Fluke 5128A RHapid-Cal® Humidity Generator

Seven key features of the 5128A RHapid-Cal® Humidity Generator

1. Best-in-class system accuracy for dependable Humidity Generator probe calibration

The 5128A RHapid-Cal® offers a best-in-class humidity system accuracy of ± 1.0 % RH (7 to 80% RH, 18 °C to 28 °C) which includes all known sources of error such as stability, uniformity, drift, and calibration uncertainty.  Calibrate with confidence using an instrument with comprehensive specifications.  Most competing generators don’t specify system accuracy.  Their specifications can be complicated and confusing, making it difficult to know how to apply them to a particular calibration process.
The Fluke 5128A also provides the flexibility to improve calibration uncertainty by using an external humidity reference such as a chilled mirror hygrometer.

2. Rapid humidity and temperature stabilization time for high calibration throughput

The materials and airflow design used in the Fluke 5128A RHapid-Cal® are selected to ensure that response time to humidity or temperature step change is fast. The rate of change for temperature increase is typically 10 °C/minute and temperature, the decrease is 1.5 °C/minute. The rate of change for humidity increase is typically 10% RH/minute and for humidity, the decrease is 5% RH/minute. A typical six-point calibration can be done in two hours using the Fluke 5128A RHapid-Cal®. In contrast, a two-pressure humidity generator takes longer to respond to humidity or temperature changes. A similar six-point calibration with a typical two-pressure generator would take more than three hours.

3. Supports on-site, multi-point calibration of humidity probes

Spot-check or one-point probe calibration using a handheld humidity meter in the field is convenient but limited in value. Calibration with a handheld meter needs to be carefully managed. Temperature differences between the probe and its environment, technician body heat, and moisture from breath can all cause RH measurement errors.

Further, one-point tests may cause out-of-tolerance readings when ambient conditions change. Using the Fluke 5128A RHapid-Cal® for a multi-point calibration gives a more reliable test and truer characterization of how a humidity probe actually operates over its working range in the field.

4. Versatile design accommodates a large workload

A large variety of humidity sensors can be accommodated in the Fluke 5128A test chamber. The Fluke 5128A comes with a five-port door for calibrating up to five RH probes, meters, and transmitters at a time. An optional transparent door with a shelf is available. Data loggers are placed on the shelf inside the chamber for calibration. The mixing insert can be removed to accommodate larger devices in the chamber.

5. Compact size and lightweight for easy transport

The 5128A RHapid-Cal® measures 237 mm high x 432 mm wide x 521 mm deep (9.3 in x 17 in x 20.5 in) and weighs just 15 kg (33.06 lbs). It can be easily carried to any desired bench space in the lab or transported on-site to a fieldwork location. Its front-loading internal desiccant cartridge design adds convenience and ruggedness. In comparison, a two-pressure humidity generator is practically limited to laboratory use because of its large size. It includes a generator, compressor, and supporting equipment. A “small” two-pressure generator requires about eight times the space that the 5128A RHapid-Cal® does and weighs about four times as much. The 5128A RHapid-Cal® is easily placed on a small cart for plant-wide transport and a convenient wheeled transport case is offered for shipment or transport to on-site calibrations.

6. ISO 17025 accredited system calibration included standard

Prior to shipment, each 5128A RHapid-Cal® receives an accredited system calibration by Fluke of the humidity chamber with its internal reference probe, using a chilled mirror hygrometer as the reference standard. This system calibration provides the assurance that the 5128A and its internal reference probe have been optimized for the best performance when they leave the factory. In contrast, some humidity generator suppliers only provide a reference probe calibration, but not a complete system calibration that ensures uniformity and accuracy delivered at the location of your device under test.

7. Easy to maintain

5128A RHapid-Cal® uses a mixed-flow method to generate relative humidity. A desiccant cartridge provides a source of low humidity and an internal humidifier generates high humidity. A display light indicates when the desiccant cartridge needs to be replaced. The 5128A front-loading internal desiccant cartridge design adds convenience and ruggedness. The 5128A front-loading internal desiccant cartridge design adds convenience and ruggedness. The desiccant cartridge can be easily changed by removing the front cap and sliding in a new one.

