A chemical dosing line RFQ should pin the meter as a DN3-DN10 micro-bore Coriolis with ±0.1% of reading accuracy, 100:1 turndown, Hastelloy C-22 or 316L wetted parts, 4-20 mA HART plus Modbus output, and ATEX/IECEx Zone 1 certification for installation downstream of the metering pump.
The minimum dosing envelope to lock in the data sheet is: line size DN1-DN15, dosing rate 0.5-50 L/h, fluid temperature and concentration range, process connection type, required accuracy class, wetted material, output protocol, hazardous area classification, and proof pressure. Omitting any of these typically forces a requote cycle because the vendor cannot size the sensor or quote the right electronics variant [S1][S4]. For background on the operating principle, the coriolis flowmeter encyclopedia entry covers the underlying mass-flow physics and why it outperforms volumetric meters under density or temperature shifts.
Line Size, Turndown, and the Dosing-Pump Interface
Micro-bore tube sizes DN1-DN6 are specified when the dosing flow sits in the 0.5-50 L/h band typical of coagulant, flocculant, hypochlorite, and antiscalant injection, because standard-size Coriolis sensors cannot resolve the small Coriolis force at these low flows and need micro-tube geometry with high-performance DSP to stay stable at milligram-per-second mass flows [S4]. On a 3/8 inch (DN10) dosing skid, the meter is mounted downstream of the metering pump where Coriolis tolerates the periodic pulsation of plunger and diaphragm pumps up to a defined limit, with a pulsation attenuator added between pump and meter to keep zero drift in check [S3][S4].
Turndown should be specified at 100:1 minimum so a single meter covers the full ramp from commissioning flow to full production rate, and the operating flow should be set at 60-80% of nominal so the sensor never sits at the bottom of its range. A typical sizing line on the RFQ reads: minimum flow 1 L/h, normal flow 8 L/h, maximum flow 80 L/h, with turndown not less than 100:1 [S4].
Accuracy, Wetted Materials, and Pressure Rating
Accuracy on dosing Coriolis meters is stated as ±0.1% of reading rather than full scale, which matters on small flows because a 1% full-scale error on a 10 L/h line is a 0.1 L/h deviation, while 1% of reading stays flat across the range [S4]. Wetted materials should be declared by the buyer, not left to the vendor, with 316L stainless steel as the default and Hastelloy C-22 specified explicitly for hydrochloric acid, ferric chloride, sodium hypochlorite above 10%, and any other halide-bearing service where 316L would pit [S3].
Pressure rating on a DN10 dosing line is commonly 100 bar process / 150 bar proof, with the process connection typically threaded NPT or flanged per DIN/ANSI, and the electronics enclosure rated to at least IP66/IP67. Buyers in chemical dosing should also pin the operating temperature window on the RFQ, since wetted elastomers and transmitter derating both shift at the temperature extremes typical of polymer and acid lines [S3][S4].
Output, Diagnostics, and Hazardous Area
Specify dual output: 4-20 mA HART primary variable (mass flow) plus a digital secondary channel (Modbus RTU or HART multidrop) carrying density and temperature for redundant monitoring of dosing quality, because a HART signal is FSK superimposed on a 4-20 mA analog loop and does not require a separate digital bus. Output #1: 4-20 mA + HART 7, primary variable mass flow kg/h, secondary variables density g/cm³ and temperature °C; Output #2: Modbus RTU on RS-485 for DCS integration [S1][S3][S4].
Hazardous area rating must be written as ATEX II 2G Ex d IIC T6 Gb and IECEx equivalent for Zone 1 dosing rooms, with Ex ia on the signal/power side per IEC 60079-11 practice, and ATEX/IECEx certification always required when the dosing line sits inside a classified area of a chemical or pharmaceutical plant [S3][S4]. Add diagnostics for empty pipe, low density (pump lost prime), entrained gas, and tube coating to the RFQ as selectable outputs, since a low-density alarm is the one that catches pump cavitation a turbine meter never sees [S3].
