Level Measurement and Inventory Control in Pharmaceutical Plants

The question in a pharmaceutical plant is not how much is in the vessel but what number can be defended. A level reading converted to a mass requires a bulk density, and bulk density for a pharmaceutical powder varies with handling, compaction and humidity. The resulting figure has an error nobody can state, which makes it unsuitable for a batch record.

Weighing gives a figure that can be defended, and it introduces a different set of problems. Load cells measure force, and anything that applies force to the vessel other than its contents becomes part of the reading: a rigid pipe, a conduit, a grounding strap, a cable tie across a flexible connection, or a pressure difference across a connection.

Cleanability applies to whatever is chosen. A level sensor intruding into the vessel is a product contact surface and a potential crevice; load cells sit outside, which is one of the quieter reasons they are preferred.

And everything has to be qualified: calibrated, traceable, with a documented method and a place in change control.

Quantity derived from weighing wherever the figure enters a batch record, with the installation designed so the load path carries only the vessel and its contents.

Where level is genuinely needed, a flush hygienic mounting with no crevice, included in the cleaning validation.

Calibration traceable, the method documented, and the instrument under the same change control as the rest of the process equipment.

In a chemical plant the dielectric constant governs the antenna, and an antenna sized for a typical value gives a reading that drifts with the product.

Weighing is designed as a mechanical problem first. The vessel sits on its load cells and everything else that touches it has to be compliant: flexible connections that are genuinely flexible, no rigid conduit, grounding straps arranged as a loop rather than a tie, and no cable or hose crossing a flexible joint. A force shunt does not produce an obvious error; it produces a drift that follows temperature and process pressure.

Pressure effects are then considered. A vessel connected to a system under vacuum or pressure has a force applied across every flexible connection, and that appears as mass. Venting or compensation removes it.

Where level measurement is used alongside weighing, as an operational indication or a safety function, it is specified for cleanability: flush mounting, no internal ledge, and a place in the cleaning validation with defined swab points.

Calibration is then set up properly: traceable weights, a documented procedure, a defined interval, and a record. That is what turns a number into evidence.

• Weighing used where the number enters a batch record.
• Load path kept free: compliant connections, no rigid conduit, grounding as a loop.
• Pressure differences across connections vented or compensated.
• Level sensors, where used, flush mounted with no crevice and included in cleaning validation.
• Calibration traceable, with a documented procedure and interval.
• Instrument under change control with the rest of the process equipment.

• Use of the number: operational indication, inventory or batch record, which decides level versus weight.
• Accuracy and traceability required by the record it feeds.
• Cleaning validation and whether the instrument adds a surface that must be swabbed.
• Force shunt risk from pipework, conduit and grounding at the installation.
• Process pressure at the vessel connections.
• Containment band, which affects whether a vessel penetration is acceptable at all.