DP level, wet legs and hydrostatic measurement

My DP level transmitter reading is frozen and won't move — is the impulse line plugged?

Probably. Equalize the transmitter at the manifold and the reading should move to whatever output corresponds to 0 inH2O differential. If it moves, the transmitter is clear and the impulse lines are the fault.

Know what "0 inH2O" looks like on your loop before you do this. On a dry leg with LRV = 0, equalizing drives the output to 4 mA. On a closed tank with a wet leg and an elevated zero — say LRV −120 inH2O, URV −30 inH2O — zero differential sits above URV, so the output goes into high saturation instead: 20.8 mA on a Rosemount standard configuration, 20.5 mA configured to NAMUR NE 43. It does not go to 21.75 mA or 22.5 mA. Those are the high alarm currents on those same two configurations. An over-range input is a valid measurement outside the range, not a device fault, so the transmitter saturates rather than alarms. NE 43 puts measuring information between 3.8 and 20.5 mA and failure information at ≤3.6 mA or ≥21.0 mA. The bands between 3.6–3.8 mA and 20.5–21.0 mA are left empty on purpose, so a saturated signal can never be mistaken for a fault. Alarm and saturation levels are ordering options and are configurable. Confirm the ones on the tag before you read anything into the number you get.

Use the correct manifold sequence so you do not over-range one side. Close the low-side block, open the equalizer, close the high-side block. Reverse it on the way back: with the equalizer open, open the high-side block, close the equalizer, open the low-side block. The order exists so the cell never sees full static pressure on one side while the other side is vented. Never open the equalizer with both blocks still open. On a wet leg that is how you fix a plug and create an evaporated-wet-leg fault in the same trip — the higher head on the high side pushes fill fluid up and out of the reference leg. If the manifold manufacturer's instructions or the site procedure give a different order, follow those.

The other tell is noise. A healthy DP level signal is never perfectly still; it carries a few tenths of a percent of movement from agitation, boiling, and pump pulsation. A plug behaves like a low-pass filter, so the signal goes quiet before it goes dead. A reading that has not changed a digit in an hour is a plug until proven otherwise. Allow for the configured damping before you call the equalize test failed, and check that the number you are watching is live. A value held in the DCS, a stale HART multiplexer poll, or an output already sitting at the same saturation limit will not move either, and none of those is a transmitter fault. If the reading truly does not move at the transmitter terminals, the candidates are the transmitter, a passing equalizing valve, or a plugged sensor port — not an impulse line. Slope impulse lines back toward the tap. 1 in per foot is the common working figure, but use the site's installation standard. Blow down each leg separately.

Why does my DP level transmitter read high when the wet leg fluid evaporates?

The wet leg is on the low side. Every inch of fill it loses takes that inch times the fill SG off the LP head, and the transmitter sees an equal rise in differential pressure. It reports that as level.

Put numbers on it. A 120 in wet leg filled with water at SG 1.00 sits at 120 inH2O on the LP side. Lose 12 in of fill and the LP head drops to 108 inH2O, so the differential rises by 12 inH2O. On a product of SG 0.90 that is 13.3 in of level that does not exist. Water is the easy case — an inch of fill is an inch of water column. With a glycol/water fill at SG 1.05 or a silicone fill at SG 0.93, multiply the lost height by the fill SG first.

Direction matters more than magnitude. A draining wet leg tells the operator there is more liquid than there is, so on a boiler drum, a knockout pot, or a flare seal drum, real level falls while the indication looks fine. Usual causes: a weeping bleed valve or fitting on the LP side, a hot service that boils the leg off, a fill fluid too volatile for the temperature, a condensate fill pot that has stopped condensing because someone insulated or heat traced it. Refill it, then find the loss. Re-zeroing a wet leg that is losing fluid buys a few days and then lies again.

Why does my level reading change when the specific gravity of the product changes?

A DP transmitter measures head, not level. Indicated level is head divided by the specific gravity you calibrated for, so a percentage change in density moves the reading by the same percentage.

