How INT_P Uses TI to Totalize a PCS 7 Flow Rate

In the documented PCS 7 Standard Library V71 INT_P block, TI is an integration scale in seconds, not a delay that makes the block update once per TI interval. The block integrates at each cyclic sample. If its input is scaled as a flow rate per hour, TI = 3600 seconds makes a constant rate accumulate to approximately that rate's numerical value over one hour. The ticket does not state the channel's engineering unit, so it cannot confirm whether 3600 is correct for this specific signal.

What TI means in INT_P

INT_P forms the time integral of its input U. In the Siemens PCS 7 Standard Library V71 manual, TI is the integration time in seconds, and the block forms the integral with sum generation using the trapezoid rule at each sampling interval SAMPLE_T. The block is evaluated cyclically; it does not wait until the configured TI interval expires before updating V.

For a constant input, the per-cycle increase is approximately U × SAMPLE_T / TI. This expression follows from the documented integral behavior and seconds-based parameters; the exact trapezoidal calculation uses the current and previous input samples.

Why TI = 3600 is used for a rate per hour

If U is a flow rate expressed per hour, setting TI = 3600 s makes the normalization interval one hour. With constant rate U = 5 engineering units/hour and one-second sampling, the output increases by about 5 × 1 / 3600 engineering units per cycle. After 3600 seconds, the accumulated change is about 5 engineering units.

The value 3600 is therefore appropriate only when the input units and desired totalization basis match that relationship. It is not chosen merely because the integration output should change once per hour. The ticket does not state the engineering units used by CH_AI or the measured flow, so VHRANGE = 15 alone is not enough to prove that 3600 is the correct project setting.

What changes if TI = 10

At the same constant input and sampling interval, TI = 10 s scales each integration increment 360 times larger than TI = 3600 s. For U = 5 units/hour sampled every second, the mathematical result after one hour would be about 5 × 3600 / 10 = 1800 in the block's normalized output units, rather than about 5. This illustrates why TI must correspond to the input rate's time basis; it is not a response delay or a one-time accumulation period.

This example assumes a constant rate, one-second sampling, no reset or hold, and no output-limit saturation. It is a calculation, not a test result.

Prerequisites for meaningful flow totalization

Before choosing TI, establish:

What physical quantity U represents after channel scaling, including its engineering unit (for example, volume per hour).
The configured scaling of the upstream analog channel. A 4–20 mA signal is an electrical representation; it must be scaled to the intended process quantity before the total has engineering meaning.
The block type and library version. The cited manual documents the classic PCS 7 Standard Library V71 INT_P block (FB40); another block revision may expose different parameters or behavior.
The block's sampling interval and cyclic interrupt OB. The V71 manual requires INT_P to be installed in a cyclic interrupt OB and documents SAMPLE_T in seconds.
The output limits V_LL and V_HL, reset/tracking/hold behavior and the desired totalization range.

How to select TI from the input units

Use the following dimensional check before downloading a change:

Write the scaled input as a rate, such as m³/h, L/min, or kg/s.
Express that rate's denominator interval in seconds: one hour is 3600 s, one minute is 60 s, and one second is 1 s.
Set TI to the denominator interval in seconds when the desired integral is a total quantity on that basis, subject to the actual INT_P implementation and project design.
Calculate the expected increment for the configured SAMPLE_T and confirm that the output units and limits make sense.
Validate the configuration in an approved test environment or project commissioning process before applying it to a running controller.

For example, a constant rate in m³/h with TI set to 3600 s produces an output increase of about that rate's numerical value over one hour. If the channel is scaled as current, level, or a rate using a different time unit, use the appropriate engineering interpretation instead of assuming 3600.

Verification

Confirm the actual INT_P block type and manual for the installed PCS 7 library. Inspect the value at U after channel scaling, the configured TI and SAMPLE_T, the cyclic OB execution, and the output limits. Check whether RESET, HOLD, or TRACK is active; the V71 manual documents their effects and states that startup handling matters.

For a controlled constant input that remains within limits, compare the observed change over a known elapsed interval with U × elapsed seconds / TI. Also inspect QVHL, QVLL and QERR where available. A trend of V should be compared with an independently calculated expected total, not judged only by whether the output moves.

Limitations and Operational Precautions

The source gives VHRANGE = 15, but it does not identify the engineering unit, the actual flow-rate basis, the sampling interval, the output limits or the installed library version. Thus the general reason for choosing 3600 can be explained, but the correctness of that setting for this specific channel cannot be confirmed from the available details.

The Siemens reference used here is the PCS 7 Standard Library V71 manual from 2009. It documents INT_P as FB40 and lists TI and SAMPLE_T in seconds; verify the matching documentation for the installed library before making changes. The block has output limits and reset/tracking behavior, so an output that stops increasing may not be caused by TI alone.

Changing a totalizer's time scaling changes the accumulated value substantially. Follow site change control, preserve the existing configuration and accumulated-value requirements, and validate the resulting engineering units and bounds before downloading to a production controller.

Engineer FAQ

Does TI = 3600 make INT_P update once per hour?

No. In the documented implementation, the block integrates at each sampling interval in its cyclic execution. TI scales the integration amount; it is not a one-hour wait timer.

Why is 3600 used with a flow signal in units per hour?

The classic INT_P documentation expresses TI in seconds. One hour is 3600 seconds, so that value normalizes the integration of a per-hour rate to an accumulated quantity over time. The input must actually be scaled as that rate.

What does TI = 10 do to the result?

For the same input and sample time, its increments are 360 times larger than with TI 3600. With a constant input of 5 units/hour and one-second sampling, the idealized increase over an hour is about 1800 rather than 5, before any output limit or control input changes.

Does VHRANGE = 15 prove that TI should be 3600?

No. VHRANGE describes an upstream channel scaling parameter, but the source does not state the engineering unit assigned to that range. Choose TI from the scaled input's physical units and the required totalization basis.

Does INT_P add the full input on every scan?

It adds a sample-time-scaled integration increment. The V71 manual describes trapezoidal sum generation per SAMPLE_T; for a constant input, the increment is approximately input multiplied by sample time and divided by TI.

Technical References

Siemens, PCS 7 Standard Library V71, Function Manual, 03/2009, A5E02102227-01, §3.13 “INT_P: Integration,” especially pp. 175–178. It identifies INT_P as FB40, defines its time-integral behavior, describes trapezoidal summation per SAMPLE_T, requires cyclic interrupt OB execution, and lists TI and SAMPLE_T in seconds. https://cache.industry.siemens.com/dl/files/733/36201733/att_83127/v1/s7jli71b_en-US.pdf
Siemens, PCS 7 SMART Getting Started – Part 1 (V8.1 with APL), 11/2015, A5E36619628-AA, §5.6, includes an INT_P example with TI identified as integration time. This is an example configuration, not a universal setting recommendation. https://cache.industry.siemens.com/dl/files/794/109478794/att_863820/v1/ps7gs1_b_en-US.pdf