LI19 is a high accuracy handheld read-out unit / datalogger. It is used to make mobile measurements, for short term datalogging (as a static logger), and as an accurate millivolt amplifier directly connected to a PC. LI19 can be used with a variety of sensors. LI19’s most common application is with heat flux- and  solar radiation sensors. LI19 battery life and memory allow continuous measurement for up to 50 days.

  • High accuracy handheld read-out unit / datalogger
  • Easy transport and protection during field measurement campaigns
  • Supplied in a practical transport case
  • Fresh batteries allow more than 50 days of operation
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Output on display Heat flux, solar radiation
Input analogue voltage
Conversion Division by the sensor sensitivity
Display definition 4 digits with sign
Display refreshment rate 1 s⁻¹
Calibration uncertainty 0.1 %
Temperature dependence < 0.5 % + 3 x 10⁻⁶ V over rated range
Sample rate 2 s⁻¹
Rated input range 6.25 to 200 x 10⁻³ V (selectable)
A/D conversion 16 bits
Stored measurement definition minimum maximum and average over storage interval with conversion to W/m²
Storage capacity 3518 measurements
Storage interval range 2 to 65535 s (selectable)
Compatibility with Hukseflux sensor models LP02, HFP01, SBG01, HF03
Battery type 2 x AA
Internal power supply voltage 3 VDC
Battery life > 50 days (on fresh batteries)
Rated operating temperature range -10 to +40 °C
System requirements for use with PC Windows XP and higher
Connection to PC USB 1.1 / 2.0 low speed
User interface on PC LI19 software
IP protection class IP40
Connection to sensor 2 x (female chassis plug for 4 mm banana with screwed signal wire clamp)
Weight 0.175 kg (net) / 1.3 kg with carrying case
Dimensions LI19 (70 x 146 x 25) mm
Dimensions transport case (400 x 300 x 120) mm


LI19 May be Used with Pyranometers and Heat Flux Sensors
LI19 is typically used to display the measured solar radiation or heat flux. It measures a DC voltage. Once programmed with the sensitivity of the connected sensor, the display will show the actual value of the heat flux or solar radiation in W/m². LI19 is programmed through its PC user interface. We recommend downloading the latest software. LI19 is battery powered, using 2 x AA-type batteries. Fresh batteries allow more than 50 days of operation. The system is supplied in a practical transport case, for easy transport and protection during field measurement campaigns. LI19 is built for easy use with a large size LCD, displaying quantities in W/m², and a USB connection. LI19 may be used with pyranometers and heat flux sensors.

Suggested Use

  • Short-term field measurement of solar radiation or heat flux
  • Amplification of heat flux sensor signals
  • Education in solar energy

Areas of Application

  • Industrial monitoring and control / heat flux and heat transfer measurement
  • Solar energy / PV system performance monitoring

Frequently asked questions

How to measure heat flux?

Heat flux sensors measure energy flux onto or through a surface in [W/m²].
The source of the heat flux may be:

  • conduction
  • radiation
  • convection

Convective and conductive heat transfer are associated with a temperature difference. Heat always flows from a source to a sink, from a hot to a cold environment. Convective and conductive heat flux is measured by letting this heat flow through the sensor. Radiative flux is measured using heat flux sensors with black absorbers. The absorbers converts radiative to conductive energy. Hukseflux started in 1993 with sensors for measurement of heat flux in soils and on walls. In the course of the years, we have added specialised sensors and systems for many other applications.
Heat flux sensors manufactured by Hukseflux are optimised for the demands of different applications:

  • rated temperature range
  • rated heat flux range
  • sensitivity
  • response time
  • chemical resistance, safety requirements
  • size, shape and spectral properties

Hukseflux is the world market leader in heat flux measurement. We have prepared a white paper briefly explaining the fundamentals of measuring with heat flux sensors. It also offers general directions what to watch out for and some, perhaps surprising, applications of heat flux sensors. Take a look at our white papers.

What matters most when measuring with a heat flux sensor?

