CPT Procedure — Equipment, Push Rate and Data Output Explained

The Cone Penetration Test (CPT) is performed by pushing a standardised instrumented cone into the ground at a controlled rate, measuring the resistance of the soil continuously from the surface to the termination depth. Unlike the Standard Penetration Test — which stops every 1.0–1.5 metres to take a blow count — the CPT provides an essentially uninterrupted depth profile of soil resistance, measured at depth intervals as small as 1–2 cm. That continuous resolution is the test’s primary advantage and the reason it has become one of the dominant in-situ tests in modern geotechnical practice.

This article explains the complete CPT procedure from the ground up: how the equipment works, the specific cone geometry and standard specifications, the controlled push rate, exactly what the sensors measure and why, how data is logged in the field, and how the raw output files are structured for import into analysis software. It is intended both as a procedural reference for engineers planning a CPT investigation and as a technical explanation for those interpreting CPT results who want to understand the measurement conditions behind the data.

The operating principle #

The CPT works on a straightforward mechanical principle: a cone-shaped probe is pushed vertically into the ground at a controlled rate by a hydraulic ram, and sensors in the cone measure the resistance to that penetration continuously as it advances. The greater the resistance, the denser or stronger the soil. The lower the resistance, the softer or looser the soil. Because this measurement is continuous — recorded electronically at every centimetre of advance rather than at discrete test intervals — the CPT produces an exceptionally detailed vertical profile of soil behaviour from ground surface to termination depth.

There is no drilling, no hammer, and no borehole. The test is entirely mechanical push, which gives it its characteristic repeatability: the same cone pushed through the same soil at the same rate will produce essentially the same readings every time, regardless of which driller operates the rig. This is one of the most important differences between CPT and SPT, where operator technique and hammer energy delivery significantly affect results.

Cone types — mechanical vs electronic #

Two generations of CPT cone exist, though only the electronic cone is used in modern practice:

Mechanical cone (Dutch cone) #

The original CPT cone, developed in the Netherlands in the 1930s and standardised under ASTM D3441. The mechanical cone uses a dual-rod system in which an inner rod advances the cone tip independently of the outer rod carrying the friction sleeve. Measurements are taken by advancing the cone tip and friction sleeve separately, reading hydraulic pressure from a gauge at the surface. Mechanical cone testing is slow, limited in depth, and produces far less data per unit depth than electronic testing. It is rarely used today except in regions without access to electronic CPT equipment.

Electronic (piezocone) cone #

The modern standard, governed by ASTM D5778 and ISO 22476-1. The electronic cone contains strain gauge load cells that measure tip resistance and sleeve friction simultaneously and continuously as the cone is pushed, transmitting data electronically through a cable running through the push rods to a data acquisition unit at the surface. Electronic CPT data is recorded digitally at depth intervals of 1–2 cm, producing thousands of data points per metre of penetration. The CPTu variant adds a pore pressure transducer — almost universally included in modern investigations.

All references to CPT procedure in this article refer to the electronic CPT under ASTM D5778 / ISO 22476-1, which is the current standard of practice globally.

Standard cone geometry and specifications #

The cone geometry is precisely defined to ensure consistent, comparable results between different cones, operators, and rigs. Deviations from these specifications affect the measured values and make comparison between soundings unreliable.

Specification	Standard value	Tolerance
Cone tip angle	60° (apex half-angle 30°)	±0.5°
Cone tip area (A_c)	10 cm² (standard); 15 cm² (less common)	±0.5 cm² (10 cm²); ±0.75 cm² (15 cm²)
Cone diameter	35.7 mm (for 10 cm² cone)	±0.3 mm
Friction sleeve area (A_s)	150 cm² (for 10 cm² cone)	±1.5 cm²
Friction sleeve position	Immediately above the cone shoulder	0 mm gap between cone and sleeve
Push rods	AW, BW, or NW flush-coupled steel rods	Straight within 1/1000 of length
Cone area ratio (a)	Typically 0.70–0.85 (cone-specific, from lab calibration)	Must be measured for q_t correction in CPTu
Applicable standard	ASTM D5778 / ISO 22476-1	—

The cone area ratio a is a critical calibration constant for CPTu testing. It accounts for the fact that the pore pressure acting on the annular shoulder area behind the cone tip generates a net upward force on the load cell that partially offsets the measured tip resistance. The corrected cone resistance q_t = q_c + u₂(1 − a) requires a precisely known a value, determined by placing the cone in a pressure vessel filled with water and comparing the pressure applied to the change in load cell reading. Each cone has a unique a value that must be provided by the cone manufacturer and verified periodically.

