Direct answer: a vibration transmitter 4-20mA 0-25 mm/s gives one number per point: overall RMS vibration velocity on the bearing housing, scaled so 4 mA = 0 mm/s and 20 mA = 25 mm/s.

That range suits most general-purpose motors, fans and pumps, because typical alert and trip levels in the ISO 20816 framework sit well inside 25 mm/s. When buying one, fix five things on the datasheet: measured quantity (RMS velocity), range, frequency band, mounting method, and how you will set alarm limits.

This guide is educational; it does not describe an Embedos product.

Who this is for: maintenance and instrumentation engineers writing a spec or purchase request for loop-powered vibration transmitters on motors, fans, blowers and pumps.

If you have not yet decided between 4-20 mA and RS-485 Modbus, read 4-20mA vs RS485 vibration sensor for motor monitoring first. 

What the transmitter measures

The usual trend value for rotating machines is broadband RMS vibration velocity in mm/s, measured on non-rotating parts such as bearing housings. ISO 20816-1 sets out general guidelines for measuring and evaluating machine vibration on these parts (ISO 20816-1). Velocity is used because, over the middle frequency range where imbalance, misalignment and looseness show up, it tracks vibration severity reasonably evenly.

A 4-20 mA transmitter contains an accelerometer, integrates the signal to velocity, filters it to a band, calculates RMS, and drives the loop. The 4 to 20 mA live-zero range is standardised in IEC 60381-1 (IEC webstore).

Check on the datasheet: RMS, not peak or peak-to-peak. Some transmitters offer “peak” velocity or “equivalent peak” (RMS × √2). Mixing them gives a 41% error when you compare against ISO-style limits.

Why 0-25 mm/s is the common range

Two reasons:

1. Resolution where it matters. On a 0-25 mm/s range, 1 mm/s is 0.64 mA (16 mA ÷ 25). Healthy machines often run at a few mm/s, so a wider range such as 0-50 mm/s halves the current change per mm/s and makes small trends harder to see on a 12-bit input.
2. Headroom above trip. Set alert and trip levels by evaluating against ISO 20816-3 zone boundaries for the machine group and foundation (ISO 20816-3). A 25 mm/s top leaves room above trip so the signal does not saturate during a fault.

Pick 0-50 mm/s or wider only for machines that run rough by design (some crushers, vibrating screens) where the maker states higher normal levels.

Scaling: velocity = (mA − 4) ÷ 16 × 25. At 8 mA, velocity = 4 ÷ 16 × 25 = 6.25 mm/s. Check your own with the 4-20 mA loop scaling calculator.

Frequency band

The broadband measurement band depends on machine speed per ISO 20816; read the band for your machine from the relevant part (ISO 20816-1).

Buyer checks:

• Does the datasheet state the band and does it suit your machine speed per ISO 20816? If not, ask.
• Slow machines: a fan at 300 rpm runs at 5 Hz. A transmitter whose lower cut-off sits near or above 5 Hz will miss most of its 1× vibration. Choose a transmitter with a lower cut-off well below running speed.
• Bearing defects: early rolling bearing faults show at higher frequencies and often in acceleration, not velocity. A 0-25 mm/s velocity transmitter is a general health trend, not a bearing diagnostic tool.

Mounting

Mounting changes the reading more than most spec items.

• Location: on the bearing housing, close to the load zone, in the direction you want to watch (horizontal on most horizontal machines, since stiffness is usually lowest there).
• Method: a stud on a spot-faced flat surface gives the best frequency response. Adhesive mounting pads are a common second choice. Magnets are for surveys. Vendor application notes show how the usable upper frequency drops as mounting gets less rigid; check the accelerometer maker’s mounting manual for the response of each method.
• Thread: confirm thread size (often 1/4-28 UNF or M8) and whether an adaptor is supplied.
• Cable: armoured or screened twisted pair, screen grounded at one end per your plant practice. On VFD-driven motors keep the run away from motor power cables.
• Enclosure: state the IP rating needed. Degrees of protection are defined in IEC 60529 (IEC webstore). For hosed-down areas ask for IP66 or IP67 and check the cable gland too.
• Hazardous areas: if the motor is in a classified zone, you need a certified transmitter and the right barrier. Do not substitute.

