Thunder Scientific Corporation - Humidity Generation and Calibration Equipment NVLAP - Code 200582-0

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Toll Free: (800) 872-7728
Phone: (505) 265-8701
FAX: (505) 266-6203



Model 2500 Humidity Generator

Model 2500 Benchtop/Mobile

"Two-Pressure" Humidity Generator


“Due to obsolescence of components, Model 2500 humidity generators that have a serial number of 0105300 or less are no longer supported by Thunder Scientific.” End of Life, News Bulletin.



  • 0.5 %RH Uncertainty 1
  • Traceable to SI 3
  • Self Contained and Mobile
  • Automated Control of User Setpoints
  • 2500 ControLog® Automation Software
  • HumiCalc® with Uncertainty Software
  • Computerized Internal Transducer Calibration
  • RS-232C Serial Interface
  • Low Noise Air Compressor with Air Dryer (Option)


The Model 2500ST Benchtop Humidity Generator is a self contained system capable of producing atmospheres of known humidities using the fundamental, NIST proven, "two-pressure" principle. This system is capable of continuously supplying accurately known humidity values for instrument calibration, evaluation, and verification, as well as for environmental testing. Simply apply power and the 2500 will power-up ready to generate. Humidity setpoint values are input by the operator from the front panel keypad and are limited only by the range of the 2500 humidity generator. Relative humidities are calculated from the measurements of pressure and temperature with the formula:

%RH= fs/ fc · es/ ec · Pc/ Ps · 100

To generate a known humidity, the computer controls the pressure ratio Pc /Ps utilizing the enhancement factor ratio fs/fc and the effective degree of saturation es/ec. Humidity produced is solely dependent on the measurement of pressures and temperatures and does not rely on any other device (such as a dew point hygrometer, psychrometer, or humidity sensor) for the measurement of water vapor content. Precision humidity generation is determined by the accuracy of the pressure measurements and on the accuracy and uniformity of temperature throughout the generating system.



The Model 2500 Benchtop Humidity Generator operates using an on board multifunction CPU in conjunction with other peripheral cards to perform calculation and control functions. The embedded computer control system allows the 2500 to generate known humidity levels unattended, freeing the operating technician from the task of system monitoring and adjustment. A computer and/or printer may be connected via the bi-directional RS-232C interface ports allowing remote setpoint control and continuous acquisition of system data. Humidity and temperature setpoint values are input by the operator from the front panel keypad while visual indications of system status are displayed in real time on the liquid crystal display.

Control / Display Screen.

Control / Display Screen

All control and measurement parameters critical to the operation of the humidity generator are displayed on this screen. Each parameter in the left most column is identified with a brief title and corresponding units. The generator operates in a variety of user selectable pressure, temperature, and flow units. Some of these are °C, °F, psi, "Hg, Tor, mbar, kPa, L/min, l/h, cfm and cfh. Humidity is calculated and displayed in percent relative humidity (%RH). The asterisk in the left most column indicates the active humidity control parameter. The "SetPnt" column lists control setpoints and the "Actual" column lists all of the measured data and calculated parameters of the generator.

Temperature Control: The system utilizes a fluid jacketed test chamber for extremely stable temperature control. Temperature setpoint control is attained by controlling the temperature of the circulating fluid medium that jackets the test chamber and associated humidity generation components. Chamber and saturation temperatures are governed by this medium and are digitally controlled by the computer at any value between 0 °C and 70 °C using PID (proportional-integral-derivative) algorithms.

Elemental Schematic of the 2500 generator.
Elemental Schematic of the 2500 generator.

Pressure And Flow Control: Pressure control and mass flow control are accomplished through computer actuation of electromechanical valve assemblies. Pressure and flow are measured continuously and controlled using PID algorithms similar to those employed in temperature control.

Calibration: The 2500 humidity generator employs an integral programmatic calibration scheme allowing the temperature and pressure transducers to be calibrated while they are electrically connected to the humidity generator.

Calibration / Display Screen

Calibration display screen.
2500 Test Chamber
2500 Test Chamber

Coefficients for each transducer are calculated by the computer and stored in the system’s nonvolatile memory until the next calibration is performed.






The fluid jacketed test chamber can accommodate humidity sensors, hygrothermographs, chilled mirror hygrometers, and various material samples for environmental testing. Virtually any humidity and temperature point may be generated, for any length of time, within the operational limits of the generator. The output of the device under test may then be compared with the generator's printed data for analysis.

Humidity Sensors And Chart Recorders: Insert humidity probes through the two inch port in the side of the chamber or place hygrothermographs into the chamber and you can: determine humidity calibration accuracy and characterize humidity sensitivity by subjecting the sensing system to a variety of humidity levels; perform operational checks such as the sensing systems capability to correctly calculate and display other humidity parameters; determine the repeatability, stability, hysteresis, and drift characteristics of various humidity sensing systems.