The CPT rig — reaction force and setup #

Pushing a cone into the ground at 2 cm/s requires a substantial reaction force — typically between 50 and 200 kN depending on soil conditions and termination depth. This reaction force must be anchored to the ground or balanced by ballast. Three configurations are used in practice:

Ballasted truck rig #

The most common onshore configuration. A heavily loaded truck (typically 15–20 tonnes) provides the reaction force through its own weight, transferred to the push rods by a hydraulic ram mounted in the truck bed. The truck is positioned over the test location and leveled before pushing begins. Ballasted truck rigs are self-contained and highly mobile, allowing multiple soundings per day across a site.

Anchored rig #

Where truck access is restricted (inside buildings, on soft ground that cannot support a heavy vehicle, or on steep slopes), a smaller rig is used and anchored to the ground using screw anchors driven into the soil. The screw anchors provide the reaction force. Anchored rigs are slower to set up than truck rigs but access locations that trucks cannot reach.

Offshore rig #

Seabed CPT is performed using a seabed frame lowered to the seafloor from a vessel, with a hydraulic push unit operating on the frame. Offshore CPT is critical for foundation design of offshore structures, pipelines, and wind turbine monopiles, and is one of the areas where CPT’s sample-free continuous-profile advantage over SPT is most pronounced — drilling a seabed borehole is far more expensive and slower than CPT from a seabed frame.

Site setup requirements #

Requirement	Detail
Ground access	Truck rig requires firm, level ground capable of supporting 15–20 tonnes. Soft ground may require timber mats or tracked carrier.
Clearance height	Typically 4–5 m above ground surface for mast-mounted push unit. Low-clearance push units available for restricted-height locations.
Test location marking	Test points should be marked on a site plan before mobilisation, confirmed against underground service records (cables, pipes) to avoid hazards.
Leveling	The push rods must be vertical at the start of the test. Inclination sensors in modern cones monitor rod deviation during pushing and trigger a halt if deviation exceeds approximately 5° from vertical.

CPT procedure — step by step #

The following procedure applies to electronic CPT under ASTM D5778 / ISO 22476-1:

Mobilise and position the rig at the designated test location. Check underground service records for the location. Level the rig and confirm vertical alignment of the push mast.
Zero the cone sensors with the cone at surface, before penetration begins. All load cells (tip resistance, sleeve friction, pore pressure if CPTu) are zeroed to remove any pre-load bias. This zero reading is recorded in the data file as the reference baseline.
Connect the cone to the first push rod. Check the cable connection between the cone sensors and the data acquisition unit at the surface. Verify that all channels are reading correctly.
Begin pushing at the standard rate of 2 cm/s. The hydraulic ram advances the push rods at this controlled rate. The data acquisition unit begins logging sensor readings automatically.
Add push rods as the cone advances. Each push rod is typically 1 m in length. When the ram reaches the bottom of its stroke, it is retracted, a new rod is added, the rod string is reconnected to the ram, and pushing resumes. This rod addition pause is noted in the data log. Modern data files mark rod addition depths automatically; manual recording on the field log is also standard practice.
Monitor data quality in real time. The data acquisition unit displays q_c, f_s, and u₂ (if CPTu) as continuous traces on the operator’s screen as the cone advances. The operator watches for anomalies — sudden spikes in q_c suggesting a gravel or cobble obstruction, sudden drops suggesting a soft pocket, erratic friction readings suggesting sleeve damage — and records observations on the field log.
Terminate the test when: (a) the target depth is reached; (b) the thrust force exceeds the rig’s safe capacity (typically 100–200 kN depending on rig size), indicating refusal; or (c) rod deviation exceeds the permissible limit, indicating bending of the rod string in an obstructed or heterogeneous stratum.
Record the termination reason on the field log: target depth reached, refusal (with depth and estimated thrust force), or deviation.
Zero the sensors again after extraction to check for zero drift. If post-test zero readings differ significantly from pre-test zeros, the data quality for that sounding is questionable and this should be noted in the sounding report.
Extract the rods and cone. Inspect the cone tip and sleeve for damage (dents, scratches, soil ingress into the piezometer filter). A damaged cone must be returned to the manufacturer for repair and recalibration before re-use.
Move to the next test location and repeat. A standard onshore CPT to 20 m depth with rod additions takes approximately 30–60 minutes including set-up and extraction.