Download the Free “Vibration Transmitter 4-20mA 0-25 mm/s Checklist”

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Alarm limits: how to frame them

ISO 20816 evaluates vibration in two ways: absolute magnitude against zones, and change from a known baseline. Use both.

1. Classify the machine by type, power and foundation (rigid or flexible) as the relevant ISO 20816 part requires.
2. Read the zone boundaries from that part for your class. Zone A is new machines, B is acceptable for long-term running, C is limited running, D is damaging.
3. Set alert near the B/C boundary, or at baseline plus an agreed step if the machine normally runs well below it.
4. Set trip no higher than the C/D boundary, and only if your protection philosophy allows a trip from this signal.
5. Add a delay of a few seconds to ride through start-up transients.

Alert and trip values are site decisions. Write down which ISO part and table you used so the next engineer can check it.

Spec sheet checklist

Where the data goes

The 4-20 mA loop lands on a PLC analog input or a gateway. From there it can be trended and alarmed. Embedos has interfaced third-party vibration sensors with Embedos Edge, our platform of products, solutions, dashboards, insights and services.

A vibration monitoring software case records a historical EmView-named desktop deployment that logged spindle vibration over OPC UA; that page is a case note, not the current EmView product definition.

Today, readings can be shown on EmView monitoring screens (sign in at emcommandview.embedos.io). The transmitter itself is chosen from your preferred instrument vendor.

FAQ

Direct answer: use the pulse output when you need an accurate total (litres, m³ consumed per shift or batch), and use 4-20 mA when you need the live flow rate for control, trends or alarms.

Pulses are counted, so a total built from them has no integration error beyond the meter’s own K-factor. A total built by integrating a 4-20 mA signal accumulates the small offset, scaling and sampling errors of the analog chain. Many plants wire both: pulses to a counter or totaliser, 4-20 mA to the PLC loop, and a separate flow switch contact for a no flow alarm.

Who this is for: instrumentation and utility engineers wiring water, cooling water, fuel oil, chemical dosing or effluent meters into a PLC, panel totaliser or energy monitoring system.

The two signals in one line each

• Pulse (frequency) output: one pulse per fixed volume. Pulse frequency is proportional to flow rate; pulse count is proportional to total volume.
• 4-20 mA output: a current that maps the flow range linearly, 4 mA at zero (or range low) and 20 mA at full scale. The live-zero 4 to 20 mA range is standardised in IEC 60381-1:1982 (IEC webstore).

K-factor: turning pulses into litres

The K-factor is pulses per unit volume, given on the meter’s calibration certificate. A KEP application note puts it simply: the certificate gives the average K-factor, for example 200 pulses per US gallon or 150 pulses per litre, and that number is entered into the totaliser (KEP, Calculating and Using K-Factors).

Worked example (illustrative): a cooling water meter has K = 50 pulses per litre.

• Total: 1,250,000 pulses counted over a shift ÷ 50 = 25,000 L = 25 m³.
• Rate: pulse frequency 125 Hz ÷ 50 = 2.5 L/s = 9 m³/h.

Field check of K, as Seametrics describes: reset the total, set K = 1, run a known volume into a container, read the pulse count, divide pulses by volume (Seametrics field calibration bulletin). For example 500 pulses into a 10 L can gives K = 50 pulses/L.

Some meters, such as turbine meters, have a K-factor that varies across the flow range. Meter manuals state linearity; one turbine sensor manual lists ±3% of reading linearity and ±0.5% repeatability (GF Signet 2100 manual). If that matters, use a totaliser with multi-point K-factors.

4-20 mA scaling

Rate = range low + (mA − 4) ÷ 16 × (range high − range low).

Example: 0 to 50 m³/h range, signal 12 mA. Rate = (12 − 4) ÷ 16 × 50 = 25 m³/h. You can check your own numbers with our 4-20 mA loop scaling calculator.

Why integrating 4-20 mA drifts

To get a total from 4-20 mA, the PLC samples the rate and adds rate × time each scan. Errors add up:

1. Zero offset: a meter at true zero sending 4.05 mA reads 0.05 ÷ 16 × 50 = 0.16 m³/h. Over 24 h that is about 3.75 m³ of “flow” that never happened. Use a low flow cut-off.
2. Span error: analog input and transmitter tolerances scale every reading.
3. Resolution: an input with limited counts over 4 to 20 mA quantises the rate.
4. Sampling: if flow pulses or changes fast between samples, the sum misses it.
5. Restart loss: a PLC restart without retained memory can reset the total.