Options: Coriolis vs Oval Gear vs Mag vs Turbine
Coriolis is the first choice when ±0.1% mass accuracy, density/temperature output, and insensitivity to fluid property shifts are required; oval gear is the budget pick at roughly 60% of the Coriolis price for viscous fluids up to 1,000 cP; mag meters are limited to conductive fluids and are unsuitable for hydrocarbons or deionized water; turbine meters drift 1-2% per 10 °C temperature change and are not recommended when the dosing fluid sees daytime temperature swings [S2][S3].
For comparison on a 3/8 inch chemical dosing skid: Coriolis (DN10, 316L or Hastelloy C-22, 100 bar, ±0.1% of reading, 4-20 mA HART, ATEX/IECEx Zone 1, pulsation-tolerant) vs oval gear (DN10, PPS or PTFE gears, ±0.5% of reading, frequency or pulse output, no density output, viscosity-limited above 1,000 cP) vs mag (DN10, PTFE or rubber liner, ±0.5% to ±1% of reading, conductive fluid only, no hydrocarbons) vs turbine (DN10, 316L, ±0.5% of reading, clean fluid only, drifts with temperature). The trade-off is cost vs accuracy vs fluid compatibility vs diagnostic richness [S2][S3][S4].
Failure Modes and Process Integration
The four failure modes that cause the most field complaints on chemical dosing Coriolis meters are: entrained gas and two-phase flow, which distorts the Coriolis signal and forces the transmitter into a bubble-handling mode; pulsation-driven zero drift on poorly attenuated metering-pump lines; crystallising or plating fluids that foul the measuring tube and shift the zero baseline; and air ingress on the suction side of the metering pump, which is the leading cause of low-flow instability [S3][S4].
Mitigations to write into the RFQ are: full single-phase pipe flow guaranteed by the process layout, a pulsation attenuator sized for the pump stroke volume between pump and meter, tube material selected to the most aggressive chemical in the matrix (Hastelloy C-22 for halides and strong oxidisers, 316L for mild acids and bases), and a transmitter with selectable digital filtering for pulsation. Buyers specifying a chemical dosing line can also reference the dosing-system integration on a packaged skid, which is covered in the automatic molding line entry on process-skid architecture, as well as the molding line encyclopedia page for skid-side flow and material handling context [S4].
RFQ Line-by-Line: What to Write on the Inquiry
Line 1, fluid: chemical name, concentration range, density range, viscosity range, temperature range, and solids/gas content. Line 2, line: size (DN3 / DN6 / DN10), process connection, max flow, normal flow, min flow, line pressure, and proof pressure. Line 3, meter: Coriolis, micro-bore, accuracy ±0.1% of reading, turndown 100:1, wetted parts 316L or Hastelloy C-22 as the chemical dictates, pressure rating to 100 bar, IP66/IP67 enclosure. Line 4, output: 4-20 mA HART 7 primary, Modbus RTU secondary, density and temperature on digital channel. Line 5, hazardous area: ATEX II 2G Ex d IIC T6 Gb, IECEx equivalent, Ex ia signal side. Line 6, diagnostics: empty pipe, low density, entrained gas, tube coating. Line 7, accessories: pulsation attenuator, mounting bracket, calibration certificate. Omitting lines 1-2 typically triggers a clarification loop and inflates the quote by 10-20% because the vendor sizes conservatively [S3][S4].
Two trackable signals to watch over the next quarter: a continued shift in the Emerson Micro Motion G200M-class transmitter supply chain as EPC contractors standardise dosing line specs across new chemical-plant builds [S6]; and OEM guidance from Endress+Hauser on the Promass F300 as a default dosing and blending spec, including refinery and petrochemical applications where mass-based custody transfer accuracy is mandatory [S5].
Background reading: Safety Interlock Switch Selection for Mining Operations.