Take a transmitter ranged 0 to 100 in on a product of SG 0.90, so 0 to 90 inH2O. Fill the same tank to a true 100 in with SG 0.85 product and it produces only 85 inH2O. The transmitter reports 94.4 in. That is 5.6% low, exactly the density error. Temperature does this quietly on hydrocarbons. A 30 API crude has a thermal expansion coefficient of roughly 0.0004 per °F, so a 100 °F swing moves density on the order of 4% and takes the level indication with it. For anything that has to be right, use the actual API MPMS Chapter 11.1 correction for the product rather than a single coefficient.

Steam drums are the extreme case. At 1000 psia, saturated water has a specific volume of about 0.0216 ft³/lb — 46.3 lb/ft³ — against 62.4 lb/ft³ cold, and the saturated steam above it is about 2.24 lb/ft³. A drum level calibrated on cold water at hydrostatic test will not agree at operating pressure. Two fixes: density compensation from a pressure or temperature input, or a technology that measures the surface position rather than the head. Guided wave radar is the usual choice, and it is indifferent to density. It is not indifferent to the vapor space. Steam slows the pulse, so a GWR calibrated in air reads low on a pressurized drum, and vendors put the uncompensated error as high as 20% on high-pressure steam. The cure is a transmitter that measures the vapor dielectric against a reference reflector at a fixed distance on the probe and corrects for it, sold under names like dynamic vapor compensation. Specify that feature on a drum rather than assuming it. Non-contacting radar is rarely workable on a drum because of the internals and the nozzle geometry.

Why does my 4-20mA level signal jump around when the VFD is running?

The drive is coupling common-mode noise into the loop through cable routing and grounding. The cure is in the cable and the ground. It is not in the transmitter's damping setting.

A VFD switches at a carrier of 2 to 16 kHz, and the IGBT edges are fast — rise times on the order of 50 to 200 ns, faster still on SiC devices. On a 480 V system with a DC bus near 650 V, that is thousands of volts per microsecond on the motor leads. Long leads cause a reflected wave, and peak voltage at the motor terminals reaches two to three times bus voltage. That energy couples capacitively into anything running beside it. Confirm the source before you chase it: stop the drive. Signal goes quiet, it is the drive. Signal stays noisy, you are looking at real process movement from agitation or a pump.

• Shielded twisted pair for the 4-20 mA, with the shield grounded at one end only — the control system or power supply end — and floating at the field device. A shield grounded solidly at both ends is a circulating current path, not a shield. One exception: where high-frequency pickup dominates, some standards call for a capacitor at the field end to drain RF while still blocking the power-frequency loop current.
• Separate signal cable from motor leads and other power conductors — 12 in is the figure most site standards use as a minimum — and cross them at 90° where a crossing is unavoidable. Never share a tray or conduit with VFD output. Use the separation your own installation standard specifies.
• VFD output should be on VFD-rated cable with a symmetrical ground conductor, and the motor frame bonded back to the drive chassis, not just to local steel.
• Check for a second ground on the loop. One reference point only.
• Verify with a scope across the 250 Ω sense resistor rather than a handheld meter — a meter averages the noise away and tells you nothing.

Raising transmitter damping to 8 or 16 seconds makes the display look better. It is the wrong answer. It hides the noise from the operator, and it hides real process upsets along with it.

Why is my submersible hydrostatic level transmitter drifting — is the vent tube blocked?

Check the vent tube first. A submersible transmitter is a gauge sensor: the back of its diaphragm references atmosphere through that tube, and any barometric change it cannot follow shows up directly as level error.

The scale is worth knowing. 1 inHg of barometric change is about 13.6 inH2O, which is 1.13 ft of water. Ordinary weather moves the barometer 0.3 to 0.5 inHg in a day. A fully blocked vent therefore drifts roughly 4 to 7 inH2O, tracking the weather and nothing in the tank. That correlation is the diagnosis. Plot the level against barometric pressure; if they move together with no flow in or out, the vent is the fault.

Pull the termination and look at the desiccant cartridge. Indicating desiccant changes color when it is saturated, and a saturated cartridge lets moisture into the tube. Water in the tube gives step changes rather than smooth drift — the slug moves, then sticks. Check the cable for a crush or a kink. Confirm the junction box is not sealed airtight with no desiccant vent path. If the vent checks out, what is left is silt or grease on the diaphragm, a product density change, and sensor temperature effects.

