Thermocouple STP01

soilfor soil thermal profile measurement

Overview
STP01 accurately measures the temperature profile of the soil at five depths close to its surface. It is used for scientific-grade surface energy balance measurements. The sensor is buried and usually cannot be taken to the laboratory for calibration. The on line self-test using the incorporated heating wire offers a solution to verify STP01’s measurement stability.

Key features
• Scientific-grade gradient measurement: Measures accurate differential temperatures across five fixed depths (0.02, 0.05, 0.1, 0.2, and 0.5 m) with precise thermocouple joint positioning (uncertainty ±0.001 m).
• Integrated on-line self-test: An incorporated heating wire performs automated heating step-response tests to verify sensor stability over time, eliminating the need for periodic laboratory recalibrations.
• High differential accuracy: Uses five matched type T thermocouples referenced against an internal Pt100 sensor at 0.5 m depth, achieving temperature difference uncertainties as low as ±0.02 °C to ±0.05 K.
• Minimal soil disturbance: Ultra-thin foil construction (0.6 x 10⁻³ m thickness) preserves natural soil layering and thermal characteristics upon installation.
• Standard copper cabling: Internal referencing allows the use of standard copper-conductor signal cables without requiring special thermocouple extension connectors.
• Experimental thermal conductivity estimation: Step-response temperature analysis during the heating cycle can be utilized for experimental in-situ soil thermal conductivity measurements at three depths (0.05, 0.1, and 0.2 m).

Applications
• Scientific-grade soil energy balance and surface energy flux monitoring networks.
• Micrometeorological research, climate observation, and agricultural research stations.
• High-accuracy soil temperature gradient (K/m) and heat storage assessments.
• Long-term environmental studies combined with soil heat flux sensors (such as HFP01SC) and net radiometers.

Specifications
• Measurand: Soil temperature at 5 depths (0.02, 0.05, 0.1, 0.2, and 0.5 m below surface)
• Temperature sensors: 5 matched type T thermocouples
• Reference temperature sensor: Pt100 (IEC 751:1983 Class B, 4-wire connection at 0.5 m depth)
• Measurement range: -30 to +70 °C
• Uncertainty of temperature difference measurement: 1.5% of measured value plus measurement system uncertainty in x 10⁻⁶ V / 40 (attainable uncertainty ±0.02 °C / ±0.05 K)
• Uncertainty of relative sensor position: ±0.001 m
• Uncertainty of relative distance between sensors: ±0.0005 m
• Sensor foil thickness: 0.6 x 10⁻³ m (2.5 x 10⁻³ m at Pt100 location)
• IP protection class: IP67
• Standard cable configuration: 2 x 5 m copper-conductor cables (see options)
• Rated operating environment: Surrounded by soil
• Heater resistance (nominal): 200 Ω
• Heater power supply: 9 to 15 VDC
• Heater power consumption (daily average): 0.005 W
• Self-test interval: Typically 24 hr
• Self-test duration: 600 s
• Data acquisition requirements: 4 millivolt channels, 1 Pt100 channel, 1 heater voltage channel, and 1 relay (12 VDC nominal) for heater control
• Order code standard version: STP01/cable length in m

Options and accessories
• Standard 2 x 5 m cables
• Extended cable lengths in multiples of 5 m (lengths above 20 m available in multiples of 10 m)
• IT01 insertion tool for fast soil slit creation with minimal ground disturbance
• Combination setups with HFP01 / HFP01SC soil heat flux sensors and NR01 net radiometers