The 2 cm/s push rate — why it matters #

The requirement for a push rate of 2 cm/s (±0.5 cm/s) is one of the most precisely specified aspects of the CPT procedure, and understanding why it is controlled matters for interpreting the results correctly.

Drainage conditions during penetration #

The drainage conditions during cone penetration — whether the soil around the advancing cone can drain excess pore pressure as fast as it is generated — depend on both the permeability of the soil and the rate at which the cone displaces soil. At 2 cm/s, the penetration is:

Undrained in clays and silts: The low permeability of fine-grained soils means that excess pore pressure generated by the advancing cone cannot dissipate at 2 cm/s. The measured q_c reflects undrained resistance. This is the intended condition for CPT in fine-grained soils, and all correlations for undrained shear strength from CPT (s_u = (q_t − σ_v0) / N_kt) assume undrained penetration.
Drained in clean sands and gravels: The high permeability of coarse-grained soils allows pore pressures to dissipate essentially instantaneously even at 2 cm/s. The measured q_c reflects drained resistance, and all correlations for relative density and friction angle from CPT assume drained penetration.

What happens at non-standard push rates #

Pushing faster than 2 cm/s in clean sands can generate partial undrained conditions, increasing q_c above its drained value. Pushing slower than 2 cm/s in soft clays allows partial pore pressure dissipation, reducing q_c below its fully undrained value. In both cases, the standard correlations no longer apply correctly. Rate-controlled CPT studies have confirmed that the 2 cm/s standard keeps sand behaviour drained and clay behaviour undrained across the full range of natural soils encountered in practice — which is why the standard is 2 cm/s and not faster or slower.

Rate monitoring in practice #

Modern CPT rigs with hydraulic control systems maintain the 2 cm/s rate automatically and record the actual push rate alongside the sensor data. If the rate deviates significantly — for example, slowing when the cone enters very dense material or speeding up in soft zones — this is flagged in the data file. Manual push rigs used in developing markets or for lightweight low-penetration investigations may struggle to maintain a constant rate, which adds uncertainty to the results.

What CPT measures — q_c, f_s, R_f #

The electronic CPT cone contains two independent load cells that measure resistance continuously throughout the push:

Cone tip resistance (q_c) #

q_c is the total force acting on the cone tip divided by the cone tip area (A_c = 10 cm²). It is the primary CPT measurement and the most direct indicator of soil strength and density — in loose, soft soils it is low; in dense, strong soils it is high. It is reported in MPa (megapascals).

Soil type and condition	Typical q_c range
Soft clay (s_u < 25 kPa)	0.1–0.5 MPa
Firm to stiff clay (s_u 25–100 kPa)	0.5–2.0 MPa
Very stiff to hard clay (s_u > 100 kPa)	2.0–5.0 MPa
Loose sand (D_r < 35%)	2–10 MPa
Medium-dense sand (D_r 35–65%)	10–20 MPa
Dense sand (D_r 65–85%)	20–35 MPa
Very dense sand / gravel	35–50+ MPa

Sleeve friction (f_s) #

f_s is the total frictional force acting along the friction sleeve (A_s = 150 cm²) divided by the sleeve area. It represents the skin friction between the steel sleeve surface and the surrounding soil and is reported in kPa. In clean sands, f_s is low because the cone tip displaces the soil rapidly and efficiently, leaving relatively little residual contact stress on the sleeve. In clays, f_s is proportionally higher because undrained adhesion between the clay and the steel sleeve is more significant.

Friction ratio (R_f) #

R_f is a derived parameter: the ratio of sleeve friction to cone tip resistance, expressed as a percentage:

R_f = (f_s / q_c) × 100%

R_f is the primary soil-type indicator in CPT data. Because it is a ratio, it normalises for the fact that both q_c and f_s increase with depth independently of soil type. Clean sands and gravels have very low R_f (typically 0.2–1.0%) because q_c is high relative to f_s. Clays have high R_f (typically 3–8%) because q_c is low but f_s is proportionally large. R_f forms one of the two axes of the Robertson Soil Behaviour Type (SBT) chart used for soil classification from CPT data — covered in full in Robertson SBT chart — how to classify soils from CPT data.