Why does my capacitance level probe read high when there's product coating on it?

The coating bridges the probe to the vessel wall above the real surface. That adds capacitance and conductance, the instrument counts both as product, and a plain capacitance probe cannot tell a film from a level.

Conductive coating is the worst case. Water, brine, and most aqueous products couple resistively up the length of the probe, and the reading can pin at 100% with the tank half empty. A non-conductive film gives a smaller error, set by its dielectric constant. The direction is the same either way: high, never low. Dielectric constant drives the whole measurement — water is about 80 at room temperature and most hydrocarbons are 1.9 to 2.5, so a probe calibrated on one and used on the other is wrong before any coating appears.

The fix is an RF admittance probe with a driven shield. A guard section sits between the process connection and the sense element, held at the same potential as the sense electrode. No current flows through the film on the guard, so the coating stops contributing. In conductive liquids the probe also has to be fully insulated in PTFE or PFA, or the measurement collapses. Hot, conductive, and heavily coating together beat every capacitance probe eventually. Go to a different technology: DP with flush-face seals, or non-contacting radar.

What's the difference between a wet leg and a dry leg, and how do I calculate the DP level transmitter calibration?

A dry leg is an empty low-side impulse line, used where the vapor space will not condense. A wet leg is that same line filled on purpose with a known liquid, so condensing vapor cannot build an unknown and changing head of its own.

The choice comes down to whether the vapor condenses. Steam, water vapor, and most condensable hydrocarbons will fill a dry leg with an uncontrolled and variable head. That is the failure the wet leg exists to prevent. A dry leg needs a drip pot or condensate trap and a slope back toward the tank so anything that does condense drains away. A wet leg needs a fill that stays put — glycol/water, the process condensate itself, or a fluid chosen to be heavier than the product on an interface service — plus heat trace and insulation if the site ever goes below the fill's freeze point.

The calibration is one equation: DP = HP − LP. Three heads go into it. The product head (level × product SG). The wet leg head (wet leg vertical height × fill SG). Any elevation of the transmitter below the lower tap (that distance × the SG of whatever fills the HP line). A dry leg on a tank ranged 0 to 100 in of SG 0.90 product, transmitter at the tap elevation, gives LRV = 0 inH2O and URV = 90 inH2O. Add a wet leg and the LRV goes negative. That is zero elevation, and it is the next section.

How do I calculate LRV and URV for a closed tank DP level transmitter with a wet leg?

LRV is the differential pressure at empty, URV is the differential pressure at full. With a wet leg both are negative: LRV = −(wet leg height × fill SG), and URV = LRV + (level span × product SG).

Worked example. Closed tank, HP tap at the bottom, LP tap in the vapor space, wet leg running from the LP tap down to the transmitter, transmitter mounted at the HP tap elevation. Wet leg vertical height 120 in, filled with water at SG 1.00. Product SG 0.90, level range 0 to 100 in above the HP tap.

• Empty: HP = 0 inH2O, LP = 120 × 1.00 = 120 inH2O, DP = −120 inH2O. That is the LRV, and it is 4 mA.
• Full: HP = 100 × 0.90 = 90 inH2O, LP = 120 inH2O, DP = −30 inH2O. That is the URV, and it is 20 mA.
• Span: 90 inH2O. Zero elevation: 120 inH2O.

Sanity check every time. URV − LRV must equal level span × product SG, or 90 inH2O here. If it does not, the arithmetic is wrong. Mount the transmitter below the HP tap and you add that vertical distance times the SG of the HP line fill to both LRV and URV; that is zero suppression, and it shifts both ends equally. Confirm the sensor's URL covers the negative range — you need a bidirectional cell, not a 0-to-full-scale one. A 250 inH2O URL cell rated −250 to 250 inH2O, say a Rosemount 3051CD Range 2, handles −120 to −30 at a turndown of about 2.8:1. Confirm the reference temperature behind your inH2O numbers too: 1 psi is 27.68 inH2O at 39.2 °F, 27.71 at 60 °F, and 27.73 at 68 °F.

