RH CGS-240 User manual

CGS-240
RH Systems, LLC
1225 W Houston Ave.,
Gilbert AZ, 85233
Manual

Introduction
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TABLE OF CONTENTS
Introduction.............................................................................................................................. 4
1.1 Background of Humidity Generation .............................................................................................................4
1.2 Scope .............................................................................................................................................................4
Theory of operation ................................................................................................................. 5
2.1 Two-pressure principle ..................................................................................................................................5
2.2 Combined two-pressure and two-temperature principle .............................................................................6
2.3 Common Defining Equations .........................................................................................................................6
2.4 Humidity Equations .....................................................................................................................................10
Installation.............................................................................................................................. 13
3.1 Facility Requirements ..................................................................................................................................13
3.2 Utility Connections ......................................................................................................................................14
3.3 Initial Preparation ........................................................................................................................................17
Operation ............................................................................................................................... 22
4.1 Introduction to Software .............................................................................................................................23
4.2 Connection Window ....................................................................................................................................24
4.3 Enter Set Point.............................................................................................................................................25
4.4 Selecting Control mode and initiating Generate .........................................................................................26
4.5 Viewing Data Numerically and Graphically..................................................................................................29
4.6 Change Units................................................................................................................................................31
4.7 Connection of Other Instruments................................................................................................................31
4.8 Data Logging ................................................................................................................................................32
4.9 System Settings............................................................................................................................................33
4.10 Chamber ......................................................................................................................................................34
4.11 Door .............................................................................................................................................................35
Calibration.............................................................................................................................. 36
5.1 Pressure Calibration.....................................................................................................................................36

Introduction
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5.2 Temperature Calibration .............................................................................................................................46
Subsystems............................................................................................................................. 53
6.1 Electrical System..........................................................................................................................................53
6.2 Pneumatic System .......................................................................................................................................58
6.3 Fluid System.................................................................................................................................................62
6.4 Refrigeration System ...................................................................................................................................65
6.5 Heaters.........................................................................................................................................................68
6.6 Connections to the ECB ...............................................................................................................................71
6.7 Temperature Measurements.......................................................................................................................75
Troubleshoot/Maintenance................................................................................................... 76
7.1 Preventative Maintenance ..........................................................................................................................76
7.2 Troubleshooting...........................................................................................................................................82
Illustrated Parts Breakdown................................................................................................... 90
8.1 Parts List.......................................................................................................................................................90
8.2 Illustrations ..................................................................................................................................................97
Drawings............................................................................................................................... 119
RHS Plateau Software ....................................................................................................... 134
10.1 Introduction...............................................................................................................................................134
10.2 Sequence of Events....................................................................................................................................136
10.3 Data File Selection .....................................................................................................................................139
10.4 Evaluating for Plateaus ..............................................................................................................................141
10.5 Selecting a Manual Reference ...................................................................................................................143
10.6 Calculating Calibration Coefficients...........................................................................................................144
10.7 Plotting Residuals ......................................................................................................................................146
10.8 Export Data ................................................................................................................................................147
Document History............................................................................................................. 148

Introduction
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INTRODUCTION
The RH Systems CGS-240 Humidity Generator is a system capable of continuous, high-accuracy humidity
generation over a wide range of humidity, temperature, and flow rates. The CGS-240 design is an RHS hybrid two-
pressure two-temperature system based on the fundamental two-pressure principle originally developed by NIST.
The basis of this principle and its original development are discussed below.
1.1 BACKGROUND OF HUMIDITY GENERATION
In 1948 at the U.S. National Bureau of Standards (now the National Institute of Standards and Technology), E.R.
Weaver and R. Riley developed a “pressure method” for the generation and control of humidity. Their method,
termed the two-pressure principle, was derived from measurements of temperature and pressure rather than
requiring measurements of water vapor.
Using their technique, air or some other gas was saturated with water vapor at high pressure and then expanded
to a lower pressure. When saturation and expansion were performed under constant-temperature conditions, the
resulting relative humidity of the gas was simply the ratio of the lower pressure to the higher pressure (or at least
very nearly).
Their equipment was designed for low rates of gas flow and was used under ambient temperature conditions.
Their saturator was a small cylinder containing water and filled with fragments of pumice or stream-washed gravel
through which the gas could be bubbled under pressure. This device was developed primarily for the calibration of
electrically conductive hygroscopic films used in the measurement of water vapor in gases.
In 1951, also at the National Bureau of Standards, the two-pressure principle was the foundation on which A.
Wexler and R.D. Daniels developed a new “pressure–humidity apparatus” with higher air-flow capability. Another
significant improvement was the incorporation of temperature control. Developed primarily for hygrometer
research and calibration, it was capable of producing atmospheres of known relative humidity from 10 %RH to 98
%RH over a fairly wide temperature range –40°C to +40 °C.
Wexler also designed and constructed a low-humidity generator based on a method termed the two-temperature
principle. With the two-temperature principle, a stream of air or other gas is saturated with water vapor at a given
temperature; then its temperature is raised to a specified higher value.
1.2 SCOPE
The CGS-240 Humidity Generator is a portable, self-contained system which produces stable atmospheres of
known humidity and temperature using the two-temperature, two-pressure saturated vapor stream principle. Its
capability and intended purpose is to provide accurate humidity and temperature for the calibration of humidity
sensors, hygrothermographs, chilled mirror hygrometers, and other environmental measuring devices.

