Tech Support Document
Thunder Support Document

Chilled Mirror Hygrometer Calibration using the Model 2500 Two-Pressure Humidity Generator

Dew point mirror & Hygro M4.

Figure 5.1

A typical dewpoint hygrometer with air temp sensor (AT) Series Hygro M4 is a general purpose optical condensation hygrometer used for industrial and laboratory applications.

Optical condensation hygrometry is a precise technique for determining the water vapor content in gases by measuring dew or frost temperatures. Optical condensation hygrometry works on the chilled-mirror principle. A metallic mirror surface is cooled until it reaches a temperature at which condensation begins to form on it. The dew layer is optically detected and the mirror is held at that temperature. The mirror temperature, measured with a platinum resistance thermometer is an accurate indicator of the dew or frost point.

Typical accuracy for this hygrometer is specified at ±0.2 °C DP.

The calibration of the dewpoint hygrometer as described and illustrated, will be a 3 point sample calibration at 20, 50, & 80% RH at a test temperature of 25 °C.

Dew point values will be calculated using HumiCalc Humidity Conversion software.



Hygro M4 Operations Manual.

Figure 5.2

It is always a good idea to review the operations manual for the instrument you will be calibrating.



Air sample pump.

Figure 5.3

You will need a sample air pump as illustrated. The air pump is used for drawing a sample from the Model 2500 test chamber and flowing this sample over the dewpoint mirror. It is important to have adequate flow rate over the dewpoint mirror. A flow rate of less than 1 SLPM is specified by the manufacturer. To much flow over the mirror will cause instability and accuracy problems.

Please refer to Fig 5.43 & 5.44 for an alternate method of providing metered flow over the dew point mirror.



Manifold in chamber.

Figure 5.4

Install the manifold accessory in the test chamber of the 2500. The manifold will thread into the chamber inlet port with a ¼ inch NPT male thread. It's not necessary to seal or tighten the manifold tight.

The manifold accessory will reduce the calibration test time.



2500 access port plug.

Figure 5.5

Remove the 2500 foam access port plug and guide the air temp probe and sample tube into the test chamber.

The tubing used for the inlet and outlet from the sample air pump should be a clean ¼ inch Teflon tube. It is not a good idea to use rubber or plastic tubing such as PVC because of the hygroscopic nature of the material.



Air sample tube & temp probe.

Figure 5.6

Bundle the sample tube, air temp probe and the chamber temp sensor from the 2500 together in the manifold as illustrated. It is important that the air temp probe from the hygrometer is positioned with the 2500 chamber temp sensor.



Insulation foam insert.

Figure 5.7

Install the 2 inch foam insulation piece in the 2500 chamber opening behind the door. Additional insulation will reduce calibration test time.



Dew point mirror, pump & Hygro M4.

Figure 5.8

The sample tube from the 2500 test chamber is connected to one side of the dew point mirror using ¼" Swagelok fitting. The outlet of the dewpoint mirror is connected to the air sample pump. You will note the outlet of the pump has been fitted with a flow meter.



Tighten the fittings.

Figure 5.9

Tighten the fittings as shown.



Ready for Calibration.

Figure 5.10

The sample tube and air pump as illustrated are ready for calibration.

Connect the dewpoint mirror and air temp probe to the connections on the display.

Apply AC power to the sample air pump and dew point hygrometer.



Flow meter adjustment.

Figure 5.11

The flow meter connected to the outlet of the sample air pump is adjusted for a value less than 1 SLPM. It is important to have adequate flow rate over the dewpoint mirror. A flow rate of less than 1 SLPM is specified by the manufacturer. To much flow over the mirror will cause instability and accuracy problems.



Change the setpoint to 20.00% RH.

Figure 5.12

Apply AC power to the Model 2500 and air compressor accessory.

Change the setpoint on the 2500 control screen to 20% RH at PC/TC, change the chamber temperature setpoint to 25 °C.



HumiCalc set @ 20% RH.

Figure 5.13

Using HumiCalc we will calculate the dewpoint for a humidity setpoint of 20% RH.

In the Normal mode select %RH as the known value, input the chamber test temperature of 25 °C, input the test pressure as displayed on the 2500 screen Chamber PSI.

The calculated dewpoint for 20% RH as displayed in the example is 0.54224 DP.



Starting the 2500 generator.

Figure 5.14

Press RUN to start the 2500 and begin the calibration. If you are using ControLog Automation software click Run and select Generate. As a rule of thumb we will allow the 2500 system and dew point hygrometer to warm up for 60 minutes before taking our first test point. The test point interval will be 30 minutes after the initial warm up.



Calibration worksheet.

Figure 5.15

Thunder metrologists use a calibration worksheet to record data from the 2500 and the device under test. Ambient test conditions in the lab are noted and dated on the work sheet. Data collected from the 2500 will include the headings as listed in this example.

“List the headings from TSC calibration worksheet Date Time etc”.



Recording calibration readings.

