Panel Meter for Rate, Totalizer with Functions A+B, A-B, AxB, A/B, A/B-1 Applications P/N L7041BFR

Rate, Totalizer & Ratio Panel Meter Frequently Asked Questions

What is the difference between ratio (A/B) and draw (A/B-1)?

Ratio is simply Channel A's rate divided by Channel B's rate. Draw subtracts 1 from that ratio, which is useful specifically for elongation or shrinkage applications where a ratio of exactly 1.0 (no stretch) should display as zero rather than as 1.0.

Why would I use A/B ratio instead of just comparing two separate rate displays manually?

Manually comparing two separately displayed rates requires an operator to do the math and notice a deviation, whereas the meter calculating and directly alarming the A/B ratio catches an out-of-tolerance condition automatically and immediately — critical in applications like chemical mixing or film coating where a delayed manual catch could waste material or produce an out-of-spec product.

Can the ratio or draw result be scaled to a specific precision or multiplied by a factor?

Yes. The A/B or A/B-1 result can be multiplied by a power of 10 ranging from 0.00001 to 100000, with the decimal point set to display the result at the desired precision, up to six digits — useful for expressing a ratio close to 1.0 (as in draw applications) with meaningful resolution.

Can this meter sum two flows for a total, and separately calculate their ratio, at the same time?

The function applied to Channels A and B (A+B, A-B, AxB, A/B, or A/B-1) is a single selected math mode rather than several running in parallel — so summing (A+B) and calculating ratio (A/B) would require selecting one function at a time rather than displaying both simultaneously from the same configuration.

What sensor types are compatible with the two input channels for a ratio application?

The FR signal conditioner accepts NPN or PNP proximity switches, TTL/CMOS logic, magnetic pickups, contact closures, low-level turbine flow meter outputs down to 12 mV, and AC line inputs up to 250 Vac — so a ratio application can compare, for example, two turbine flow meters, two proximity-switch speed sensors, or a mix of sensor types across the two channels.

Does the meter need external power to run the proximity switches or sensors feeding Channels A and B?

Not necessarily. A built-in isolated excitation supply (5, 10, 12, or 24 Vdc, jumper-selectable) can power both sensors directly, eliminating the need for separate external power supplies.

How is the ratio or draw alarm typically used in a coating thickness or draw control application?

With the relay output option, the A/B or A/B-1 result can trigger an alarm when the ratio drifts outside an acceptable band — for example, flagging when coating thickness (via the flow-to-film-speed ratio) or film elongation (via the draw calculation) moves out of tolerance, so an operator or automated control loop can correct it before significant off-spec material is produced.

Can the ratio or total be transmitted to a PLC or SCADA system rather than only displayed locally?

Yes. The calculated ratio, draw, sum, difference, or total can be transmitted via RS232, RS485 (Modbus RTU), Ethernet (Modbus TCP), or an isolated 4-20 mA/0-10V analog output, in addition to the local front-panel display.

How accurate is the ratio calculation itself, separate from the sensors feeding it?

The meter's internal time base is crystal-calibrated to ±2 ppm with long-term drift of only ±5 ppm/year, so the ratio math itself introduces negligible error — the practical accuracy of a ratio application depends far more on the accuracy and matching of the two sensors and flow meters feeding Channels A and B than on the meter's own calculation.

Is a separate meter needed for each math function, or can the same meter be reconfigured between A+B, A-B, AxB, A/B, and A/B-1 later?

The same physical meter can be reconfigured between these functions in the field via the front panel or Instrument Setup Software, since they're all software-selected modes of the same Extended counter hardware rather than separate physical configurations — so a meter set up for A+B summing today can be reconfigured for A/B ratio later without needing different hardware.

Rate, Totalizer & Ratio Panel Meter Questions From the Field

Why does my A/B ratio reading spike to an implausible value occasionally, even though both flows look steady?

This is commonly caused by one channel briefly dropping to zero or a very low value — since A/B is a division, even a momentary near-zero reading on the B channel (from a signal dropout, slow startup, or noise) can produce a momentary, wildly large or erratic ratio result. Checking whether the spike coincides with a brief signal interruption on either channel, rather than assuming a persistent calibration issue, usually explains it.

My coating thickness ratio (A/B) is correct most of the time but drifts during startup or line speed changes — why?

Transient mismatches between the two channels' response times are a frequently reported cause — if one sensor or flow meter responds faster to a speed change than the other, the ratio can briefly show an incorrect value during acceleration or deceleration even though both channels settle to correct readings once speed stabilizes. Reviewing whether ratio alarms are specifically occurring during speed transitions, rather than during steady-state operation, helps distinguish this from a genuine sensor problem.

Why does my draw (A/B-1) reading show a small nonzero value even when I believe the film isn't stretching at all?

A small residual nonzero draw reading at a supposedly 1:1 ratio is commonly explained by minor mismatch in scale factors between the two channels, or genuine small mechanical differences between rollers (such as slightly different diameters) rather than an actual process issue. Verifying both channels' scale factors are correctly matched to their actual pulses-per-revolution, and confirming roller diameters against the assumed values used in scaling, resolves most of these small residual offsets.

Why does my A/B ratio alarm trigger falsely when both individual flow rates still look reasonable on their own?

This is often traced to the deviation limit around the target ratio being set too tight relative to normal process variation, since even small, individually-acceptable fluctuations in each flow can combine into a ratio deviation that crosses a narrow alarm threshold. Reviewing the alarm deviation band against actual observed ratio variation during known-good operation, rather than assuming both flows must be individually out of range to trigger a ratio alarm, clarifies this.

Can noise on one channel affect a ratio calculation more than it would affect that channel's rate reading alone?

Yes — because ratio calculations amplify small errors through division, noise-driven jitter on either input channel can produce a proportionally larger effect on the calculated ratio than it would on that channel's rate display alone, particularly when the two channels' rates are close to each other (as in draw applications). Applying the adaptive digital filter or a longer gate time on both channels is generally more important for ratio and draw applications than for simple rate display.

Why does my conveyor line synchronization ratio work fine at full speed but become unreliable at low or startup speeds?

At low speeds, the pulse rate from proximity switches sensing gear teeth or drive wheel spokes drops proportionally, and if the gate time isn't long enough to capture a meaningful number of pulses at that lower rate, the ratio calculation becomes noisier and less stable. Confirming the gate time is adequate for the lowest expected operating speed, not just the normal running speed, addresses this class of low-speed instability.

My A+B summed total is consistently a bit lower than what I calculate by manually adding the two individual totals — why?

This is commonly traced to each channel having a slightly different scale factor or rounding behavior applied before the sum is calculated, so small individual rounding on Channel A and Channel B compounds into a small but measurable discrepancy versus manually adding two already-rounded displayed totals. Comparing the meter's internal (pre-rounding) scaled values for each channel, rather than the rounded displayed totals, against the summed result usually accounts for the difference.

Why did my A-B (difference) reading go negative unexpectedly during normal operation?

A negative A-B result simply means Channel B's rate has exceeded Channel A's rate at that moment, which is a legitimate and often meaningful condition — for example, in a net inflow/outflow application, a negative A-B (inflow minus outflow) genuinely indicates the tank is emptying rather than filling. Confirming whether a negative reading reflects a real process condition, rather than assuming it's an error, is the first thing to check before troubleshooting further.