Theory of operation
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THEORY OF OPERATION
2.1 TWO-PRESSURE PRINCIPLE
In an ideal two-pressure system, a stream of gas at an elevated pressure is saturated with respect to the liquid or
solid phase of water and then expanded isothermally to a lower pressure. Measurements of the pressure and
temperature of the gas stream, both at saturation and after expansion, are all that is required to determine the
resulting humidity content of the expanded gas stream. The two-pressure system is shown in Figure 1.
Figure 1—Simplified schematic diagram of the two-pressure principle where Tsis saturator temperature, Tcis
chamber temperature, and Ts= Tc.
A two-pressure generator is commonly used in the generation of a range of relative humidity values at fixed
temperatures. The saturator and chamber share a common fluid bath temperature and are, ideally, in thermal
equilibrium with each other. In this case, where saturator temperature, Ts, and chamber temperature, Tc, are of
nearly the same value, the generated relative humidity may be approximated by the ratio of the measured
saturated gas-stream pressure (saturator pressure) to the measured chamber pressure by using the simplified
expression
RH = 
P
P
c
s
100
where Pcis the absolute chamber pressure
Psis the absolute saturation pressure.
Please note that the above expression ignores the use of enhancement factors which account for non-ideal
properties of the air / water-vapor mixture, and is therefore only an approximation. More exacting equations
which include enhancement factors are used internally within the system.
Saturation
Pressure, Ps
Chamber
Pressure, Pc
Expansion
Valve
Inlet
Valve
Exhaust
Valve
Saturator
Chamber
Constant Temperature Bath, Ts= Tc
High
Pressure
Source
Atmosphere or
Vacuum Source

Theory of operation
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2.2 COMBINED TWO-PRESSURE AND TWO-TEMPERATURE PRINCIPLE
The two-pressure principle described above can be combined with a two-temperature principle to form a
combined Two-Pressure, Two-Temperature system which exploits the benefits of both architectures. In addition to
measurement of saturation and chamber pressures (as in the case of a two-pressure system), a combined two-
pressure two-temperature system also relies on measurement of saturation and chamber temperatures to fully
determine the resulting humidity. A combined two-pressure two-temperature system is shown in elemental
schematic form in Figure 2—Simplified schematic diagram of the two-pressure, two-temperature principle where Ts
≠Tc and Ps ≠Pc.
Figure 2—Simplified schematic diagram of the two-pressure, two-temperature principle where TsTcand PsPc.
Here, the temperature of saturation, Ts, and temperature of the chamber, Tc, may be controlled independently of
each other if desired. By allowing the saturator and chamber to operate at different temperatures from each
other, a wider variety of humidity values may be generated with reduced requirements on the height of saturation
pressure required. Regardless of whether the saturator and chamber are operated at the same temperature, or at
temperatures different from each other, all humidity calculations rely on the two pressures (Ps and Pc) and on the
two temperatures (Ts and Tc).
2.3 COMMON DEFINING EQUATIONS
The following equations of R. Hardy for saturation vapor pressure, enhancement factor, and temperature (from
saturation vapor pressure) are common and fundamental to most of the humidity calculations presented here.
2.3.1 SATURATION VAPOR PRESSURE OVER WATER
Saturation vapor pressure over water at a given ITS-90 temperature in the range –100 °C to +100 °C is defined by
the formula







+= −
=
TgTge i
iilnexp 7
2
6
0
(1)

Theory of operation
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where eis the saturation vapor pressure (in Pascals) over liquid water in the pure phase
Tis the temperature in Kelvin
and g0= –2.8365744·103
g1= –6.028076559·103
g2= 1.954263612·101
g3= –2.737830188·10-2
g4= 1.6261698·10–5
g5= 7.0229056·10–10
g6= –1.8680009·10–13
g7= 2.7150305
2.3.2 SATURATION VAPOR PRESSURE OVER ICE
Saturation vapor pressure over ice at a given ITS-90 temperature in the range –100 °C to 0.01 °C is defined by the
formula







+= −
=
TkTke i
iilnexp 5
1
4
0
(2)
where eis the saturation vapor pressure (in Pascals) over ice in the pure phase
Tis the temperature in Kelvin
and k0= –5.8666426·103
k1= 2.232870244·101
k2= 1.39387003·10–2
k3= –3.4262402·10–5
k4= 2.7040955·10–8
k5= 6.7063522·10–1
2.3.3 ENHANCEMENT FACTOR
The “effective” saturation vapor pressure over water or ice in the presence of other gases differs from the ideal
saturation vapor pressures given in Equations 1 and 2. The effective saturation vapor pressure is related to the
ideal by
é = e·ƒ (3)
where éis the “effective” saturation vapor pressure
eis the ideal saturation vapor pressure (as given in Equation 1 or 2)
and ƒis the enhancement factor

