testo 521-3 - Differential pressure measuring instrument (up to 2.5 hPa)

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The testo 521-3 high-precision differential pressure measuring instrument easily determines even the smallest differential pressures (up to +2.5 hPa). Its high accuracy and a resolution of 0.1 Pa make the instrument ideal for differential pressure measurements in clean rooms or for flue inspections.

  • Integrated temperature-compensated differential pressure sensor with accuracy of up to ± 0.5 Pa

  • Calculation of the flow velocity and the volume flow via Pitot tube (optional)

  • Direct zeroing of the display values of pressure probes

  • 2 probe inputs for additional pressure and temperature probes


Product Description

The testo 521-3 differential pressure measuring instrument is designed for high-precision differential pressure measurements in air conditioning and ventilation technology. It can be used, for example, for measurements in clean rooms and for flue inspections – thanks to a high level of accuracy and outstanding resolution. In conjunction with a Pitot tube (optional), the integrated pressure sensor measures flow velocities of +1 to +20 m/s.

The integrated temperature-compensated differential pressure sensor performs measurements to an accuracy of up to ± 0.5 Pa. In addition, you can attach up to 2 further optional probes to the measuring instrument. This means you can flexibly adapt the differential pressure measuring instrument to your application. You have the choice of the following probes:

  • Differential pressure probes
  • Absolute pressure probes
  • Relative pressure probes
  • Temperature probes (e.g. surface probes or immersion and penetration probes)

The practical features of the testo 521-3 differential pressure measuring instrument

The integrated differential pressure sensor enables the testo 521-3 measuring instrument to calculate flow velocity and volume flow using a Pitot tube (which can be ordered separately). The differential pressure measuring instrument also offers timed and multi-point mean calculation.

The optional ComSoft software enables you to save the measurement data so that it is linked to the measuring location and analyze it on the PC. You can use the optional testo fast printer to print out measurement data directly at the measuring location.

You can choose between the following units for pressure measurement: mbar, hPa, bar, Pa, kPa, inH2O, mmH2O, torr and psi. The mean value, the last measured value and the min./max. values can be displayed at the press of a button.

Delivery Scope

testo 521-3 differential pressure measuring instrument, including calibration protocol and batteries.

Temperature - NTC
Measuring range
-40 to +150 °C
Accuracy ±0.2 °C (-10 to +50 °C)*
±0.4 °C (Remaining Range)*
Resolution 0.1 °C


Temperature - TC Type K (NiCr-Ni)
Measuring range
-200 to +1370 °C
Accuracy ±0.4 °C (-100 to +200 °C)*
±1 °C (Remaining Range)*
Resolution 0.1 °C


Differential pressure (internal sensor) - Piezoresistive
Measuring range 0 to 2.5 hPa**
Accuracy ±0.5 Pa (0 to 20 Pa)
±(0.5 Pa ±0.5 % of mv) (20.1 to 250 Pa)
Resolution 0.1 °C
Static pressure 1000 hPa
Overload 50 hPa
Zeroing to 0.5 hPa


Differential pressure (external probe) - Piezoresistive
Measuring range 0 to 2000 hPa
Accuracy ±0.1 % of mv*
Resolution 0.1 Pa (0638 1347)
0.01 hPa (0638 1547)
0.001 hPa (0638 1447)
0.1 hPa (0638 1847 / 0638 1647)


Differential pressure (external probe) - Ceramic
Measuring range  -1 to 400 bar
Accuracy ±0.2 % of fsv*
Resolution 0.01 bar


General Technical Data
Dimensions 219 x 68 x 50 mm
Operating temperature 0 to +50 °C
Product-/housing material ABS
Channels 2; 1
Measuring rate 0.04 s
Connection Hose: inner Ø 4 mm outer Ø 6 mm
Battery type 9 V (6LR61)
Battery life
Continuous operation w/ internal pressure sensor: 30 h; With rech. battery: 10 h; With carbon battery: 18 h
Display type LCD
Display function 7 segment display and point matrix
Display update 2x per second, in fast measurement 4x per second
Power supply Battery/Rechargeable battery,Mains unit 12 V
Interface RS232
Memory 100 kB; 25,000 measuring values
Storage temperature  -20 to +70 °C
Weight 300 g


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