Pulses avoid points 1 to 4 because each pulse is a counted, fixed volume. They still need retained counters and protection against missed pulses.

PNP, NPN and open collector outputs

Pulse outputs are typically open collector (NPN, sinking), PNP (sourcing), or a passive contact (reed). The input must match:

• NPN / open collector: the output pulls the line to 0 V. The input must be a sourcing type (it supplies current that the output sinks), or you add a pull-up resistor to supply. Check the meter’s maximum switching current and voltage.
• PNP: the output drives the line to supply voltage. Use a sinking type PLC input (current flows from the output into the input).
• Reed or mechanical contact: low frequency only, and contact bounce needs input filtering.

IEC 61131-2:2017 defines Type 1, Type 2 and Type 3 24 V DC digital inputs, each with its own on and off voltage and current limits; read the exact thresholds from your PLC input datasheet (IEC webstore). A pulse output has to cross both thresholds cleanly.

Maximum frequency

At full flow, pulse frequency = K × max flow. Example: K = 50 pulses/L, max flow 5 L/s gives 250 Hz. Check three limits:

• The meter’s maximum output frequency and minimum pulse width
• The input’s maximum counting frequency (ordinary PLC digital inputs are filtered; a high speed counter input may be needed)
• The input filter time, which must be shorter than the pulse width

If frequency is too high, many meters let you set a pulse scaler (one pulse per 10 L). Each pulse then represents more volume, which lowers total resolution but stays accurate.

Download the Free “Flow Signal Wiring and Totaliser Checklist”

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Cable length and noise

• 4-20 mA is a current loop, so voltage drop along the cable does not change the reading as long as the loop supply covers transmitter minimum voltage plus load and cable resistance. That makes it tolerant of long runs. Check the loop voltage budget on the transmitter datasheet.
• Pulse signals are voltage edges. Long cables add capacitance that rounds edges and can pick up spikes from VFD and motor cables, causing false counts. Use shielded cable, earth the shield at one end, keep it away from power cables, and use input filtering suited to the max frequency.
• In a steel plant or rolling mill bay with large drives, route signal cables in separate trays from power. In process plants with long field runs, 4-20 mA for rate is often the easier signal to keep clean.

Use both, plus a flow switch

A common pattern in Indian utility and process plants:

The flow switch is a separate discrete device (or a configurable meter relay output) so that a loss of cooling water trips a furnace or compressor even if the PLC analog chain fails. Wire it fail safe: contact closed when flow is healthy.

If the meter supports a fieldbus or Modbus output, it may give rate and total over one cable. That is outside this guide’s scope.

If you want a panel display for this, the Em Bose EB series has digital inputs for pulse signals and analog inputs for 4-20 mA, and a Flow Totalizer application. Check the live product page for input ratings against the maximum pulse frequency you worked out above.

For PLC and SCADA telemetry (tag values, alarms, remote diagnostics) a 4G LTE industrial router with dual SIM failover is usually enough, because the data is small and LTE coverage is wider.

Choose 5G when you have a measured need for high bandwidth (several camera streams, large file transfers) or lower latency, and you have tested 5G signal at the exact panel location on more than one operator.

Whatever you buy for use in India, check its TEC MTCTE certification status.

Who this is for: plant engineers and IT/OT teams choosing a cellular router for remote monitoring of PLCs, SCADA nodes, pump stations, DG sets or solar sites.

Start with the data, not the radio

Estimate what you actually send. A worked example (illustrative):

• 300 tags, each a 4 byte value plus about 50 bytes of protocol overhead, sent every 10 s
• 300 × 54 bytes = 16.2 kB per update, 8,640 updates per day
• About 140 MB per day, roughly 4.2 GB per month, before compression or report by exception

Even this generous case is a small, steady stream, well within LTE uplink capability. Report by exception (send only on change, as MQTT and many SCADA protocols support) can cut it further.