Only clean distilled water is needed to operate the 5128A RHapid-Cal®. Compressed air or other additional fluids are not required. A water level indicator on the front panel shows the status of the water level in the humidity generator. When the water level falls below the minimum level, use clean distilled water to fill the reservoir.

No special shut-down routines are required after use, so you can move on to the next job quickly.

Set up your own, in-house humidity calibration and measurement lab with the Fluke 5128A

Should you bring humidity measurement and calibration in the house? Get answers to this question from the perspective of ROI, requirements, and workload at our blog post: Is it Time to Set Up Your Own In-House, Humidity Calibration, and Measuring Lab?

Fluke 5128A General specifications

AC mains voltage 100 V to 240 V ±10%
Standard frequency range 47 Hz to 63 Hz
Power consumption 300 VA
Required test fluids Distilled water
Warm-up period Twice the time since last warmed up, to a maximum of 30 minutes
Mains Fuse Rating F4A 250 V (fast blow)
Operating humidity range, 18 °C to 28 °C Up to 80 % RH
Storage temperature and relative humidity -20 °C to 50 °C, 0 % to 95 % RH, non-condensing
Transducer Power Output 12 V dc, 1 A maximum, fuse: F 1A 250V (fast blow)
Computer Interface USB
Safety IEC 61010-1, Installation Category II, Pollution Degree 2, Indoor use only
Altitude 2000 m

Electromagnetic Compatibility (EMC)

International IEC 61326-1: Controlled Electromagnetic Environment

CISPR 11: Group 1, Class A
Group 1: Equipment has intentionally generated and/or uses conductively-coupled radiofrequency energy that is necessary for the internal function of the equipment itself.

Class A: Equipment is suitable for use in all establishments other than domestic and those directly connected to a low-voltage power supply network that supplies buildings used for domestic purposes. There may be potential difficulties in ensuring electromagnetic compatibility in other environments due to conducted and radiated disturbances.

Korea (KCC) Class A Equipment (Industrial, Broadcasting, & Communication Equipment)

Class A: Equipment meets requirements for industrial electromagnetic wave equipment and the seller or user should take notice of it. This equipment is intended for use in business environments and not to be used in homes.

USA (FCC) 47 CFR 15 subpart B. This product is considered an exempt device per clause 15.103.
Chassis only
15 kg (33.06 lbs)
Chassis 237 mm x 432 mm x 521 mm (9.3 in x 17 in x 20.5 in) (height x width x depth)
Overall Chamber Dimensions 200 mm (7.87 in) (depth) x 150 mm (5.90 in) (diameter)
Working Volume Dimensions 109 mm (4.3 in) (depth) x 125 mm (4.92 in) (diameter)
Temperature resolution
Display 0.1 °C
USB data 0.1 °C
Humidity resolution  
Display 0.1 % RH
USB data 0.1 % RH
Dew point  
Resolution 0.1 °C (for indication only)

Note:  The displayed dew point reading (DP) is calculated from the Product’s actual temperature and %RH readings. It is calculated as a dew point (water vapor over water) over the entire range, regardless of the dew point is equal to or lower than 0°C, and at a nominal pressure of 101.325 kPa (1 atmosphere).

Humidity and temperature chamber technical specifications

The Product specifications describe the Absolute Instrumental Uncertainty of the Product. The Product specifications include stability, ambient temperature, and humidity (within specified limits), linearity, line regulation, the reference standard measurement uncertainty, and long term stability of one year. The product specifications are provided at a 99 %, k=2.58, normally distributed level of confidence unless otherwise noted.