How do I calibrate a level transmitter with remote diaphragm seals and capillaries?

With the seals attached, in the orientation they will be installed in. The seals, capillaries, and fill fluid are part of the measuring element, and the fixed head the fill puts on the loop belongs in your LRV rather than being trimmed away blind.

Seal head is the vertical separation between the two seal diaphragms multiplied by the fill fluid SG. The sign follows which seal is higher. On a closed tank with the LP seal up in the vapor space it behaves exactly like a wet leg and drives the LRV negative. Silicone 200, the most common fill, is SG 0.934 at 25 °C with a service range of −45 to 205 °C (−49 to 401 °F). So 100 in of vertical separation between seals puts 93.4 inH2O of static offset on the low side before there is any product in the tank. In the field, zero trim in place with the vessel confirmed empty. That catches the seal head, the mounting orientation, and any residual in one step. On the bench, apply pressure at the process face of the high-side seal with the assembly held in its installed orientation, using a test flange or a water column.

Two things ruin a seal system. The first is breaking the capillary or the seal-to-transmitter joint. The fill is charged under vacuum at the factory and cannot be replaced in the field. A kinked capillary, a dented diaphragm, a system that has lost fill — those are replacements, not repairs, and anyone offering to top one off in a shop is not doing you a favor. The second is mismatched capillaries. Route both together and keep them the same length so ambient temperature effects cancel; a sun-exposed low-side capillary paired with a shaded high-side capillary produces a level indication that follows the sun. Seal temperature effect is specified against a stated ambient change — Rosemount data sheets use 50 °F — and it grows with capillary length and bore. Silicone 200 at about 10 cSt at 25 °C gets slow in long, small-bore lines in cold weather. Other makers state seal temperature effect differently. Compare like for like before you trust a number off a data sheet.

Why is level control so hard to tune, and why does normal PID tuning make an integrating process swing?

Level is an integrating process. Step the valve and level ramps; it does not settle at a new value. That gives the loop a lower gain limit as well as an upper one, which a flow or pressure loop does not have — so the reflex of cutting controller gain to calm a swing makes the swing worse.

On a self-regulating loop, lowering gain always adds stability. Not here. The controller's integral action and the process's own integration combine, and below a certain gain they roll into a slow cycle whose period only gets longer the more you detune. You have a window of workable gains and a window of workable reset times, not just an upper bound on each. Gain and reset move together. Halve the controller gain, at least double integral time. Decreasing gain without lengthening reset in proportion is what produces that near-sustained slow rolling oscillation people then try to fix with still less gain.

Match the tuning to what the tank is for. A surge or averaging tank exists to absorb variation. Use P-only, gain 1 to 2, bias at 50%, and accept the offset — holding a surge tank at setpoint just passes the upset downstream. Tight level, on a drum or a separator interface, wants PI with lambda tuning: pick the closed-loop arrival time you actually want and compute from it. Derivative is the wrong tool on level as a working rule. The signal is noisy and derivative amplifies noise. Where a drum needs help with shrink and swell, the answer is three-element control with feedforward on steam and feedwater flow, not a derivative term. And before you touch any constant, check the valve. A control valve with 2% of stiction will cycle a level loop forever, and no set of tuning constants fixes a sticking valve.

When do you use P-only, PI, or full PID on flow, level, pressure, and temperature loops?

PI on flow and liquid pressure. P-only, or PI with a long reset, on level and gas pressure. Full PID on temperature. The split follows whether the process is fast and noisy, integrating, or slow with real dead time.

• Flow: PI. Fast, noisy, essentially no dead time. Low gain — typically well under 1, often 0.1 to 0.5 — with a short reset, commonly 0.05 to 0.5 min. No derivative; it just amplifies the turbulence in the signal.
• Level: P-only for surge and averaging duty (gain 1 to 2, bias 50%); PI for tight control, with reset long enough to satisfy the integrating rule. No derivative.
• Pressure, liquid-filled system: behaves like flow. PI, fast, no derivative.
• Pressure, gas in a large vessel: integrating, same as level. Tune it like level, not like flow.
• Temperature: PID. Large lag, real dead time, and a signal smooth enough that derivative earns its place — Td is usually set at Ti/4 to Ti/6.