R_f (%)	Indicated soil behaviour type
< 0.5	Dense gravel, very coarse material
0.5–1.0	Clean sand to silty sand
1.0–2.0	Sandy silt, silty sand — transition zone
2.0–4.0	Silt, clayey silt, sandy clay
4.0–8.0	Clay, silty clay, organic soil
> 8.0	Sensitive fine-grained soil, peat, or organic material

Depth intervals and data resolution #

Electronic CPT data is recorded at depth intervals of 1–2 cm by default in modern data acquisition systems. This means a 20 m sounding produces 1,000–2,000 data points — each with a depth, q_c, f_s, and (for CPTu) u₂ value. This resolution is what allows CPT to detect thin soil layers that SPT testing — with its 1.0–1.5 m interval — completely misses.

Thin-layer effect #

The CPT cone has a finite diameter (35.7 mm for a 10 cm² cone), which means the cone “sees” an influence zone of approximately 10–30 times the cone diameter above and below the cone tip at any given moment. For a thin layer — for example, a 50 mm sand lens in a clay sequence — the cone tip begins responding to the sand before it fully enters the layer and reverts to clay response before it fully exits. As a result, q_c in thin layers is underestimated relative to what it would be in a thick layer of the same material. This “thin-layer effect” means CPT data from thin layers must be corrected using established methods (Robertson et al., 1983; Ahmadi & Robertson, 2005) before using correlations calibrated on thick uniform layers.

Practical data management #

A complete CPT dataset for a multi-location investigation may contain millions of rows of data. Field data loggers store data in proprietary binary formats (specific to the rig manufacturer) or in standard text formats. The most common export formats for import into analysis software are:

CSV / TXT: Depth, q_c, f_s, u₂ in columns, comma or tab delimited. The most universally compatible format.
GEF (GeoTechnical Exchange Format): A structured text format developed in the Netherlands and widely used in European CPT practice. Contains a header with metadata and a data block.
AGS (Association of Geotechnical and Geoenvironmental Specialists): UK and international standard for geotechnical data exchange. CPT data is stored in the SCPT data group.
Excel (.xlsx): Many field loggers export directly to Excel, making data immediately accessible without format conversion. DartiGeo accepts CPT data import directly from Excel.

Field data acquisition and data logger output #

The data acquisition unit (DAQ) at the surface receives the analogue electrical signal from the cone’s strain gauge load cells through the cable running through the push rods, converts it to a digital signal, and records it with the corresponding depth measurement from a draw-wire encoder or incremental encoder on the push ram.

Typical DAQ display during a CPT push #

During the test, the DAQ displays real-time traces of q_c, f_s, and u₂ (for CPTu) plotted as depth profiles that grow downward as the cone advances. The operator monitors these traces for:

Sudden spikes in q_c indicating gravel, cobbles, or cemented lenses — potential refusal zones
Abrupt q_c drops indicating soft zones or cavities
Anomalous f_s readings suggesting sleeve friction damage or clay smearing on the sleeve
Sustained high u₂ in CPTu indicating a fine-grained layer generating excess pore pressure
Negative u₂ below hydrostatic in CPTu — indicating dilating dense sand, overconsolidated clay, or a defective piezometer filter

Zero drift monitoring #

Electronic strain gauges are subject to thermal drift — as the cone warms from the ambient temperature at the surface to the ground temperature at depth, the zero readings of the load cells shift slightly. ASTM D5778 requires zero readings before and after each push. A drift of more than 5% of the maximum measured value is cause to question the reliability of the data. Pre-heating the cone to approximate ground temperature before zeroing reduces drift in deep soundings.

Data file contents #

The raw data file from a CPT push contains, at minimum:

Field	Description	Units
z	Depth from reference datum (usually ground surface)	m
q_c	Cone tip resistance	MPa
f_s	Sleeve friction	MPa or kPa
R_f	Friction ratio (derived: f_s/q_c × 100)	%
u₂	Pore water pressure at cone shoulder (CPTu only)	MPa or kPa
Inclination	Rod deviation from vertical (modern cones)	degrees
Temperature	Cone temperature (some cones)	°C

The file header typically includes: sounding ID, test date and time, location coordinates, ground level, water table depth, cone type and serial number, cone area ratio (a), pre-test zero readings, operator name, and rig type.