Chilled Mirror Hygrometers: Install the actual chilled mirror head into the chamber or insert a sample tube through the test port and draw a sample through the chilled mirror head and you can: verify mirror temperature measurement accuracy (calibration) when the hygrometer is in thermal equilibrium with its environment; perform operational checks of the heat pump and optical components before and after mirror cleaning and balancing; determine whether the hygrometer is controlling the mirror deposit in the liquid phase or ice phase when operating at dew and frost points below 0 °C; determine if the hygrometer is correctly calculating other humidity parameters; determine the hygrometer's repeatability, stability, and drift characteristics.

Environmental Testing: The 2500 can serve as a test bed for evaluation and R&D of humidity sensors, humidity sensing systems, and humidity sensitive products, e.g., polymers, composites, film, magnetic medium, blood gas analysis, pharmaceuticals, soil hydrology, consumables, electronics, optics, etc. Depending on the temperature and humidity being generated, the 2500 may operate continuously from hours to months. With continuous generation of a nominal 50 %RH at 21 °C, the reservoir will last about two weeks between refills.



Relative Humidity Range: 10 to 95 %RH
Relative Humidity Uncertainty: @ Pc Tc 1 0.5 %RH
Chamber Fluid Temperature Range: 0 to 70 ºC
Chamber Fluid Temperature Range: (Optional) –10 to 70 ºC
Chamber Temperature Uniformity: 2 ±0.1 ºC
Chamber Temperature Uncertainty: 1 0.06 ºC
Chamber Pressure Uncertainty: 1 0.15% FS
Chamber Pressure Range: Ambient
Display Resolution: 0.01
Gas Flow Rate Range: 5 to 20 L/min
Gas Flow Rate Resolution: 0.02 L/min
Gas Flow Rate Uncertainty: 1 2% FS
Gas Type: Air or Nitrogen
Gas Pressure Rating (MAWP): 175 psiG
Heating: Stainless Steel Immersion Heater
Heating/Cooling Rate: 2.5 Minutes Per ºC Average
Cooling: 1/3 hp R-134A Refrigeration System
Chamber Window: 6" x 6" (152 mm x 152 mm)
Physical Dimensions: H 22" (559 mm) W 36" (914 mm) D 23" (584 mm)
Physical Dimensions of Cart: H 33.25" (845 mm) W 43" (1.09 m) D 26" (660 mm)
Physical Dimensions 2500 with Cart: H 56" (1.42 m) W 43" (1.09 m) D 26" (660 mm)
Chamber Dimensions: H 15" (381 mm) W 15" (381 mm) D 12" (305 mm)
Access Port: SEE TABLE A

1 Represents an expanded uncertainty using a coverage factor, k=2, at an approximate level of confidence of 95%. Uncertainty for relative humidity is not specified below 0 °C.

2 When operated at temperatures within 10 °C of room ambient temperature.

3 Traceable to the International System of Units (SI) through a national metrology institute (NIST) recognized through a CIPM MRA.

Other custom options are available.

Access Port Dimensions
Option # Ports Port Diameter Location
Standard 1 1.9" (48 mm) Right Side
-TPA 2 1.9" (48 mm) Right Side

1 1/4" Swagelok® Right Side
-LPD 6 1.93" (49 mm) In Door



Electrical Power: 200/240 V~, 8 A, 60 Hz
Electrical Power: (Optional)

200/240 V~, 8 A, 50 Hz

Air Compressor: 100/115 V~, 10 A, 50/60 Hz
Air Compressor: (Optional) 220/240 V~, 6.3 A, 50/60 Hz
Air Supply (External): Clean Oil Free Instrument Air @ 175 psiG & 20 L/min



Operating Temperature: 15 to 30 ºC
Storage Temperature: 0 to 50 ºC
Humidity: 5 to 95% Non-condensing



Thunder has various options available for the Model 2500ST.

Tri Port Array (TPA) Has 2 access ports with a diameter of 1.9" (48 mm) in the right side of the chamber and 1 additional hygrometer sample port with a 1/4" (6.35 mm) fitting.


Low Temp (LT) Expanded chamber temperature range of –10 °C to +70 °C.


Large Port Door (LPD) Has 6 each 1.93" (49 mm) diameter access ports through the door for larger multi probe calibration.


Chamber Fluid Temp Port (FTP) provides external temperature probe access through the top panel for measurement of the saturation temperature.


High Air Flow (HAF) option provides a gas flow rate of 10 to 40 Liters per minute.


Door without window (NWD) The No Window Door option provides the system with improved chamber temperature uniformity.


Mobile Cart (CART-ST) Is specifically made for the Model 2500 but can be used to haul around any of your lab equipment.


2500 ControLog Automation Software (2CL) Allows the 2500 to communicate with various test instruments.

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Tri Port Array View
  Tri Port Array View
Large Port Chamber Door
  Large Port Chamber Door
Chamber Fluid Temp Port
  Chamber Fluid Temp Port
No Window Door
  No Window Door
2500 Mobile Cart
  2500 Mobile Cart
2500 ControLog Software
  2500 ControLog Software
  1 Represents an expanded uncertainty using a coverage factor, k=2, at an approximate level of confidence of 95%. Uncertainty for relative humidity is not specified below 0 °C.
  2 When operated at temperatures within 10 °C of room ambient temperature.
  3 Traceable to the International System of Units (SI) through a national metrology institute (NIST) recognized through a CIPM MRA.


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