Those are common working ranges, not standard values. They move with valve sizing, transmitter span, and vessel residence time. The loop on your unit gets tuned from its own step test.

Derivative is the exception, not the default. If you cannot see a clean trend on the PV, you cannot tune with derivative. Add it to a noisy loop and the valve never stops moving; the packing pays for it. P-only on an averaging level loop is a deliberate design choice, not an unfinished job.

What level sensor works best in a slurry tank with buildup, foam, and an agitator?

Two hold up here: a DP transmitter with flush-face or extended remote seals, and 80 GHz non-contacting radar. If the foam is thick and it stays, neither one is reliable, and the honest answer is vessel weighing or a gamma gauge.

• DP with flush or extended diaphragm seals: a flush diaphragm keeps measuring head through a film, and a 2 in or 4 in extended diaphragm pushes past the nozzle so the pocket cannot silt up. Weakness: it measures head, so any change in solids loading changes the reading with no change in level.
• 80 GHz non-contacting radar: a narrow beam — about 3° off an 80 mm (roughly 3 in) antenna on an 80 GHz device — can be aimed into the gap between the agitator and the wall, and it measures distance, so density does not matter. Beam angle is antenna-specific, so read it off the data sheet for the model and antenna you are actually buying. Weakness: dense foam absorbs the microwaves and the echo drops into the foam or is lost, and wet buildup on the antenna kills it. Use a flushed or PTFE-faced antenna.
• Guided wave radar: usually the wrong choice here. The probe coats and a strong agitator will bend or break a single rod.
• Capacitance: coats, and reads high when it does.
• Gamma: works through the wall and ignores everything inside, but the license, the wipe tests, the survey records, and eventual disposal are permanent obligations. Last choice.

If the vessel sits on legs and the piping can be flexible-connected, price load cells before any of the above. Nothing inside the vessel can coat them or foam over them. Know what they give you, though. Load cells weigh contents, so you still need density to turn mass into level or volume. Rigid piping, wind load, and thermal growth on the supports all steal accuracy if the installation does not account for them. If the number has to be right for a mass balance or custody, weigh it. Nothing that looks through foam is going to give you that.

Tuning fork level switch vs float switch — which is better for a high level alarm?

The fork, in almost any dirty service. It has no moving linkage to stick, and it keeps working through coating, foam, turbulence, and bubbles. A float depends on a mechanism, and that mechanism's failure mode is to sit silently in the last position it held.

A fork is driven at its own resonant frequency, and that frequency drops when the tines are wetted; the electronics switch on the change. The frequency itself is a few hundred hertz and varies with fork geometry and manufacturer, so do not carry a number from one model to another. What matters on a spec sheet is density and viscosity. On the Endress+Hauser Liquiphant M family the minimum medium density is 0.5 g/cm³ (0.4 g/cm³ available as an option, and some coated models ship set for 0.7 and are switched down). Viscosity is capped at 10,000 mm²/s. Above that the fork damps out and stays latched wet. Mount the tines out of the fill stream and with clearance from the wall so buildup cannot bridge them.

Floats are still the right call in clean water, in a sump, and where no power is available, because a mechanical float switch needs no electronics at all. They are the wrong call anywhere the product coats. A stuck float reads normal. The proof test usually settles the argument: fork switches are available for use in safety systems to IEC 61508/61511 with an electronic test that exercises the sensing element in place, while a float that requires draining a vessel to verify will not actually get tested on schedule. Read the safety manual for the specific model — SIL 2 is typically the single-device rating, and SIL 3 usually requires redundancy.

How does a displacer level transmitter work and how do you calibrate one?

A displacer does not float. It hangs fixed and gets lighter as liquid rises around it, and a torque tube converts that weight loss into a small rotation the transmitter reads. Span comes from the displacer's volume and the product's specific gravity, not from level alone.

