Calibration of the Two-Pressure Humidity Generator

Figure 8.1

Dew point standard with an accuracy of ±0.1 °C, heated hose accessory with controller. Dew point instrument is designed with an internal sample pump to provide air flow over the dew point sensor.

Temperature probe with an accuracy of ±0.03 °C accuracy.

Figure 8.2

Laptop computer running ControLog© Automation software.

HumiCalc© Humidity Conversion software will be used to calculate dew point values measured from the dew point standard.

Figure 8.3

Tape the 2500 chamber temperature sensor together with the external temperature probe to the back wall of the test chamber or insert the probes into the calibration manifold.

Figure 8.4

Install the calibration manifold, insert the dew point sample tube thru the access port into the manifold fixture.

Figure 8.5

Insert the dew point sample tube thru the access port.

Figure 8.6

Position the dew point sample tube into the manifold fixture.

Figure 8.7

Press CHNG SETP key. Change the %RH value to 20.0% RH, change the satur test temp to 23.0 °C on the 2500 control screen, press ENTER. Allow the system to stabilize for 60 minutes.

Figure 8.8

Select saturation temperature, change the set point to 23.0 °C test temperature, select %RH set point, change the value to 20.0% RH.

Figure 8.9

Test temperature, select %RH set point. Change the value to 20.0% RH.

Figure 8.10

Select RUN click on Generate to begin the calibration comparison at 23.0 °C at 20.0% RH, allow the 2500 to stabilize for 60 minutes before taking the first test point.

Figure 8.11

2500 display screen stable at 20.0% RH at a test temperature of 23.0 °C.

Figure 8.12

Dew point standard displays a stable dew point measurement at 20.0% RH.

Figure 8.13

ControLog© graph displays a stable test temperature of 23.0 °C at 20.0% RH, note the calculated dew point value.

Figure 8.14

Calculate dew point value using HumiCalc©, input Known dew point, test temperature as measured by the external temperature sensor, test pressure as displayed on the 2500 screen. Humidity value as calculated from the measured dew point.

Figure 8.15

Press CHNG SETP key. Change the %RH value to 50.0% RH on the 2500 control screen, press ENTER. Allow the system to stabilize for 30 minutes.

Figure 8.16

Select %RH set point, change the value to 50.0% RH or change the set point value on the 2500 control screen to 50.0% RH. Allow the system to stabilize for 30 minutes.

Figure 8.17

2500 display screen stable at 50.0% RH at a test temperature of 23.0 °C.

Figure 8.18

Dew point standard displays a stable dew point measurement at 50.0% RH.

Figure 8.19

ControLog© graph displays a stable test temperature of 23.0 °C at 50.0% RH, note the calculated dew point value.

Figure 8.20

Calculate dew point value using HumiCalc©, input Known dew point, test temperature as measured by the external temperature sensor, test pressure as displayed on the 2500 screen. Humidity value as calculated from the measured dew point.

Figure 8.21

Press CHNG SETP key. Change the %RH value to 80.0% RH on the 2500 control screen, press ENTER. Allow the system to stabilize for 30 minutes.

Figure 8.22

2500 display screen stable at 80.0% RH at a test temperature of 23.0 °C.

Figure 8.23

Dew point standard displays a stable dew point measurement at 80.0% RH.

Figure 8.24

ControLog© graph displays a stable test temperature of 23.0 °C at 80.0% RH, note the calculated dew point value.

Figure 8.25

Calculate dew point value using HumiCalc©, input Known dew point, test temperature as measured by the external temperature sensor, test pressure as displayed on the 2500 screen. Humidity value as calculated from the measured dew point.

Figure 8.26

Select RUN click on Shutdown to end the calibration comparison. Save the 2500 data to a proper file, or press stop to shutdown the 2500.

Calibration – Pressure Transducers

• Proper calibration of the pressure transducers is critical to the accuracy of the generated humidity.
• Low – Pressure (0-50 PSIA) Accuracy of the calibration source should be ±0.025 PSIA.
• High – Pressure (50-150 PSIA) Accuracy of the calibration source should be ±0.10 PSIA.
• Calibration of the transducers is performed in the system as a system. Calibration coefficients are calculated automatically by the 2500 embedded computer and stored in memory with the date of the most recent calibration.

Figure 8.27

Disconnect AC power from the 2500 system before removal of the panels. First remove the reservoir fill cap before removing the top left panel. Use a #10 torx driver to remove the panels.

Figure 8.28

When removing the right rear cover, pay close attention to the fan cord plug, it must be removed before pulling the panel away. This will allow access to the low and high pressure transducers.