Theory of operation
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The enhancement factor, for an air and water-vapor mixture, is determined at a given temperature and pressure
from the formula










−+





−= 11expƒe
P
P
e

(4)
with
i
iiTa

=
=3
0

(5)
and








=
=
i
iiTb
3
0
exp 
(6)
where Pis the total pressure in the same units as e
Tis temperature in Kelvin
and ai, bidepend on temperature range and are given as
for water:
223.15 K to 273.15 K (–50 °C to 0 °C) 273.15 K to 373.15 K (0 °C to 100 °C)
a0= –5.5898101·10–2a0= –1.6302041·10–1
a1= 6.7140389·10–4a1= 1.8071570·10–3
a2= –2.7492721·10–6a2= –6.7703064·10–6
a3= 3.8268958·10–9a3= 8.5813609·10–9
b0= –8.1985393·101b0= –5.9890467·101
b1= 5.8230823·10–1b1= 3.4378043·10–1
b2= –1.6340527·10–3b2= –7.7326396·10–4
b3= 1.6725084·10–6b3= 6.3405286·10–7
for ice:
173.15 to 223.15 K (–100 °C to –50 °C) 223.15 to 273.15 K (–50 °C to 0°C)
a0= –7.4712663·10–2a0= –7.1044201·10–2
a1= 9.5972907·10–4a1= 8.6786223·10–4

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a2= –4.1935419·10–6a2= –3.5912529·10–6
a3= 6.2038841·10–9a3= 5.0194210·10–9
b0= –1.0385289·102b0= -8.2308868·101
b1= 8.5753626·10–1b1= 5.6519110·10–1
b2= –2.8578612·10–3b2= –1.5304505·10–3
b3= 3.5499292·10–6b3= 1.5395086·10–6
2.3.4 TEMPERATURE FROM SATURATION VAPOR PRESSURE
Equations 1 and 2 are easily solved for saturation vapor pressure over water or ice for a given saturation
temperature. However, if vapor pressure is known and temperature is the unknown desired quantity, the solution
immediately becomes complicated and must be solved by iteration. For ease of computation, the following inverse
equation is provided. This equation is generally used to find the dew point or frost point temperature when the
vapor pressure of a gas has been determined. When vapor pressure is known, use the water coefficients to obtain
the dew point and use the ice coefficients to obtain the frost point.
( )
( )


=
=
=3
0
3
0
ln
ln
i
i
i
i
i
i
ed
ec
T
(7)
where Tis the temperature in kelvin
and eis the saturation vapor pressure in pascals
with coefficients
for water: for ice:
c0= 2.0798233·102c0= 2.1257969·102
c1= –2.0156028·101c1= –1.0264612·101
c2= 4.6778925·10–1c2= 1.4354796·10–1
c3= –9.2288067·10–6c3= 0
d0= 1 d0= 1
d1= –1.3319669·10–1d1= –8.2871619·10–2
d2= 5.6577518·10–3d2= 2.3540411·10–3
d3= –7.5172865·10–5d3= –2.4363951·10–5

Theory of operation
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2.4 HUMIDITY EQUATIONS
The following equations are used in the CGS-240 to calculate various humidity parameters. These equations are
not approximations, but rather account for the temperature differences between the saturator and chamber, and
the non-ideal behavior of water vapor when admixed with air or other gases.
2.4.1 RELATIVE HUMIDITY
Percent relative humidity is the ratio of the amount of water vapor in a gas to the maximum amount possible at
the same temperature and pressure. For two-pressure, two-temperature systems, it is defined by the equation
100


=
c
s
s
ce
e
P
P
RH
(8)
which then expands to
100=
c
s
c
s
s
ce
e
f
f
P
P
RH
(9)
where Pcis the absolute chamber pressure
Psis the absolute saturation pressure
ƒcis the enhancement factor at chamber temperature and pressure
ƒsis the enhancement factor at saturator temperature and pressure
ecis the saturation vapor pressure at chamber temperature
and esis the saturation vapor pressure at saturator temperature.
2.4.2 DEW POINT TEMPERATURE
Dew point temperature is the temperature to which a gas must be cooled to initiate condensing water vapor in the
form of dew (note that dew point temperature can exist above or below 0°C). Dew point temperature is obtained
with the following iterative steps.
a. Make an educated guess at the dew or frost point enhancement factor d. Setting d = 1 is a suitable first
guess.
b. Next, compute the dew or frost point vapor pressure of the gas with the formula
s
c
d
s
sd P
P
ee = ƒ
ƒ
(10)
c. Use the dew or frost point vapor pressure eddetermined in the previous step, along with Equation 7 for
water, to compute T. Call this value the dew point temperature Td.