The buoyancy force is the displaced volume times the liquid density. Water is 0.0361 lb/in³, so full-scale force change is displacer volume in in³ × SG × 0.0361 lb. The displacer length is the level span. Standard displacer lengths in the Fisher 249 family are 14, 32, 48, 60, 72, 84, 96, 108, and 120 in; check the bulletin for the specific model, because not every 249 style offers the whole list. Calibrate wet or dry. Wet: fill the cage with water in increments, set zero at empty and span at full. Dry: hang calibrated weights from the stem equal to the buoyancy at 0%, 50%, and 100%.

What catches people is that span scales directly with specific gravity. Calibrate on water at SG 1.00, put the displacer into an SG 0.80 product, and a 100 in displacer reads 80 in with the cage completely full. Calibrate at the actual process SG, or correct the weights. Interface service is its own problem. The displacer is fully submerged at all times, so the span comes from the density difference between the two phases, and a 0.15 SG difference is a very small force change that needs a long or large-volume displacer to be usable. The failure that gets blamed on the instrument is usually not the instrument. The lower equalizing connection on the cage silts up, the cage stops tracking the vessel, and it looks exactly like a plugged impulse line.

How do I build a tank strapping table to convert level to volume in a horizontal tank?

For the cylindrical shell, volume at liquid depth h is V = L × [R²·cos⁻¹((R−h)/R) − (R−h)·√(2Rh − h²)] with the arccos in radians, and the heads are added separately — for a pair of 2:1 elliptical heads their combined liquid volume is (π·h²/12)·(3D − 2h).

Worked example: an 8 ft diameter tank (R = 48 in) with a 20 ft straight side (L = 240 in), at a liquid depth of 24 in. The arccos term is cos⁻¹(24/48) = 1.047198 rad, so 2304 × 1.047198 = 2412.74. The second term is 24 × √(2304 − 576) = 24 × 41.5692 = 997.66. The difference, 1415.08 in², times 240 in gives 339,620 in³, or 1470.2 gal. The two 2:1 elliptical heads add (π × 576/12) × (288 − 48) = 36,191 in³, or 156.7 gal, for a total of 1626.9 gal. That head term assumes both heads are identical 2:1 semi-elliptical; dished, hemispherical, or flat heads need their own formula.

Build the table in fine steps near the bottom and the top, where a small change in depth is a large change in volume relative to the reading. 1/4 in or 1/2 in increments through the first and last 10% is reasonable. Then be clear about what you have. This is geometry, not the tank. Real tanks are out of round, tilted, and full of heating coils, manways, and internals. A 1° tilt over 240 in moves one end 4.2 in, and at low level on a horizontal tank that is a serious volume error. If the table has to be right for custody transfer, inventory reconciliation, or regulatory reporting, it is measured, not calculated, under API MPMS Chapter 2.2E — the US adoption of ISO 12917-1, covering manual calibration of nominally horizontal cylindrical tanks. That is a tank calibration company's work, not a control system configuration exercise.

What is the difference between Performance Level (PL) and SIL for machine safety?

Two ways of expressing the same thing: how reliably a safety function works. PL comes from ISO 13849-1 and covers safety-related parts of control systems in any technology. SIL here comes from IEC 62061, written around electrical, electronic, and programmable electronic systems.

The bridge between them is PFHd, the average probability of a dangerous failure per hour. Both standards use the same units and adjacent bands. The PFHd bands in ISO 13849-1:

• PL a: ≥10⁻⁵ to <10⁻⁴ — no SIL equivalent
• PL b: ≥3×10⁻⁶ to <10⁻⁵ — SIL 1
• PL c: ≥10⁻⁶ to <3×10⁻⁶ — SIL 1
• PL d: ≥10⁻⁷ to <10⁻⁶ — SIL 2
• PL e: ≥10⁻⁸ to <10⁻⁷ — SIL 3

They get there differently. PL is determined from architectural Category (B, 1, 2, 3, 4), MTTFd, average diagnostic coverage, and common cause failure, which must score at least 65 points on the Annex F checklist. SIL uses hardware fault tolerance, safe failure fraction, and architectural constraints. SIL 4 is not used on machinery. The current editions are ISO 13849-1:2023 (fourth edition, April 2023) and IEC 62061:2021. The 2021 edition of 62061 addresses the safety-related control system as a whole, but its subsystem requirements are still written for E/E/PE technology.