Figure 5.16

After 60 minutes we are ready to take a test point at 20% RH at 25° C. Record the readings from the 2500 and dewpoint hygrometer.



Humidity output reading @ 20%.

Figure 5.17

Temperature output reading in degrees C.

Figure 5.18

Temperature output reading in degrees F.

Figure 5.19

Dew Point output degrees C.

Figure 5.20

Dew point output degrees F.

Figure 5.21

The displayed output values from the hygrometer are illustrated in sequence Fig. 5.17-5.21.



Hygrometer readout @ 20% RH.

Figure 5.22

In addition to the manual readings the serial output from the hygrometer has been recorded using a terminal program on a PC labtop. The displayed values offer higher resolution and permanent record for the 20 % RH test point.



HumiCalc set @ 20% RH.

Figure 5.23

Using HumiCalc the dew point value has been calculated at the 20% RH test point using the measured values from the 2500.



ControLog shown @ 20% RH.

Figure 5.24

The display screen from ControLog Automation Software is shown in Fig 5.24, note the dewpoint value is displayed in the calculated humidity parameters. Using ControLog in conjunction with the serial output from the hygrometer will save time recording and storing calibration data.



Setting the 2500 to 50.00% RH.

Figure 5.25

Change the setpoint on the 2500 to 50% RH at 25 °C. When using ControLog click on the humidity setpoint and input the new setpoint of 50% RH. The setpoint stabilization time will be 30 minutes. Record the readings from the 2500 and dewpoint hygrometer at 50% RH.



Humidity output reading @ 50%.

Figure 5.26

Temperature output reading in degrees C.

Figure 5.27

Temperature output reading in degrees F.

Figure 5.28

Dew Point output degrees C.

Figure 5.29

Dew Point output degrees F.

Figure 5.30

The displayed output values from the hygrometer are illustrated in sequence Fig. 5.26-5.30.



Hygrometer readout @ 50% RH.

Figure 5.31

In addition to the manual readings the serial output from the hygrometer has been recorded using a terminal program on a PC labtop. The displayed values offer higher resolution and permanent record for the 50 % RH test point.



HumiCalc set @ 50% RH.

Figure 5.32

Using HumiCalc the dew point value has been calculated at the 50% RH test point using the measured values from the 2500.



ControLog shown @ 50% RH.

Figure 5.33

The display screen from ControLog Automation software is shown in Fig 5.33 note the dewpoint value is displayed in the calculated humidity parameters.



Setting the 2500 to 80.00% RH.

Figure 5.34

Change the setpoint on the 2500 to 80% RH at 25 °C. When using ControLog click on the humidity setpoint and input the new setpoint of 80% RH. The setpoint stabilization time will be 30 minutes.

Record the readings from the 2500 and dewpoint hygrometer at 80% RH.



Humidity output reading @ 80%.

Figure 5.35

Temperature output reading in degrees C.

Figure 5.36

Temperature output reading in degrees F.

Figure 5.37

Dew point output degrees C.

Figure 5.38

Dew point output degrees F.

Figure 5.39

The displayed output values from the hygrometer are illustrated in sequence Fig. 5.35-5.39.



Hygrometer readout @ 80% RH.

Figure 5.40

In addition to the manual readings the serial output from the hygrometer has been recorded using a terminal program on a PC labtop. The displayed values offer higher resolution and permanent record for the 80 % RH test point.



HumiCalc set @ 80% RH.

Figure 5.41

Using HumiCalc the dew point value has been calculated at the 80% RH test point using the measured values from the 2500.



ControLog shown @ 80% RH.

Figure 5.42

The display screen from ControLog Automation software is shown in Fig 5.42 note the dewpoint value is displayed in the calculated humidity parameters.

The initial as found calibration is complete. The dew point mirror should be cleaned and inspected as per the manufacturers recommendation before testing the as left calibration of the dew point hygrometer.

The as left calibration will be performed using the same test points beginning at 20% RH, test point interval will be 30 minutes.



An alternate test setup when the air sample pump is not available is shown in the following illustrations.

Needle valve placement in chamber.

Figure 5.43



Needle valve adjustment.

Figure 5.44

A tee fitting with needle adjustment valve has been assembled and threaded into the 2500 chamber inlet port. The inlet of the tee fitting is open to the test chamber. The sample tube is connected to the needle valve at one end, the other end is connected to the dew point mirror. The 2500 chamber temp sensor and hygrometer temp probe have been taped to the bottom of the test chamber.



Flow valve attached to the dew point mirror.

Figure 5.45

The outlet from the mirror is shown connected to the flow meter. It will be necessary to run the 2500 at the first test point and adjust the needle valve for proper flow rate through the dew point mirror as shown in Fig 5.44 Wait 5 minutes for the 2500 to stabilize at the setpoint before attempting the flow rate adjustment.


Click here to continue with the next calibration session.

Please contact support@thunderscientific.com should you have questions or problems using the procedure. We always appreciate your comments and feedback.

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