Servo Level Measurement

Servo Level Measurement

Endress+Hauser’s servo level portfolio (Proservo) is built for the highest‑accuracy custody transfer and inventory control on liquid tanks. A small displacer is lowered on a measuring wire and positioned by a servo motor; when it contacts the liquid, buoyancy reduces the apparent weight and changes the torque at a magnetic coupling. The electronics detect this via Hall‑sensor arrays and hold the displacer right at the liquid surface, computing the level from the drum position. Because the measurement is mechanical at the interface, it is largely unaffected by media properties such as conductivity or dielectric constant, and it remains stable in the presence of vapors. The servo gauge’s guided displacer technique also avoids drift from changing dielectric constant or vapor composition, and because the head sits on the tank roof, there is no wetted diaphragm to clog or age.

Portfolio coverage is straightforward: the Proservo NMS80, NMS81 and NMS83 address most custody‑transfer and inventory scenarios, and the Digital Transmitter TMD1 complements them in tank‑gauging systems. Across the Proservo family you get accuracy up to ±0.4 mm, wide process envelopes (typically −200…+200 °C) and engineered pressure capability by model, together with NMi/PTB approvals and compliance with OIML R85 and API MPMS 3.1B. Beyond level, Proservo can determine interfaces between up to three liquid layers, the tank bottom, and spot/profile densities. Model‑specific envelopes cover the installed base: NMS80 measures to 36 m, NMS81 to 47 m, and hygienic NMS83 to 22 m; all list main wetted parts in 316L, Alloy C‑276 and PTFE. Temperature capability is −200…+200 °C, with pressure ranges from 6 bar abs (NMS80/83) up to 25 bar abs (NMS81).

Proservo NMS81 is the high‑capacity workhorse when tall tanks, higher static pressures, or liquefied gases are in scope. It covers measuring distances up to 47 m and process pressures up to 25 bar abs, with wetted parts such as 316L and Alloy C‑276 to handle sour and aggressive services. The method is especially well‑suited to LNG/LPG custody transfer because it is not influenced by dielectric constant or boil‑off gas, critical in cryogenic tanks that challenge non‑contact technologies. Safety is integral: hardware and software are developed to IEC 61508 with SIL 3 achievable in homogeneous redundancy, and international explosion‑protection and overfill‑prevention certificates are available. Like other Proservo devices, NMS81 covers the full −200…+200 °C process temperature range, making it suitable for cryogenic and hot‑product service alike.

Proservo NMS80 brings the same metrology to mainstream crude/product depots and chemical tank farms. With ranges to 36 m and accuracy to ±0.4 mm, it satisfies OIML R85 and API custody‑transfer requirements and provides the same rich approvals portfolio. A key advantage is the ability to position the displacer at defined depths to identify up to three interfaces and to determine spot and profile densities, enabling sharper inventory reconciliation, pipeline interface tracking, and settlement monitoring. Hardware and software are developed in accordance with IEC 61508, enabling SIL up to 3 in homogeneous redundancy, and international Ex, WHG and country‑specific custody‑transfer certificates are available. Like NMS81, NMS80 supports SIL practices and WHG overfill prevention and is available with flanged process connections and common materials of construction.

For sanitary production environments, Proservo NMS83 extends servo accuracy to hygienic tanks in food and beverage. It keeps the ±0.4 mm performance with a 22 m range and mirrors the same legal‑metrology heritage, while being specified for hygienic applications and referenced for high‑value beverages such as beer, whiskey, wine, and sake. Where frequent cleaning, low conductivity, or foam would complicate other principles, the displacer‑based measurement remains precise and repeatable. The unit carries the same approvals set (NMi/PTB, OIML R85, API MPMS) and uses flanged connections with materials compatible with sanitary processes, so it can be integrated without compromising cleanability.

System and lifecycle integration are pragmatic. The Proservo instruments support conventional 4–20 mA/HART as well as tank‑gauging protocols, including Sakura V1 and Whessmatic 550; Modbus RS‑485 broadens host connectivity. The Digital Transmitter TMD1 fits into these systems to enable remote tank calibration and to transmit level, temperature, and alarms, with convenient features such as handheld‑terminal configuration and an input for a HART temperature sensor. On the protocol side, the catalog lists Sakura V1, Whessmatic 550 and Modbus for the gauges; TMD1 adds legacy compatibility (V1/BCD/Sakura) so modernized systems can aggregate level, temperature, and alarms even where older field wiring remains in place. This combination allows upgrades of legacy gauging networks while preserving modern diagnostics and serviceability.

Where do you apply servo? Wherever ultra‑high precision and regulatory certainty are mandatory or where process conditions unsettle other technologies. Typical use cases include custody‑transfer and inventory‑control duties on crude oil, gasoline, and diesel; petrochemical and chemical tank farms; LNG/LPG tanks and terminals with cryogenic temperatures and active boil‑off; and hygienic beverage production that demands traceable, repeatable gauging. Because the displacer can probe layers and density profiles, the same instrument supports dewatering and quality tracking within a single tank‑top penetration. In these applications, SIL‑rated operation and overfill‑prevention certification support risk reduction in tank‑farm safety instrumented functions.

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