Figure 8.29

Remove the low pressure transducer as identified as T2 using a 9/16” and 11/16" wrench set.

Figure 8.30

Connect the low pressure transducer to the calibration source.

Figure 8.31

Power on the 2500 and allow the system to stabilize for 30 minutes.

Press the EDIT / CAL key.

Figure 8.32

Press CAL, the calibration menu is displayed.

Figure 8.33

Press the PRES CAL key.

Figure 8.34

Using the MARK/CLR key mark the low range transducer for calibration, a marked transducer is indicated with an asterisk in left column.

Figure 8.35

Press OK, if a mistake was made press EXIT QUIT.

Figure 8.36

The first calibration point will be ambient test pressure as displayed on the calibration source, press LOW PRES key and enter the reference pressure then press ENTER.

If a mistake is made during the pressure entry press the OOPS to cancel the entry.

Figure 8.37

Press the LOW PRES key.

Figure 8.38

Enter the reference pressure.

Figure 8.39

Press ENTER. Note the Low value as entered for the low pressure.

Figure 8.40

Apply the mid range pressure calibration point of 30.000 PSIA, wait 5 minutes for stabilization.

Figure 8.41

Press MID PRES key and enter the reference pressure, then press ENTER.

Figure 8.42

Apply the high pressure calibration point of 50.000 PSIA, wait 5 minutes for stabilization.

Figure 8.43

Press HIGH PRES key and enter the reference pressure, press ENTER.

Figure 8.44

Press CALC COEF key. All coefficients for the marked transducer will be calculated and displayed on the 2500 calibration screen.

Figure 8.45

Press SAVE QUIT key, the coefficients will be stored to non-volatile memory.

Figure 8.46

Press PRES CAL retest as left calibration comparison with the pressure source at ambient test pressure, 20.00, 30.00, 40.00, & 50.00 PSIA test points. Press EXIT QUIT key.

Figure 8.47

Press the EDIT / CAL key, press CAL, the calibration menu is displayed.

Figure 8.48

Press the PRES CAL key.

Figure 8.49

Using the MARK / CLR key and down arrow key mark the high range transducer for calibration, a marked transducer is indicated with an asterisk in left column.

Figure 8.50

Press OK, if a mistake was made press EXIT QUIT.

Figure 8.51

Remove the high pressure transducer as identified as T3 using a 9/16” and 11/16" wrench set.

Figure 8.52

Connect the high pressure transducer to the calibration source.

Figure 8.53

The first calibration point will be 50.000 PSIA test pressure as displayed on the calibration source, wait 5 minutes.

Figure 8.54

Press LOW PRES key and enter the reference pressure then press ENTER.

Figure 8.55

If a mistake is made during the pressure entry, press the OOPS key to cancel the entry.

Figure 8.56

Apply the mid range pressure calibration point of 100.000 PSIA, wait 5 minutes for stabilization.

Figure 8.57

Press MID PRES key and enter the reference pressure, press ENTER.

Figure 8.58

Apply the high pressure calibration point of 150.000 PSIA, wait 5 minutes for stabilization.

Figure 8.59

Press HIGH PRES key and enter the reference pressure, press ENTER.

Figure 8.60

Press CALC COEF key. All coefficients for the marked transducer will be calculated and displayed on the 2500 calibration screen.

Figure 8.61

Press SAVE QUIT key, the coefficients will be stored to non-volatile memory.

Figure 8.62

Press the PRES CAL key.

Figure 8.63

Press OK. Retest as left calibration comparison with the pressure source at 50.00, 75.00, 100.00, 125.00, & 150.00 PSIA test points.

Figure 8.64

Press EXIT QUIT key.

Figure 8.65

At the calibration menu press DONE, press DONE again, the system will reinitialize to the Control/Display screen.

2500 System Calibration Temperature

• Four 10KΩ thermistors are use for temperature readout and control of the 2500.
• All are easily removed for calibration.
• Probes.
  1. Saturation Temperature (RTD0)
  2. Presaturation Temperature (RTD1)
  3. Expansion Valve Temperature (RTD2)
  4. Test Chamber Temperature (RTD3)

A small temperature circulation bath will be used for calibration of the temperature sensors over the range of 0° C to +70.0° C A standard reference thermometer (PRT or Thermistor) with an accuracy of ±0.03° C.

Figure 8.66

Disconnect AC power from the 2500 system before removal of the panels, using a #10 torx driver remove the top right cover.

Figure 8.67

Next remove the left rear cover for access to the temperature sensors.

Figure 8.68

Remove insulation material from the expansion valve box.

Figure 8.69

Loosen the 7/16“ probe fitting.