The CPTu variant — adding pore pressure measurement #

The CPTu (piezocone test) adds a piezometer filter element at the u₂ position — immediately behind the cone shoulder — that is connected to a miniature pressure transducer inside the cone body. The filter is typically made from sintered high-density polyethylene (HDPE), polyethylene, or ceramic, with a pore size of approximately 10–20 microns. Before the test, the filter must be saturated with glycerin or silicone oil to eliminate air that would otherwise compress when pore pressure is applied, causing a sluggish or erratic pore pressure response.

The additional pore pressure measurement enables:

Corrected cone resistance q_t: q_t = q_c + u₂(1 − a). In soft clays where u₂ can be several times q_c, this correction is essential for accurate interpretation. In sands, u₂ ≈ u₀ (hydrostatic) and the correction is negligible.
Pore pressure ratio B_q: B_q = (u₂ − u₀) / (q_t − σ_v0). Used as the third axis on the Robertson SBT classification chart to distinguish sensitive clays, structured soils, and other anomalous materials.
Dissipation testing: When the cone is held stationary in a fine-grained layer, the decay of excess pore pressure with time can be measured. The time for 50% dissipation (t₅₀) is used to estimate the horizontal coefficient of consolidation c_h.
Layer identification in soft sediments: Abrupt changes in u₂ — particularly negative excursions below hydrostatic — mark transitions between normally consolidated clay and sand layers or overconsolidated zones.

The piezometer filter must be maintained in a saturated state throughout setup and testing. Filter desaturation — caused by exposure to air during cone handling or rod addition — produces erratic u₂ readings that are difficult to detect without independent data. In modern practice, filter saturation under vacuum using glycerin is the standard protocol, and filters are replaced or resaturated after any test where desaturation is suspected.

For the full treatment of CPTu pore pressure measurement, interpretation of B_q, and dissipation testing, see CPTu piezocone test — pore pressure measurement and interpretation.

CPT refusal — causes and recording #

CPT refusal occurs when the thrust force required to advance the cone at 2 cm/s reaches or exceeds the rig’s safe working capacity. Unlike SPT refusal (which has a defined blow count criterion), CPT refusal is equipment-specific — a small lightweight rig may refuse at 20 m in medium-dense sand, while a heavy rig may penetrate the same sand to 40 m.

Common causes of CPT refusal #

Cause	Typical depth pattern	Field indicator
Dense gravel or cobble layer	Abrupt refusal at variable depths	Sudden spike in q_c immediately before refusal; audible resistance increase
Cemented or indurated layer	Refusal at consistent depth across site	Sharp q_c increase with no f_s increase (cemented — high q_c, low R_f)
Bedrock	Refusal at or near rock head depth	Sustained very high q_c with rod deviation, typically confirmed by borehole
Rod bending or buckling	Progressive resistance increase	Inclination sensor deviation increasing progressively; asymmetric sensor readings
Coarse fill or rubble	Refusal at shallow depth (1–5 m)	Erratic q_c spikes from early in the push; rod percussion

Recording refusal #

When refusal occurs, the sounding report records: the refusal depth, the maximum thrust force applied, the q_c value at refusal, and the probable cause. If refusal is caused by a localisied obstruction (a single cobble, for example), the cone may be retracted and re-pushed offset by 0.5–1.0 m to attempt to avoid the obstruction. If refusal is caused by bedrock or a persistent dense stratum, supplementary SPT boreholes are typically used to penetrate through and investigate below the refusal depth.

Field records and CPT sounding report #

Every CPT sounding is accompanied by a field record — either a paper form or a digital tablet entry — completed by the CPT operator during testing. The field record documents everything not captured by the data file itself:

Sounding ID and test location (coordinates, reference to site plan)
Date and time (start and finish)
Rig type, model, and maximum thrust capacity
Cone type, serial number, and area ratio a
Pre-test zero readings for all channels
Groundwater depth (observed or assumed for stress calculations)
Depth and reason for any interruptions (rod additions, temporary stops)
Termination depth and reason (target depth, refusal, deviation)
Post-test zero readings
Any anomalies observed: sudden resistance changes, rod percussion, sleeve damage
Filter saturation method (for CPTu)

The final deliverable from a CPT investigation is the sounding report: typically a PDF containing the depth profiles of q_c, f_s, R_f, and u₂ plotted on a standard layout, alongside the field record and the raw data file. Many clients also request the derived plots — normalised Q_tn vs F_r SBT classification, soil parameter depth profiles, and bearing capacity estimates — which require analysis software.

