Why INT_P Uses TI 3600 for Hourly Flow Totalization in PCS 7
TI = 3600 supplies the time-scale conversion when INT_P integrates a value expressed per hour into an accumulated quantity. It does not make the block calculate only once per hour. Siemens documents INT_P as integrating at each sampling interval using the trapezoidal rule. Confirm the engineering units of CH_AI.V first: a 4–20 mA transmitter and VHRANGE = 15 do not by themselves establish an hourly flow value.
Table of Contents
- TI Scales Integration Rather Than Scheduling It
- Choose TI from the Input Time Basis
- Verify CH_AI Scaling Before Interpreting the Total
- Verification
- Limitations and Operational Precautions
- Engineer FAQ
- Technical References
- Related Articles
- Related Tags
TI Scales Integration Rather Than Scheduling It
For INT_P FB40 in Standard Library V7.1, Siemens describes trapezoidal integration at SAMPLE_T. TI and SAMPLE_T are expressed in seconds. This means TI determines the integral's scaling, while the sampling interval determines calculation timing. It does not wait for TI seconds between updates. Standard Library V71, section 3.13.
For constant input U and ordinary integration, the increment is U × SAMPLE_T / TI. For changing input, trapezoidal integration uses the average of the preceding and current samples. These equations are mathematical interpretations of the documented algorithm, not a replacement for its limits and operating modes.
Choose TI from the Input Time Basis
If the input represents volume per hour, use TI = 3600 seconds to express the accumulated result in volume. This is a unit conversion: one hour contains 3600 seconds. It does not change how often the cyclic block is called.
The following illustrative calculation assumes constant U = 5, an actual 1-second sampling interval, ordinary integration, and no limiting, hold or tracking:
These are calculated values, not measured results. The 3600-second total applies to a complete interval of established constant samples. Changing TI from 3600 to 10 multiplies the unconstrained accumulation rate by 360; it does not correct an incorrectly scaled input.
Verify CH_AI Scaling Before Interpreting the Total
The reported signal chain is a 4–20 mA transmitter through CH_AI, with VHRANGE = 15, then CH_AI.V connected to INT_P. Do not label CH_AI.V as mA solely because the transmitter uses a current loop.
Siemens documents CH_AI conversion using VLRANGE and VHRANGE; equal ranges have a special input-signal interpretation. Therefore, VHRANGE = 15 alone does not establish the flow unit, lower range or time basis. Standard Library V71, section 4.2.
Record the transmitter's physical range and unit, both CH_AI range parameters and the interpreted unit of V. Then check whether the rate is per second, minute or hour. The total's engineering unit depends on that definition.
Verification
- Identify the actual INT_P library version and inspect its installed documentation.
- Record the block's cyclic execution interval and SAMPLE_T; compare them with the intended sample timing.
- Inspect TI, input U, output V and the states affecting normal accumulation.
- In an isolated test configuration, apply an established constant engineering value and compare the observed increase over a known interval with U × elapsed seconds / TI.
- Check the input unit and scaling before accepting the accumulated quantity.
The proposed check must account for limits and initialization; an output that stops increasing does not prove the unit conversion is wrong.
Limitations and Operational Precautions
The explanation is independently documented for Standard Library V7.1; the installed library version is not identified. Confirm applicability before changing a running project.
Hold, tracking, reset and output limits can alter ordinary accumulation. Preserve the existing total and project configuration before approved changes to TI or execution timing. Do not change a production totalizer merely to reproduce the illustration. No live calculations or laboratory tests were performed.
Engineer FAQ
Does TI = 3600 mean one update per hour? No; it supplies the hourly scale factor in seconds.
Is TI = 10 interchangeable? No; it produces a different accumulation scale.
Does VHRANGE = 15 prove an hourly flow input? No; the transmitter units and complete scaling must establish that.