Figure 8.70

Remove the expansion valve temperature sensor (RTD2).

Figure 8.71

Remove the insulation material from the chamber temp probe location.

Figure 8.72

Remove the chamber temperature sensor (RTD3).

Figure 8.73

Remove the insulation material from the back panel location labeled pre-sat temp probe.

Figure 8.74

Loosen the 7/16” probe fitting.

Figure 8.75

Remove the pre-saturation temperature sensor (RTD1).

Figure 8.76

Remove the insulation material from the chamber fluid fill port.

Figure 8.77

Remove the red cap cover.

Figure 8.78

Replace the red cap with a tight fitting rubber plug.

Figure 8.79

Loosen the chamber fluid drain hose fitting and crimp the hose to prevent fluid loss, remove the fitting cap.

Figure 8.80

Drain approximately 20 oz. of fluid in a clean container. Fluid will be reused after calibration.

Figure 8.81

Remove the insulation material from the back panel location labeled sat-temp probe.

Figure 8.82

Loosen the 7/16” probe fitting.

Figure 8.83

Remove the saturation temperature sensor (RTD0).

Figure 8.84

Bundle the 4 temperature sensors together with the PRT or thermistor probe.

Figure 8.85

Submerge in the fluid circulation bath.

Figure 8.86

Apply power to the 2500, wait for the system to initialize and display the Control/Display screen.

Press the EDIT/CAL key, the edit cal menu will be displayed.

Figure 8.87

Press the CAL key, the calibration menu will be displayed.

Figure 8.88

Press the TEMP CAL key, the probe selection screen will be displayed.

Figure 8.89

Using the MARK/CLR key and down arrow key mark the probes to be calibrated.

Figure 8.90

A marked probe is indicated by an asterisk on the left, the Refer Tmp cannot be marked and is displayed as reference only.

Figure 8.91

Once the probes are marked press OK.

Figure 8.92

The LOW, MID and HIGH temperature calibration values will be displayed at the bottom of the screen, actual data from the 4 probes will be displayed in the Count and Deg columns.

Figure 8.93

Start the calibration at the first calibration reference point of 0.0 °C allow the system time to stabilize.

Figure 8.94

After stabilization press the LOW TEMP key and input the value from the Standard Thermometer at the LOW temperature.

Figure 8.95

Use the ± key as necessary.

Figure 8.96

Press ENTER. The LOW temperature value just entered will be displayed under the LOW legend.

If a mistake is made during the temperature entry press the OOPS to cancel the entry.

Figure 8.97

Change the calibration reference point to 35.0 °C allow the system time to stabilize.

Figure 8.98

After stabilization press the MID TEMP key and input the value from the Standard Thermometer at the MID temperature.

Figure 8.99

Press ENTER. The MID temperature value just entered will be displayed under the MID legend.

Figure 8.100

Change the calibration reference point to 70.0 °C allow the system time to stabilize.

Figure 8.101

After stabilization press the HIGH TEMP key and input the value from the Standard Thermometer at the HIGH temperature.

Figure 8.102

Press ENTER. The HIGH temperature value just entered will be displayed under the HIGH legend.

Figure 8.103

Press CALC COEF key. All coefficients for the marked temperature sensors will be calculated and displayed on the 2500 calibration screen.

Figure 8.104

Press SAVE QUIT key, the coefficients will be stored to non-volatile memory.

Figure 8.105

Press TEMP CAL.

Figure 8.106

Press OK. Retest as left calibration comparison with the temperature calibration source at 0 °C, 15.0 °C, 25.0 °C, 45.0 °C & 70.0 °C or at random values over the range.

Figure 8.107

Press EXIT QUIT key.

Figure 8.108

At the calibration menu press DONE, press DONE again, the system will reinitialize to the Control/Display screen.

Printing The Report

If a printer is connected to the Printer Port a report may be printed which lists the calibration coefficients and calibration dates for the pressure and temperature transducers.

The report is printed from the EDIT/CAL menu using the PRNT REPT key.

Repeat the as left calibration inter-comparison using the dew point calibration standard and reference thermometer.

Two-Pressure Humidity Generator

Two-pressure humidity generation entails saturating air or nitrogen with water vapor at a known temperature and pressure. The saturated high-pressure air flows from the saturator and through a pressure-reducing valve, where the gas is isothermally reduced to test pressure at test temperature. Humidity generation does not depend on measuring the amount of water vapor in the air, but instead is dependent only on the measurements of temperature and pressure.

Two-Pressure Principle

Figure 8.109

Two-pressure humidity generator.