On the legal side, Machinery Directive 2006/42/EC applies until 19 January 2027 and EU Machinery Regulation 2023/1230 takes over on 20 January 2027. There is no overlap period. The lists of standards harmonized under each are amended often and the Regulation's list is still being built, so check the current OJEU citation rather than assuming a standard is listed.

This is not Bridges Industrial's lane. A required PL determination, the risk assessment behind it, and the validation record are a functional safety engineer's work, and on a CE-marked machine a notified body may be involved. Hire a certified functional safety engineer — TÜV Rheinland FS Eng, TÜV SÜD, and exida CFSE are the common credentials.

What vibration level in mm/s RMS is acceptable for an industrial motor under ISO 10816-3?

For a medium machine of 15 to 300 kW on a rigid support, ISO 10816-3:2009 puts the zone B/C boundary at 2.8 mm/s RMS. Below that is acceptable for unrestricted long-term running. 2.8 to 4.5 means plan the repair. Above 4.5 mm/s, the C/D boundary, damage is likely and the machine should come out. Note that ISO 10816-3 is withdrawn and replaced by ISO 20816-3:2022, so work from the current edition before writing a spec.

The zone boundaries in ISO 10816-3:2009, Annex A, in mm/s RMS:

• Group 2 — medium machines above 15 kW up to and including 300 kW; electrical machines with shaft height 160 mm ≤ H < 315 mm. Rigid support: A/B 1.4, B/C 2.8, C/D 4.5. Flexible support: A/B 2.3, B/C 4.5, C/D 7.1.
• Group 1 — large machines above 300 kW and not more than 50 MW; electrical machines with shaft height H ≥ 315 mm. Rigid support: A/B 2.3, B/C 4.5, C/D 7.1. Flexible support: A/B 3.5, B/C 7.1, C/D 11.0.

The same tables give displacement boundaries in µm alongside the velocity ones. Evaluate both where the spectrum is expected to contain low-frequency content; the standard says so.

Zone A is a newly commissioned machine. Zone B is acceptable for unrestricted long-term operation. Zone C is not suitable for continuous long-term running. Zone D is where damage occurs. The standard also gives limit guidance: an ALARM should normally not exceed 1.25 × the upper limit of zone B, and a TRIP should not exceed 1.25 × the upper limit of zone C.

Measure broadband velocity RMS on the non-rotating parts — bearing housings, in horizontal, vertical, and axial — with flat response over at least 10 to 1000 Hz. For machines approaching or below 600 r/min the lower limit must not be greater than 2 Hz. ISO 10816-3 covers 120 to 15,000 r/min. ISO 20816-3:2022 extends that to 30,000 r/min and folds in the shaft vibration criteria that used to live in ISO 7919-3. The support counts as rigid when the lowest natural frequency of the machine-and-support system in the measurement direction is at least 25% above the main exciting frequency, usually running speed. A motor bolted to a grouted base is normally rigid; a skid- or spring-mounted machine is flexible. A system can be rigid in one direction and flexible in the other.

Do not apply these numbers to a pump. Rotodynamic pumps are excluded from the scope of ISO 10816-3 and are covered by ISO 10816-7. So are reciprocating pumps and compressors, submerged motor-pumps, and screw compressors. And a single broadband number tells you something is wrong, not what is wrong. Diagnosis needs a spectrum and a phase measurement, and that is a vibration analyst's work — ISO 18436-2 Category II or higher.

When this page runs out

Most of what is on this page gets settled at the panel with a three-valve manifold, a calibrator, and the arithmetic above. Some of it does not. An accredited calibration certificate comes from an accredited laboratory. A custody-transfer strapping table comes from a tank calibration company working to API MPMS Chapter 2.2E. A required Performance Level determination and its validation record come from a functional safety engineer. A vibration diagnosis, as opposed to a single overall number, comes from a Category II analyst. A remote seal system that has lost its fill goes back to the manufacturer, because nobody recharges one in the field. Bridges Industrial will tell you when one of those is the right call rather than take the work.