Industrial Testing Equipment · Coating Thickness Gauges

Dual-Principle Coating Thickness Gauge

Magnetic and eddy current coating thickness gauge measuring 0 to 1250 µm on steel and aluminium, with automatic probe identification, statistics, limit alarms and 500 stored readings.

TT220 (3 models) ·

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Key specifications

Model
TT220 / TT230 / TT260 and the split-probe F and N series
Measuring principle
Magnetic induction on ferromagnetic substrates, eddy current on non-ferrous substrates, selected automatically by the probe fitted
Measuring range
0 to 1250 µm with the standard F1 or N1 probe; 0 to 400 µm with F400; 0 to 5000 µm with F5; 0 to 10000 µm with F10; 0 to 2000 µm on the integral TT220
Resolution
0.1 µm below 100 µm; 10 µm on the long-range F10 probe; 1 µm above 100 µm on the integral model
Indication error
±(1 to 3) % of reading ±1 µm after two-point calibration; ±(3 % of reading ±1 µm) after one-point calibration
Minimum substrate thickness
0.5 mm on a ferromagnetic substrate with F1 and 0.3 mm on a non-ferrous substrate with N1
Minimum measuring area
7 mm diameter with F1 and 5 mm diameter with N1
Storage and statistics
500 readings in five groups of 100, with mean, maximum, minimum, count and standard deviation
Supply and runtime
Two AA cells, stated as over 200 hours of operation with the backlight off
Languages
Chinese and English, offered on some models in the range

A coating thickness gauge exists because the alternative is a knife, a microscope and a scrapped part. Paint, plating, powder coat and anodising are all specified as a thickness, and all of them have to be checked on the delivered part rather than on a coupon - which means the measurement has to leave the part intact.

The instrument does that by measuring the layer from above, and what makes it more than a single-purpose tool is that the same body will do it on steel and on aluminium, provided the substrate is recognised for what it is.

What the instrument is

A probe and a body. The probe carries a coil and measures a property of the field between itself and the substrate; the body supplies the display, the calibration and the statistics. Between them they implement two different methods, and the probe in use is what decides which one is applied.

The range in this material is broad: a split body with a cable-mounted probe, the same arrangement in a metal housing, a version that switches between steel and aluminium automatically, a long-range version for thick coatings, and a compact integral instrument for routine work on flat parts. They share the probe family and the measurement principle and differ in what they are pointed at.

The two methods, and why the substrate decides

The magnetic induction method energises a coil and measures the reluctance of the path between the probe and a ferromagnetic substrate. The coating is a gap in that path, so the reading is essentially a distance across the layer. It requires the substrate to be genuinely ferromagnetic and the coating to be genuinely non-magnetic - which is why nickel plating is the awkward case, since a nickel layer can act as part of the substrate.

The eddy current method drives an alternating field into a conductive substrate and measures the change in impedance caused by eddy currents flowing in it. Because the standoff affects that field, the same coil becomes a thickness reading once the substrate is known. It is the method for aluminium, copper, brass and austenitic stainless steel, where a magnetic gauge reads nothing useful at all.

A gauge carrying both identifies the probe automatically and applies the matching method, which removes the most common way of getting a wrong answer: using the right instrument on the wrong substrate.

Calibration, and why two points beat one

Both methods are relative and are calibrated rather than absolute. Zeroing on the bare substrate removes the offset - the error from the probe not reading exactly zero on the substrate itself. Adding a foil of known thickness and calibrating against it corrects the gain as well.

That is the whole difference between the material’s two error figures. One-point calibration is quoted at ±(3 % of reading + 1 µm) and two-point at ±(1 to 3) % of reading ±1 µm. Practically, a one-point calibration is at its best near the thickness at which it was zeroed and drifts further away from it, so work near the ends of a probe’s range is done with two points.

