M850 Flow Computer

Spirax Sarco M850 flow computer for compensated measurement

The Spirax Sarco M850 is a programmable flow computer for calculating mass flow and energy flow in steam, gas and liquid applications. It sits between the flowmeter system and the operator or plant control network, combining transmitter signals, fluid properties and configured calculation rules into usable process values. The result can be displayed locally, totalised, recorded, sent to a supervisory system or used to trigger an alarm or simple control function.

The M850 is suited to flowmetering installations that need more than a local indication. It can receive differential-pressure, volume-flow or mass-flow signals and can use pressure and temperature information for compensated measurement. The device is documented for systems using Gilflo, ILVA, linear flowmeters and orifice plate assemblies. The selected meter, transmitter range, fluid and required calculation must still be checked against the application before an order is placed.

One flow computer, two installation formats

The M850-W-x is the wall-mounted version and is designed around a 100-240 Vac supply. The M850-P-x is the panel-mounted version and uses a 24 Vdc SELV supply. Both versions use the same measurement and calculation concept, so the physical format can be selected around the control-panel layout, available power and enclosure arrangement rather than by sacrificing the core flow-computing functions.

Each format is available with three output configurations. The -0 version has no optional analogue current output, -1 has one passive 4-20 mA output and -2 has two. Four binary outputs remain available for alarm, signalling or pulse functions, with the output hardware differing between the wall and panel variants. This nomenclature should be confirmed against the required supply, output type and local certification at quotation stage.

Measurement inputs and calculation functions

The M850 provides twelve measurement inputs. Six accept 0-20 mA or 4-20 mA current-loop signals, three accept four-wire RTD sensors, and three are configurable pulse, frequency or binary inputs. The current inputs can be used with passive transmitters supplied by the M850 or with active current-source transmitters. The device provides a 24 Vdc transmitter supply with a stated maximum of 22 mA per channel, while the input circuits and protection limits remain subject to the technical data.

The three RTD channels support Pt-100 x K and Ni-100 x K sensor arrangements. The documented ranges extend from -200 to +850 C for Pt-100 x K and from -60 to +150 C for Ni-100 x K. Wire-resistance compensation and four-wire connection are available for temperature measurement. The pulse inputs can monitor a contact state, measure frequency or count pulses from a meter with a fixed pulse value; this makes the same input group useful for different meter output arrangements.

Configured mathematical channels can use measured values, calculated values and constants to create additional process values. The quickstart guide describes flow and energy balances, averages, ratios, totalisers and event recording as examples of the functions available in the device. A one-second calculation update rate is specified for compensated flow and heat-energy measurement. The technical sheet gives typical accuracy better than plus or minus 0.5 percent, with a worst-case figure better than plus or minus 2 percent for the listed steam, water, other-liquid and technical-gas measurement. These figures do not replace a system-specific uncertainty assessment.

Local display, recording and communications

A backlit 3.5 inch colour TFT display presents measurement values, calculations, totals, alarms and configurable screens. Three two-colour indicators identify alarm, recording and USB status, while the 19-button membrane keypad supports local navigation and setup. The internal memory can record process values and events, allowing the flow computer to retain information where a permanent computer-network connection is not available.

For plant integration, the M850 provides an RS-485/RS-422 interface and an RJ-45 Ethernet port. The technical information lists ASCII, Modbus RTU and BACnet MSTP functions for the serial interface, and Modbus TCP, BACnet IP, HTTP server, DHCP server and ICMP functions for Ethernet. A USB 2.0 Type-A port supports the documented configuration and data-transfer workflow. Optional passive 4-20 mA outputs can retransmit a selected measured or calculated value to a receiver with an external loop supply.

These interfaces allow one configured instrument to serve several information tasks: local operator viewing, a plant historian or building-control network, hardwired alarm signalling and analogue retransmission. The exact protocol, address, baud rate, loop supply and data map should be agreed with the control-system designer before commissioning.

Engineering value for steam and utility systems

For steam measurement, the M850 can use the pressure and temperature signals needed to distinguish saturated and superheated conditions in a configured application. For water and other liquids, the calculation uses the relevant fluid characteristics entered during setup. For technical gases, the meter and transmitter data must be matched to the configured measurement method. In every case, the flow computer calculates from the signals it receives; it does not correct an incorrectly sized meter, a misranged transmitter or an unsuitable sensor installation.

The flowmeter is therefore part of the selection decision. Visit the flowmetering product range and compare the target flowmeter range to review upstream meter options, then confirm the M850 input arrangement, pressure and temperature compensation, pulse value or calibration coefficients. The M850 documents describe ILVA, Gilflo and M410/orifice differential-pressure examples for the supplied quickstart software. Other meter types, extra inputs and extended output assignments require the full installation and maintenance reference.

Installation and environmental limits

The M850 is specified for indoor installation. The wall version uses an IP65 enclosure when the cable-entry arrangement is correctly installed; the panel version provides IP65 protection at the front and IP20 at the rear. The documented operating temperature is 0 to +55 C with 5-95 percent non-condensing relative humidity, at altitudes up to 2000 m. Storage is specified from -30 to +70 C.

Choose a location away from excessive heat, vibration, shock, flooding, motors, large transformers and other sources of electrical or magnetic interference. A wall-mounted unit uses three 5 mm mounting points and requires suitable IP65 glands or conduit. A panel-mounted unit requires a 186 x 92 mm aperture, its supplied seal and fixing clamps. The rear of a panel unit needs clearance for wiring and viewing access; the installation guide illustrates a minimum 10 cm clearance around the rear unit in an enclosure.

Electrical work must be completed by suitably qualified personnel. The installation guide requires mains isolation before opening, appropriate overcurrent protection, an accessible disconnect device, compliant wiring and correct cable-screen connection. The wall variant uses hazardous mains voltage, while the panel variant uses a 24 Vdc SELV supply. The technical and installation sections below retain the terminal, isolation, wiring and safety conditions in full.

What to confirm before ordering an M850

• Whether the wall-mounted M850-W-x or panel-mounted M850-P-x enclosure and supply match the installation.

• The flowmeter technology, nominal range, output signal and calibration information.

• Whether the application needs pressure, temperature or both for compensation.

• The required number of optional passive 4-20 mA outputs and the available external loop supply.

• The binary output load, communication protocol, network topology and recording requirements.

• The required certification or inspection report, which the technical sheet says must be specified when ordering.

• The indoor location, panel aperture or wall fixing arrangement, IP protection and cable-entry method.

When specifying the Spirax Sarco M850, treat the product code, meter data, sensor ranges and control-system interfaces as one engineering selection. This keeps the flow computer, transmitters and flowmeter aligned and gives commissioning personnel the information needed to reproduce the intended calculation.