There are quite a few general considerations when starting a heat flux measurement.

  • Representativeness in time and space; average!
    A heat flux sensor measures at a certain location. Is this location representative of what you need to measure? If possible, use a relatively large sensor, rather than a small one, and consider use of multiple sensors. Thermal processes often have large time constants; instantaneous measurements may be misleading. Average to get the full picture.
  • Optical properties
    When heat flux sensors also measure radiation, pay attention to the surface color. If needed paint the sensor surface. Please mind that shiny metallic surfaces reflect both infra-red and visible radiation. Paints may have different colors in the visible range, but are usually “black” absorbers in the far-infra-red.
  • Sensor thermal resistance
    A heat flux sensor distorts the local heat flux. In order to minimize this effect, use the sensor with the lowest possible thermal resistance.
  • Edge effects
    A heat flux sensor locally distorts the heat flow pattern, in particular around the edges of the sensor. A passive guard, i.e. a non-sensitive part around the sensor is essential to avoid errors due to edge effects.

There are more characteristics that matter. Please find them in our white paper on heat flux fundamentals and applications.

Which sensor(s) to use for surface energy flux measurement?

Hukseflux manufactures a range of sensors for surface energy flux measurements. All have proven reliability.
These state-of-the-art sensors are made for the global fluxnet community:

  • NR01 is a market leading 4-component net radiometer.
  • HFP01 and HFP01SC measure soil heat flux.
  • STP01 offers an accurate temperature profile measurement.
  • TP01 is the leading sensor for soil thermal conductivity.

Sensors made by Hukseflux are designed for compatibility with most common datalogger models. For many models we have example programs and wiring diagrams available.

How to measure R-value and U-value of buildings?

On-site measurements of thermal resistance, R, are often applied in studies of buildings. Alternatives are to measure its inverse value, the thermal conductance which is called the Λ-value, or the thermal transmittance which includes ambient air boundary layer thermal resistance, the U-value. The measurements of R are based on simultaneous time averaged measurement of heat flux Φ and differential temperature, ΔT, (using two temperature sensors on each on a different side of the wall).

R = ΔT / Φ

Hukseflux provides a range of sensors and measuring systems for use in measurement of the energy budget of buildings and characterization of construction materials.

HFP01 heat flux sensor and TRSYS01 measuring system are widely used for on-site measurements on walls, windows and other construction elements in building physics.

  • HFP01 can be used for in-situ measurement of building envelope thermal resistance (R-value) and thermal transmittance (H-value) according to ISO 9869, ASTM C1046 and ASTM 1155 standards. HFP01 is the world’s most popular sensor for heat flux measurement in the soil as well as through walls and building envelopes. HFP01 measures heat flux through the object in which it is incorporated or on which it is mounted, in W/m². More information? Visit the HFP01 product page.
  • TRSYS01 is a high-accuracy system for on-site measurement of thermal resistance, R, thermal conductance, the Λ-value, and thermal transmittance, the U-value, of building envelopes. TRSYS01 is mostly used for measurements according to standard practices of ISO 9869 and ASTM C1155 / C1046. The system is equipped with high-accuracy electronics, two heat flux sensors of model HFP01 as well as two pairs of matched thermocouples. The two measurement locations provide redundancy, leading to a high level of confidence in the measurement result. The high accuracy of the heat flux sensors and temperature difference measurements ensures that TRSYS01 continues measuring when other systems no longer perform; in particular at very low temperature differences across the wall.

Where can I find complete heat flux measuring systems?

Hukseflux, market leader in heat flux measurement, offers both sensors and systems.

These measuring systems typically include a Measurement and Control Unit and one or more sensors for measuring heat flux as well as other measurands, such as temperature and humidity. Examples are the TCOMSYS01 Hot Cube thermal comfort measuring system, including a TCOM01 sensor, and the TRSYS01 measuring system, incorporating two HFP01 heat flux sensors and two pairs of matched thermocouples.

Cannot find what you are looking for? Please contact us.