For tank-farm spot-temperature arrays, an 8-input multichannel transmitter replaces eight individual single-channel devices and eight cable runs with one bus-powered field device and one trunk, with the Rosemount 2240S explicitly offering 4, 8, or 16 spot-element variants with ±0.05 °C conversion accuracy for custody-transfer applications [S4].
The 2240S connects up to sixteen 3- or 4-wire RTD spot elements and an integrated water level sensor, then publishes spot and average liquid temperature plus free water level to the Tankbus over FOUNDATION fieldbus, IEC 61158 [S1][S4]. It is the canonical reference design for an 8-channel-class device in tank gauging, and a useful baseline for sizing and specifying alternatives.
Where 8-Channel Architecture Pays Off
Multi-channel transmitters make economic sense when several sensors are clustered within a few meters of each other, as documented in application guidance from May 2026 [S2].
Four common cases dominate: storage tanks with multi-depth thermowells, vertical reaction or heat-exchange columns where top, middle, and bottom points must be read together, coking furnaces with skin thermocouples distributed across the wall, and rotating equipment where bearing and stator-winding sensors are co-located on one machine [S2]. In each case, the cabling delta between eight single-channel transmitters and one 8-channel transmitter is roughly 8 cable runs down to 1, with corresponding reductions in tray entries, gland plates, and termination density at the cabinet [S2].
Channel Count and Sensor Wiring
The 2240S is offered in 4, 8, or 16 spot-element input versions, with 3-wire or 4-wire RTD options and individual or common return, so an 8-channel class device is a stock SKU rather than a custom build [S4].
Available spot-element counts are explicitly listed as 3, 7, or 15 in regional distribution data, reflecting the way multiple-spot thermowells such as the Rosemount 565, 566 (cryogenic/LNG), and 765 (with integrated water level) are bundled [S4]. For an 8-input tank-farm application, the 8-element variant covers the typical 6–8 depth array used for net-standard-volume correction under API chapter 7.3 without paying for unused channels.
Accuracy and Custody-Transfer Suitability
Conversion accuracy of ±0.05 °C (±0.09 °F) is the published Premium performance-class number for the 2240S, qualified against the demanding net-standard-volume calculation that combines spot temperatures with a radar level reading [S4].
The 2240S is approved for custody transfer when paired with the Rosemount 2410 Tank Hub under the corresponding Custody Transfer Type Approval, and the data sheet explicitly cites API chapter 7.3 compliance as the design driver [S4]. A useful design check: an 8-channel transmitter used only for monitoring, not custody transfer, can usually tolerate the same accuracy class but does not need the 2410 hub, the 765 water-level sensor, or the type-approval paperwork, which materially changes the BoM.
Protocol Choice and Cable Count
Bus-powered protocols such as PROFIBUS PA and FOUNDATION Fieldbus H1 carry both power and data on one twisted pair, so an 8-channel transmitter on PA or FF H1 needs only that single pair from field to control room; data-only protocols such as Modbus RTU (RS-485) and PROFINET require a separate power feed and therefore a second cable run [S2].
For tank farms where a Tankbus or H1 segment is already present, the bus-powered variant is the lowest-cable option. Ethernet-based PROFINET is easier to integrate on greenfield sites that already route Ethernet to the field cabinet, accepting the extra power run as a tradeoff for simpler network configuration [S2]. Field enclosure ratings should be checked at the same time: the 2240S carries IP66/67 and NEMA 4X certifications, which is the level typically required for outdoor tank-farm mounting [S4].
Selection Criteria vs Single-Channel and Higher-Density Devices
The decision between a single-channel temperature transmitter, an 8-channel device, and a 16- or higher-channel data logger is set by three variables: sensor density per asset, cable-tray congestion, and protocol infrastructure already in place. [S2]
Compact panel-mount loggers are offered in 8, 16, 24, and 32 input channels for mixed thermocouple and RTD inputs, which suits test benches and laboratory work but trades Ex-certification and bus-protocol integration for channel count [S5]. General-purpose DAQ units with 8 configurable inputs add current, voltage, and pulse measurement on the same terminals, useful when temperature is only one variable among several, but they are not bus-powered and are rarely specified for hazardous-area tank-farm service [S7]. For process-tank applications, the bus-powered 8-channel FOUNDATION fieldbus or PROFIBUS PA transmitter remains the lowest-cable, Ex-rated, custody-transfer-capable option.
Field Integration: Hub, Sensor, and Radar Pairing
An 8-channel tank-farm transmitter is rarely a standalone device: it sits on a Tankbus with a radar level gauge, a pressure transmitter for hydrostatic back-up, and a 2410 Tank Hub that hosts the custody-transfer approval, as shown in the 2240S reference architecture [S4].
The pairing logic matters: spot temperatures are read by the 2240S, level is read by a 5900S radar gauge, free water is read by a 765 multiple-spot sensor, and pressure backup is supplied by a 3051S [S4]. For brownfield tank farms, integrating the temperature array with existing level and pressure instruments on the same Tankbus is usually the lowest-risk migration path. Related selection problems across the site, such as deciding between HART and FOUNDATION fieldbus for a differential pressure transmitter on the same segment, follow the same bus-power and protocol-availability logic that drives the 8-channel choice.
Hazardous Area Certification and Installation
Tank-farm service routinely requires intrinsic safety, and the 2240S is offered with ATEX (I1), Brazil Inmetro (I2), FM-US (I5), FM-Canada (I6), and IECEx (I7) hazardous-location certifications in the same model code [S4].
2-wire IS bus power is a defining feature of the 2240S installation: the same pair that carries FOUNDATION fieldbus communication also powers the device from the Tankbus, simplifying Ex-loop design by removing the local power supply [S4]. For LNG and cryogenic tanks, the Rosemount 566 multiple-spot sensor is the matched low-temperature probe, and the same 8-channel transmitter can be specified without changing the bus or hub configuration.
Trackable signals for the next planning cycle: confirm whether the 2410 Tank Hub approval list has been refreshed for the 8-element 2240S variant in your target jurisdiction, and verify on the regional distributor datasheet whether the 7- or 15-spot thermowell bundles are still stocked in your preferred lead time, since channel-count options occasionally change with manufacturing batches [S4].
The underlying component specifications are covered under level transmitter.
This topic is covered further in 7-decade vs 6-decade resistance box: 0.01 Ω to 11 MΩ selection guide.