The conditions that bias the reading

The field spreads sideways as well as downwards, and everything that changes how much material surrounds the measurement point changes the reading a little. Curvature bends the field, an edge cuts it off, and a substrate thinner than the method expects stops behaving as an infinite body. The material gives a figure for each of these per probe: a minimum area of 7 mm diameter for F1 and 5 mm for N1, a minimum radius of 1.5 mm convex for F1 and 3 mm for N1, and a minimum substrate thickness of 0.5 mm on steel and 0.3 mm on aluminium.

None of these announces itself when it is violated. The number simply drifts, and it drifts in the direction that makes a thin coating look acceptable. That is why the conditions are checked against the part before the probe is chosen rather than after a reading looks wrong.

Measurement limitations and selection considerations

The material’s figures conflict in three places and all three are carried rather than resolved. The minimum substrate thickness differs between the split probes and the integral model. The body dimensions are given four ways across the range. And the indication error is quoted with three different bases, which turns out to be a difference of calibration procedure rather than a contradiction - but only once the procedure each figure belongs to is attached to it.

There is also a limit the material implies rather than states. It lists coating types - paint, plating, powder, anodising, films - without mapping them to a method, and while most are straightforward, the ones that are not are exactly the ones that go wrong: a magnetic layer on a ferromagnetic substrate, or a conductive layer on a conductive substrate. Where a coating is neither clearly magnetic nor clearly non-magnetic, saying so in the enquiry is what gets it settled.

Ordering

The items that fix the answer are the substrate material and grade, its thickness, and its smallest radius and narrowest area; the coating material and how it is applied, and whether any layer is magnetic; the expected thickness with its tolerance band; whether steel and aluminium parts both have to be inspected, which decides between a single-method instrument and one carrying both; how many readings per part the acceptance criteria require; and whether readings have to be stored, exported or printed. Where the part is small or curved, the geometry decides the probe, so a rough sketch of the part is worth more than a written description. Quantity, destination port and required delivery date complete the enquiry.

Features

  • Both measuring methods in one instrument, with the probe identified automatically so the wrong method cannot be applied by mistake
  • Five probe types covering 0 to 400 µm, 0 to 1250 µm, 0 to 5000 µm and 0 to 10000 µm, plus a right-angled probe for work against a shoulder
  • A minimum substrate thickness of 0.5 mm on steel and 0.3 mm on aluminium, which is thin enough for sheet and formed parts
  • 500 stored readings in five groups, each group carrying its own statistics
  • Five statistics shown on one screen: mean, maximum, minimum, count and standard deviation
  • Upper and lower limit alarms for screening a batch without reading every value
  • A minimum measuring area of 7 mm on F1 and 5 mm on N1, so small and narrow features can be measured
  • Over 200 hours of operation on two AA cells with the backlight off

Technical specifications

Every value below is taken from source documentation. Where a parameter is not stated in the available material it is marked as information required rather than estimated.