Error and Uncertainty

Error and uncertainty calculations based on the calibration results of a Model 2500 Two-Pressure Humidity Generator.

• In this section we will analyze the error and uncertainty based on the results from the calibration process.
• We will also help identify the difference between error and uncertainty and the effect the different errors have on the generation of humidity in a Two-Pressure Humidity generator.

It is important not to confuse the terms ‘error’ and ‘uncertainty’.

Error is the difference between the measured value and the ‘true value’ of the item being measured. Whenever possible we try to correct for any known errors; for example, by applying corrections from the calibration process.

Uncertainty is a quantification of the doubt about the measurement result. The uncertainty in a stated measurement is the interval of confidence around the measured value such that the measured value is certain not to lie outside this stated interval.

The Difference Between Error and Uncertainty

A sample assumed to be about 15 %RH is measured by five different instruments, each taking five different readings. The readings, averages and standard deviations from each instrument are shown in the following table:

Assuming that none of the instruments have been calibrated and are basically new off the factory floor,

To get the answers, mouse over the question and click.

1. Which instrument has the greatest error?
Unknown
2. Which instrument has the greatest uncertainty?
Answer = D

The sample is now taken to a highly regarded lab which certifies it is 15.36 %RH.

1. Which instrument has the smallest error?
Answer = D
2. Which instrument has the smallest uncertainty?
Answer = C

The Effect Different Errors Have on the Generation of Humidity

Relative Humidity in a two-pressure humidity generator is determined from the measurements of temperature and pressure only and is expressed by the following formula:

Where the f functions are enhancement factors, eS is the saturation vapor pressure, hS is the % efficiency of saturation, Tc and Ts are the chamber and saturation temperatures, and Pc and Ps are the chamber and saturation pressures.

Pre-Saturation Temperature Probe –

The air stream of a two-pressure generator must be 100% saturated with water vapor at test temperature on the high-pressure (saturator) side of the expansion valve. This is accomplished by first passing the air stream through a "pre-saturator". The pre-saturator is a vertical pressure vessel presenting a water surface to the incoming air stream and is maintained constant at a temperature 15 to 20 °C warmer than the desired system (chamber) temperature. The Pre-saturator temperature probe is used in the control of the pre-saturator heaters, which when activated, are used to control this 15 to 20 °C temperature offset. As long as the pre-saturator temperature error is well below this temperature offset, it will have no influence on the computed value of the generated humidity.

Expansion Valve Temperature Probe –

The expansion valve temperature probe is used in the control of the expansion valve heaters, which when activated, are used to warm the expansion valve body, offsetting the cooling effects due to gas expansion. This expansion valve temperature is always maintained around 0.5 °C above the saturation temperature. As long as the expansion value temperature error is well within the 0.5 °C tolerance, it will not influence the computed value of the generated humidity.

Saturator and Chamber Temperature Probes –

During normal operation the chamber and saturator temperatures are very close. In the relative humidity equation, temperatures are computed as a ratio of the corresponding saturation vapor pressures. If no mismatch between the chamber and saturation temperatures exists, then very little error is contributed due to the temperature probe errors. This is true even if the temperature measurement of the two probes is actually incorrect or out of specification, provided that the relative difference between them is zero. Given this, even if both probes were out of specification, but indicated the same numeric value at the same equal temperature, there would be very little error contributed due to temperature.

Saturator and Chamber Pressure Transducers –

The pressure ratio, Pc/Ps, in a two-pressure humidity generator is the major %RH determining factor. Again, this is computed as a ratio, but since the saturator and chamber pressures can very greatly, it does not have the same negating effect that the temperatures ratio does.

Flow Meter –

The flow rate has no influence on the computed value of the generated humidity, but flow rate errors may have an affect on the devices you are calibrating with the Model 2500 Two-Pressure Humidity Generator.

Some Suggested Reading –

NCSL - RISP- 5

Two-Pressure Two-Temperature Humidity Generator

NIST - Handbook 150-2H

Calibration Laboratories Technical Guide for Thermodynamic Measurments

ASTM D 4230-02

Standard Test Method of Measuring Humidity with Cooled-Surface Condensation (Dew-Point) Hygrometer

ASTM E 104-02

Standard Practice for Maintaining Constant Relative Humidity by Means of Aqueous Solutions

Caution -Saturated salt solutions are extremely corrosive, and care should be taken in their preparation and handling. There is also the possibility of corrosive vapors in the atmospheres over the saturated salt solutions.

ASHRAE Standard - Spc. 41.6-1994R

Standard Method for Measurment of Moist Air Properties

Water Vapor Measurment - Methods and Instrumentation

Author: Peter R. Wiederhold

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