement errors caused by turbulence, since the meter's accuracy depends on a properly developed flow profile reaching the rotor; elbows, valves, reducers, and other pipe fittings upstream are documented as distorting that flow profile, which is why a defined length of undisturbed straight pipe is specified before the meter.

What are the commonly documented turbine flow meter straight-run figures, and do they differ between upstream and downstream?

Yes — documented industry guidance commonly cites roughly 15-20 pipe diameters upstream (sometimes including the length of an upstream strainer) and around 5 pipe diameters downstream as general figures for turbine meters, with upstream requirements documented as consistently longer than downstream, since upstream disturbances have a greater documented effect on the flow profile actually reaching the meter's rotor.

Does having two elbows upstream of a turbine meter require more straight run than a single elbow, and does the specific arrangement matter?

Yes — documented guidance specifically distinguishes these cases: two elbows in the same plane are documented as requiring a smaller increase in straight run than two elbows in different planes, with the latter specifically cited as potentially requiring as much as 50 pipe diameters upstream, since out-of-plane elbows are documented as inducing a more complex, harder-to-settle swirling flow pattern.

Can a flow conditioner or straightening device reduce the documented straight-run requirement for a turbine meter installation?

Yes — documented guidance specifically describes flow conditioners (such as tube-bundle or perforated-plate types) as capable of reducing required upstream straight run, with one documented example specifically citing a reduction from around 10 pipe diameters down to roughly 5 when a conditioner is used, at the cost of some additional documented pressure loss through the conditioner itself.

Why is a strainer commonly recommended upstream of a turbine flow meter specifically, beyond general flow profile concerns?

Documented guidance specifically identifies this as protection for the meter's moving parts — a turbine meter's rotor and bearings are documented as vulnerable to damage from contaminants entrained in the flow, so an upstream strainer is specifically recommended to capture debris before it reaches the rotor, independent of the separate straight-run requirement addressing flow profile distortion.

Why is maintaining adequate downstream backpressure specifically important for a turbine flow meter, beyond simply having enough straight run?

Documented guidance specifically explains that insufficient downstream backpressure can cause cavitation or flashing at the meter, both distinct from straight-run flow-profile issues — one documented rule of thumb specifically recommends downstream pressure of at least 1.25 times the fluid's vapor pressure, plus twice the pressure drop through the meter itself, specifically to prevent these damaging conditions.

Does even a small amount of entrained air or gas in the flow stream meaningfully affect turbine meter accuracy?

Yes — documented guidance specifically cites entrained air or gas above roughly 100 mg/l as capable of producing a documented measurement error, with the meter tending to read high in the presence of entrained gas; larger quantities of entrained air are further documented as capable of causing physical damage to the rotor itself, beyond just an accuracy error.

Do all flow meter technologies share the same straight-run sensitivity as turbine meters, or do some technologies avoid this requirement?

No — documented comparison specifically notes that Coriolis flow meters measure mass flow and density directly and are documented as insensitive to fluid flow profile, requiring no upstream or downstream straight pipe run at all, in clear contrast to turbine meters' documented sensitivity to flow profile distortion; positive displacement meters are similarly documented as measuring volume mechanically and being unaffected by flow profile, though with different documented maintenance tradeoffs from their moving parts.