Fluke 5128A Chamber specifications

One Year, Ambient Temperature Range 23 °C ± 3 °C1
Range: Chamber temperature  Range: Chamber humidity  Humidity specification Temperature specification
18 °C to 23 °C 7 % to 80 % RH
>80 % to 95 %RH
±1.0 % RH
±1.25 % RH
±0.2 °C
±0.2 °C
>23 °C to 28 °C 7 % to 80 % RH
>80 % to Hmax% RH
±1.0 % RH
±1.25 % RH
±0.2 °C
±0.2 °C
1 For ambient conditions of 23 °C ± 5 °C, multiply the specifications by 1.5.

2 Hmax is the maximum humidity value at which the specification applies.

Note: Specifications apply to the Working Volume shown in the Working Volume Template found at the end of the Operators Manual, and is referenced to the “Actual” reading on the Product display.

Chamber Uniformity and Stability

Ambient temperature range: 23 °C ±3 °C1
Chamber temperature Chamber humidity Chamber temperature uniformity2 Chamber humidity uniformity2 Chamber humidity stability3 Chamber temperature stability 3
18 °C to 28 °C 7 % ±0.12 °C ±0.3 % RH ±0.15 % RH ±0.05 °C
The following specifications are typical for chamber conditions shown4
5 °C to <18 °C5
>28 °C to 30 °C
>30 °C to 35 °C
>35 °C to 40 °C
>40 °C to 50 °C
15 %
7 %
7 %
7 %
7 %
±0.5 °C
±0.2 °C
±0.3 °C
±0.5 °C
±0.5 °C
±1.5 % RH
±0.6 % RH
±0.9 % RH
±1.5 % RH
±1.5 % RH
±0.5 % RH
±0.3 % RH
±0.4 % RH
±0.5 % RH
±0.5 % RH
±0.5 °C
±0.2 °C
±0.3 °C
±0.5 °C
±0.5 °C
1  For ambient conditions of 23 °C ± 5 °C, multiply the specifications by 1.5.
Defined as the uniformity of the Working Volume.
3  Defined as 1-sigma standard deviation of measurement readings over a 5-minute span.
4 Chamber humidity uniformity is listed for mid-level humidity settings. Lower humidity settings will give better uniformity while higher humidity settings will give worse uniformity.
5 The chamber control range is 5 °C to 50 °C. Achievable low temperature may be limited to 15 °C below ambient dependent upon stabilization time and temperature and humidity settings.

Fluke 5128A Operational specifications

Temperature rate of change – down 1.5 °C/minute (Typical)
Temperature rate of change – up 10 °C /minute (Typical)
Humidity rate of change – down 5 % RH/minute (Typical)
Humidity rate of change – up 10 % RH/minute (Typical)
Model Name Description
5128A RHapid-Cal® Humidity Generator


  • 5-Port Door- 1 Square
  • Desiccant Cartridge-1
  • 1 Fill Syringe with ExtensionTube
  • 5 Grommets (one each of 1/4 in, 3/8 in, 1/2 in, 3/4 in, 7/8 in sizes)
  • 1 Mains Power Cord, 2-Meter
  • Fluke ISO 17025 Accredited System Calibration, 115 V AC / 230 V AC

Fluke 5128A Accessories common to all models:

Accessory Description
5128-2686 Desiccant Material Kit (includes desiccant plus 2 caps and 2 filters)
5128-2685 Desiccant Cartridge (including desiccant material)
5128-2681-R5 Round Door, 5 ports
5128-2681-S0 Clear Square Door with two shelves (small and large)
5128-2681-S5 Square Door, 5 ports (spare)
5128-CASE 5128A Case with Wheels
5128-2682-1/4 Port Grommets, 1/4 inch, 5 each
5128-2682-3/8 Port Grommets, 3/8 inch, 5 each
5128-2682-1/2 Port Grommets, 1/2 inch, 5 each
5128-2682-3/4 Port Grommets, 3/4 inch, 5 each
Port Grommets, 7/8 inch, 5 each
5128-2683 Port Plugs, 5 each
5128-2684 Fill Syringe with Extension Tube


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