One 1080p stream, by contrast, typically needs several Mbit/s continuously (the YouTube encoding guide recommends 8 Mbit/s for 1080p at 30 fps; H.265 CCTV streams are often lower, so check the camera) (YouTube recommended upload encoding settings).

That is where bandwidth starts to matter.

Measure your own: most routers show data counters. Run a pilot for a week and look at the monthly total and peak.

What 4G gives you

• Coverage: LTE networks in India are much wider than 5G, especially in industrial estates outside city centres. Check operator coverage maps, then test at site.
• Bandwidth: LTE category (Cat 1, Cat 4, Cat 6 and above) sets peak rates; for telemetry even Cat 1 is often enough. Categories are defined by 3GPP in TS 36.306 (3GPP specifications).
• Power and heat: 4G modules generally draw less than 5G modules, which helps in sealed panels and solar powered sites. Compare datasheet figures for the exact models.
• Cost: 4G routers and plans are generally cheaper. We do not quote prices here.

What 5G adds

5G NR is specified by 3GPP from Release 15 onward (3GPP Release 15). For a plant router it can add:

• Higher uplink and downlink throughput, useful for video, large log uploads or remote desktop to many HMIs
• Lower air interface latency, which helps interactive remote work. End to end latency over public networks still depends on the core network and internet path
• Release 17 introduced reduced capability (RedCap) devices for mid range IoT (3GPP Release 17); check operator support before relying on it

What it does not do: make a closed loop control link over a public network safe. Keep control in the PLC; use cellular for supervision and data.

Most 5G routers fall back to 4G when 5G is not available. Confirm the fallback bands too.

Coverage is the deciding factor

Plants are hard RF places: steel sheds, concrete, panels inside panels. Before you buy:

1. Use a phone or a test router at the panel location, not at the gate.
2. Record signal values (RSRP, RSRQ, SINR) for each operator, for 4G and 5G. These are defined in 3GPP TS 36.214 for LTE and TS 38.215 for NR.
3. Test at different times of day.
4. If signal is weak inside, plan an external antenna on the roof with low loss cable. Antenna gain does not fix a dead site.

Check which bands each operator uses in your area and that the router supports them. India’s spectrum assignments are published by DoT (Department of Telecommunications).

Dual SIM failover

For telemetry, reliability usually matters more than speed. Dual SIM lets the router switch to a second operator when the first fails. Ask:

• Is it dual SIM single standby (one active at a time, switch on failure) or dual modem (both active)? Single standby is common and cheaper; switching takes time.
• What triggers failover: loss of registration only, or a failed ping to a set host? Ping based checks catch more real failures.
• Does it fail back to the primary SIM, and when?
• Does the VPN reconnect automatically after a switch?

Pair it with local buffering at the gateway or PLC so data during an outage is sent later, not lost.

Security basics

Whatever the radio, follow the same rules as any remote access to control systems:

• No open ports from the internet to PLCs or HMIs. Use a VPN or an outbound only broker connection.
• Private APN from the operator if available.
• Change default passwords, keep firmware updated, turn off unused services.
• Log remote sessions.

CISA’s primary mitigations for operational technology say to remove OT connections to the public internet and, where remote access is needed, use a private network or VPN with phishing-resistant multi-factor authentication (CISA, Primary Mitigations for OT). The IEC 62443 series is the reference for zones and conduits (ISA/IEC 62443).

India buyer check: TEC MTCTE

Under the Indian Telegraph (Amendment) Rules 2017, the Telecommunication Engineering Centre (TEC) runs the Mandatory Testing and Certification of Telecom Equipment (MTCTE) scheme.

The TEC notice dated 1 July 2024 (Annexure-I) lists Routers under Phase IV, mandatory from 1 August 2024, and IoT Gateway under Phase III, mandatory from 1 January 2024.

There is no separate cellular router category, so ask the seller which category their model is certified under (TEC MTCTE notice). Ask the seller for the MTCTE certificate number for the exact model and check it on the TEC portal (tec.gov.in).

Also ask whether a WPC Equipment Type Approval (ETA) applies to the radio in that model (DoT).

Side by side

If your need is small Modbus telemetry over 4G rather than a general purpose router, see our 4G Ethernet Modbus gateway with Wi-Fi page for its 4G SIM, Wi-Fi and Ethernet options.