Measurement
PrincipleMagnetic induction for ferromagnetic substrates of steel and iron; eddy current for non-ferrous substrates of aluminium, copper, brass and austenitic stainless steelStandard
Measuring range0 to 1250 µm as standard; 0 to 400 µm with F400; 0 to 5000 µm with F5; 0 to 10000 µm with F10; 0 to 1500 µm on the split ferrous and non-ferrous version; 0 to 2000 µm on the integral TT220Standard
Resolution0.1 µm on the short-range probes; 10 µm on F5 and F10; the integral model gives 0.1 µm from 0 to 99.9 µm and 1 µm above 100 µmStandard
Indication error±(1 to 3) % of reading ±1 µm by two-point calibration; ±(3 % of reading + 1 µm) by one-point calibration; ±(3 % of reading + 1.5 µm) for N1 and ±(3 % of reading + 10 µm) for the long-range probesStandard
Unitsµm and mil across the range, with mm additionally on the integral modelStandard
CalibrationZero on the bare substrate, then one-point or two-point calibration against supplied reference foilsStandard
Application conditions
Minimum substrate thickness0.5 mm for F1, 0.2 mm for F400, 2 mm for F5 and F10, 0.3 mm for N1; the integral model gives 0.2 mm on steel and 0.03 mm on a non-ferrous substrateStandard
Minimum area7 mm diameter for F1, 3 mm for F400, 40 mm for the long-range probes, 5 mm for N1, and 15 mm on the integral modelStandard
Minimum radius of curvatureStated per probe: above 1 mm convex for F400, 1.5 mm for F1, flat for the right-angled F1, 10 mm for F10 and 3 mm for N1Standard
Coating typesPaint and lacquer, zinc and chromium plating, copper plating, plastic coating, powder coating, films, anodised and oxidised layers, anti-corrosion layers and electrophoretic layersStandard
Substrate combinationsNon-conductive coatings on a ferromagnetic substrate with the magnetic method; non-conductive coatings on a non-ferrous substrate, and non-ferrous metal layers on a non-ferrous substrate, with the eddy current methodStandard
Measurement modeSingle-point and continuous measurementStandard
Instrument
Storage500 readings in five memory groups holding up to 100 each; the material notes that the statistics cannot be altered once a group is full, though measurement can continueStandard
StatisticsMean, maximum, minimum, count and standard deviation, shown for the group on one screenStandard
Limit alarmSettable upper and lower limits with an audible indication; the material notes that the two limits cannot be closer together than 3 % of their valuesStandard
Data connectionHost computer software on some models, with a 485 interface on some of thoseInformation required
Display and power managementMeasurement value, unit, battery state, probe type, record count, measuring mode and limits; a sleep function and automatic power-offStandard
SupplyTwo AA cells on the split and body models; two AAA cells on the integral modelStandard
Dimensions and weight115 × 67 × 31 mm and about 340 g on the split models; 151 × 76 × 33 mm on the ferrous and non-ferrous split version; 103 × 62 × 27 mm and about 60 g on the integral model; 125 × 72 × 26 mm and about 400 g on the metal-bodied modelStandard
Ingress protectionNot stated in the materialInformation required
Operating temperatureNot stated in the materialInformation required
Supplied with
Packing listInstrument, two cells, one coating probe in F1 or N1, a substrate, five coating reference foils, a USB cable, documentation and a carrying caseStandard
SubstrateOne substrate supplied, either magnetic or eddy current depending on the probe, and used for zeroingStandard
Reference foilsFive coating foils supplied for one-point and two-point calibrationStandard
ProbeOne probe supplied as standard; the material does not state the cable lengthInformation required

Available models

Selection table
Model Configuration Key ranges Availability
TT230 Split-probe body in a plastic housing with a probe on a cable, 115 × 67 × 31 mm, about 340 g 0 to 1250 µm with F1 or N1; 0.1 µm resolution; 0.5 mm minimum substrate on steel; 500 readings The general form, chosen when the probe has to reach a surface the body cannot lie flat on
TT260 The same arrangement in a metal housing, 125 × 72 × 26 mm, about 400 g 0 to 1250 µm with F1; 0.1 µm resolution; 0.5 mm minimum substrate; 500 readings The same instrument in a housing for rougher handling
Split ferrous and non-ferrous Split probe at 151 × 76 × 33 mm, switching between magnetic and eddy current automatically 0 to 1500 µm; ±(1 to 3) % of reading ±1 µm; 0.1 µm resolution; 0.5 mm minimum substrate; 7 mm minimum area The form for an inspection that has to cover steel and aluminium parts in the same shift, with the language selection and host software
Split long-range Split probe with the F5 or F10 probe fitted 0 to 5000 µm with F5 and 0 to 10000 µm with F10; 10 µm resolution; 2 mm minimum substrate; 40 mm minimum area The form for thick coatings, where the short-range probe would run out of range
TT220 Integral instrument with the probe built into the body, 103 × 62 × 27 mm, about 60 g, two AAA cells 0 to 2000 µm; ±(2 % of reading + 1 µm); 0.1 µm below 100 µm and 1 µm above; 0.03 mm minimum substrate on a non-ferrous substrate; 15 mm minimum area The compact form for routine checks on larger flat parts, and the only model in the range with a USB and Bluetooth connection to a mobile application