Importing CPT data into DartiGeo #

DartiGeo’s CPT interpretation module accepts raw CPT data imported directly from Excel, eliminating the need to re-enter data manually from the field data logger output. The import workflow is straightforward:

Export the raw CPT data from the field data logger software to Excel (or CSV, converted to Excel). The required columns are: depth (m), q_c (MPa), f_s (MPa or kPa), and optionally u₂ (MPa or kPa) for CPTu data.
Open the CPT module in DartiGeo and use the Import from Excel function to load the data. DartiGeo maps the column order from the import file.
Enter the sounding metadata: sounding ID, ground surface elevation, groundwater depth, and unit weights for the soil layers above the sounding (needed for σ_v0 and σ’_v0 calculations).
Select the analysis to run: SBT classification (Robertson chart), soil parameter estimation, bearing capacity from CPT, or settlement from CPT.
Generate the report in PDF, Word, or Excel format.

Because all CPT data and analysis results are stored in the same DartiGeo project file as the borehole logs, SPT processing, laboratory tests, and foundation design calculations, there is no re-entry of soil profile data between modules.

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Frequently asked questions #

Why is the CPT push rate specified at exactly 2 cm/s? #

The 2 cm/s rate is a carefully chosen compromise that keeps penetration undrained in fine-grained soils (clays and silts) and drained in coarse-grained soils (sands and gravels) across the range of natural permeabilities encountered in practice. This means that standard correlations — which were developed under these drainage conditions — apply without correction. Pushing faster would create partial undrained conditions in sands (inflating q_c); pushing slower would allow partial drainage in clays (deflating q_c). The 2 cm/s rate has been validated by calibration chamber studies and field comparisons over several decades and is the universal standard in ASTM D5778 and ISO 22476-1.

What is the difference between q_c and q_t? #

q_c is the raw measured cone tip resistance — the force on the cone tip load cell divided by the cone area. q_t is the corrected cone resistance, adjusted for the pore water pressure acting on the unequal area at the cone shoulder: q_t = q_c + u₂(1 − a), where a is the cone area ratio and u₂ is the measured pore pressure at the cone shoulder. In clean sands where u₂ ≈ hydrostatic, the correction is small (often less than 1–2%). In soft clays where excess pore pressure u₂ can be several times the hydrostatic value, the q_c-to-q_t correction can be significant — sometimes 20–50% — and must be applied before any parameter correlation. All modern CPT interpretation should use q_t, not q_c, as the primary variable.

How many CPT soundings are needed for a site investigation? #

The number and spacing of CPT soundings depends on the site area, variability of the ground conditions, and the design requirements. For a uniform site with known stratigraphy, one CPT per 500–1000 m² of building footprint may be sufficient for preliminary design. For a variable site — alluvial deposits, infilled land, or sites with suspected soft zones — one CPT per 100–200 m² may be needed to characterise the variability adequately. Most geotechnical investigation standards recommend a minimum of three CPT soundings to establish whether stratigraphy is consistent across the site, with additional soundings at locations of concentrated loads or known problem areas.

Can CPT be used in gravel or fill containing rubble? #

CPT is poorly suited to gravels, cobbles, or fill containing rubble, bricks, concrete, or large-diameter particles. Single cobbles or concrete fragments cause the cone to refuse at shallow depths, terminating the sounding before the target stratum is reached. In such materials, SPT boreholes with rotary drilling are more reliable because the drill bit can grind through obstructions. The standard approach on sites with suspected coarse fill or gravels is to advance SPT boreholes through the problematic material and use CPT below the refusal depth, or to use CPT only where it can reliably penetrate to the target depth confirmed by prior investigation.

What is zero drift in CPT and why does it matter? #

Zero drift is the change in the load cell’s zero reading between the pre-test measurement (cone at surface, no load applied) and the post-test measurement (cone returned to surface after extraction). It occurs because the strain gauges and electronic circuits in the cone change their electrical properties slightly as the cone heats up at depth and then cools on extraction. ASTM D5778 requires that drift for q_c and f_s does not exceed 5% of the maximum measured value in the sounding. Significant drift means that either q_c was higher than measured (if zero drifted negative) or lower (if zero drifted positive) — introducing a systematic error that cannot be corrected after the fact. Cones with excessive drift should be returned to the manufacturer for repair.