Customization

  • The probe, from F400, F1, the right-angled F1, F5, F10 and N1, chosen by the substrate and the expected coating thickness
  • Whether the probe is built into the body or on a cable, which decides whether a curved or recessed surface can be reached
  • The measuring method: magnetic, eddy current, or both with automatic identification
  • The housing, in plastic or metal, at slightly different weights
  • The data route, from internal storage only up to host software with a 485 interface on some models

Certification and compliance

Certification documents not yet supplied

Certifications are only published once a certificate copy can be produced, so this section is deliberately empty rather than populated with assumed approvals. Tell us which approvals your application requires and we will confirm what is available for this instrument.

Parameters still to be confirmed

The following are required for a complete specification but are not stated in the documentation supplied to us. They are listed rather than estimated:

  • The conformity standards the gauges are built to, such as the relevant ISO or ASTM numbers, which the material does not state
  • Ingress protection and the operating temperature range for the whole range
  • Which models carry the host computer software and the 485 interface, since the material says only that some do
  • The probe cable length
  • Whether the reference foils and the substrate supplied with the instrument are traceable to a national standard
  • The full meaning of the 3 % figure given for the minimum separation of the upper and lower alarm limits
  • Repeatability, as distinct from the indication error that is given
  • The difference in body dimensions between the split models, given as 115 × 67 × 31 mm in one table and 151 × 76 × 33 mm for the ferrous and non-ferrous version, and whether they are different instruments
  • Which coating types the eddy current method will not measure, since the material lists coating types without mapping them to a method
  • Whether a curved-surface probe is available for radii below the stated minimum
  • Warranty period and the terms that apply
  • An English-language datasheet and the full specification for each probe and body

Frequently asked questions

Does the coating have to be non-magnetic?

It has to be non-magnetic for the magnetic induction method to read it as a coating rather than as part of the substrate. That is why nickel plating is the awkward case: it is conductive and, depending on the layer, magnetic enough to disturb the reading, and the material notes that nickel layers need to be considered separately. For the eddy current method the requirement is the opposite - the substrate has to be conductive and the coating non-conductive - and the method is the one for aluminium, copper, brass and austenitic stainless steel, where a magnetic gauge reads nothing useful.

Why does the error change between one-point and two-point calibration?

Because they correct different things. Zeroing on the bare substrate removes the offset, which is the error that comes from the probe not reading exactly zero on the substrate itself. A two-point calibration adds a foil of known thickness and so corrects the gain as well, which is why the material quotes ±(1 to 3) % of reading ±1 µm for two-point calibration against ±(3 % of reading + 1 µm) for one-point. The practical consequence is that a one-point calibration is adequate near the thickness at which it was zeroed and drifts further away from it, so work at the extremes of the probe range is done with two points.

Why does the minimum measuring area matter?

Because the field spreads sideways as well as downwards, and near an edge or on a small part the field does not have enough material around it to behave as though the part were infinite. The number does not jump; it drifts, which is worse. The material gives a minimum area of 7 mm diameter for F1 and 5 mm for N1, a minimum radius of 1.5 mm convex for F1 and 3 mm for N1, and a minimum substrate thickness of 0.5 mm on steel and 0.3 mm on aluminium. Where the part falls inside those figures, a probe with a smaller footprint is the answer rather than a correction applied afterwards.

What has to be confirmed before a quotation?

The substrate material and grade, its thickness, and its smallest radius and narrowest area, because those decide the probe. The coating material and how it is applied, and whether any layer is magnetic - nickel being the usual case. The expected coating thickness with its tolerance band. Whether steel and aluminium parts both have to be inspected, which decides between a single-method instrument and one carrying both. Whether readings have to be stored, exported or printed, and how many readings per part the acceptance criteria require. And whether the reverse face of the part is accessible, since the substrate thickness condition applies to the material under the probe.

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