Measurement report on
Humidity and chemical sensors For the subject of sensors and transducers
Supervisor: ing. Antonín Platil November 23rd, 2006, 1800-1930
Yann KOWALCZUK Measurement Laboratory 61 Faculty of Electrical Engineering Czech Technical University in Prague
1. Task of the Measurement 1) Measure the relative humidity using the manual psychrometer and digital humidity meters with capacitive polymer probe Humistar. 2) From the given tables calculate the dew point, absolute humidity, and partial pressure of water vapours in air. 3) Determine the transfer function (response) of the gas sensor in relation to the concentration of ethanol (CH3 − CH2 − OH) and N-heptane CH3 − (CH2 )5 − CH3 vapours in air in the bell.
2. Schematic Diagram The schematic diagrams of the digital humidity sensors, and the mechanical humidity sensor (psychrometer) are not available. The electronic circuit diagram of the chemical sensor is shown below in Figure 1.
Figure 1 Schematic diagram of chemical sensor
3. List of Used Equipment For the purpose of humidity measurement, Smart Hygrometer Humistar HTM 98, and Humidity and Temperature Meter Humistar HTM 998 were used. No specific details are available for the psychrometer. No specific details are available for the chemical sensor.
4. Theory Humidity, in general, is the amount of water vapour present in the air. Humidity meters are used for measuring humidity of air and nonaggressive gases in various fields. Some terms worth noting regarding humidity are: Absolute Moisture [kg/m3 ]: the amount of water contained in a liquid or solid by absorption or adsorption which can be removed without altering its chemical properties. Absolute Humidity (RH) [kg/m3 ]: is the density of water vapour in air. Relative humidity [%]: the ratio of the actual vapour pressure of the air at any temperature to the maximum of saturation vapour pressure at the same temperature. Mathematically it can be defined as: µ ¶ φw H= 100 (1) φs where φw is the partial water vapour pressure at any temperature and φs is the saturated partial water vapour pressure at that temperature. Dew Point: It is the temperature at which the partial pressure of water vapour in the air would be the saturated partial pressure of water vapour. At dew point temperature the relative humidity is 100%. For a particular sample of air, if we know the dew point then we can judge the relative humidity of the sample. Our atmosphere consists of many gases and water vapour. In general we can say that all the air in the atmosphere is composed of “dry air” and “water vapour.” According to Dalton’s law of partial pressures, P = φdry air + φw
(2)
where φdry air is the partial pressure of the dry air and φw is the partial pressure of water vapour in the air.
5. Procedure 1) Moistened the blue bulb of the psychrometer. 2) Recorded the relative humidity, temperature, and dew point readings from the digital meters. 3) After 5 minutes of moistening the blue bulb of the psychrometer, set the air mechanism of the meter. After the fan came to rest, the temperature of the dry bulb and wet bulb were recorded. 4) With the help of the tables provided by the teacher, the calculations were performed. 5) Determined the concentration of ethanol (CH3 − CH2 − OH) and N-heptane CH3 − (CH2 )5 − CH3 vapours in air in the bell.
6. Measured & Calculation Values I. Humidity Measurement 1) Digital meter, Humidity and Temperature Meter HTM 998 Temperature [◦ C] 20.8
Relative Humidity [%] 48.8
Dew Point [◦ C] 9.5
Table 1 Digital meter readings
2) Digital meter, Smart Hygrometer HTM 998 Temperature [◦ C] 20.5
Relative Humidity [%] 50.4
Dew Point [◦ C] 9.9
Table 2 Digital meter readings
3) Mechanical Humidity Sensor (Psychrometer) Temperature [◦ C] dry 21.6
Temperature [◦ C] wet 15.1
Temperature [◦ C] difference 6.5
Relative Humidity [%] (from table) 50
Table 3 Psychrometer meter readings
II. Chemical Sensor Ethanol vapour were introduced into the bell with the help of a 10 ml syringe, after an interval of 1 minute. The same was done for N-heptane. Tables (4) and (5) show the voltage reading recorded after changing concentration of organic compound vapour in the bell. According to Dalton’s law of partial pressures, the total pressure exerted by a mixture of gases (PTotal ) is equal to the sum of partial pressure of each gas (φi ). Mathematically: PTotal = φ1 + φ2 + · · · + φn .
(3)
In our case the total pressure in the jar containing the volatile liquid (ethanol or Nheptane) was equal to the atmospheric pressure. The jar was closed with the cap with a small opening and from neither from inside the jar or from outside the jar, there was pressure, which means that pressure in the jar was, to high degree of accuracy, equal to
the atmospheric pressure outside the jar. We can state that: Patm = φ1 + φ2
(4)
where φ1 is the partial pressure of ethanol or N-heptane gas and φ2 is the partial pressure of air in the jar. We know the ideal gas equation, which is stated as: P V = nRT
(5)
where P is the pressure, V is the volume, n is the number of mole(s),and T is the temperJ L atm ature of the gas. R is the universal gas constant, and R = 0.082057 K mol = 8.3144 mol K . P V = nRT (φ1 + φ2 )V = (n1 + n2 )RT
) (6)
φ1 V = n1 RT and φ1 V = n2 RT 1 mole of a an ideal gas occupies a volume of 22.4 L at standard temperature and pressure (298 K) and 1 atm [101 325 Pa or N/m2 ]. At the pressure of 1 atm, 1 mole of a gas occupies a volume equal to RT, where R is the universal gas constant, and T is the temperature in Kelvins. Assuming that the atmospheric pressure of the laboratory was 1 atm at the time of measurement, we can say that n1 mole of a gas occupies (n1 × RT )L of volume. Let’s denote the volume (n1 × RT ) as V1 , that is, V1 = n1 RT.
(7)
Let CV1 represent the volume concentration of the gas, then, CV1 =
V1 . V
(8)
From equations (6), (7), and (8) we can say, µ φ1 =
¶ CV1 RT RT
φ1 (in atm) = CV1 1 atm = 101.325 kPa CV1 φ1 (in kPa) = 101.325 φ1 (in kPa) ≈ CV1 (%)
(9)
(10)
(11)
V1 100[%]. V On the basis of the text provided by the teacher and the above mentioned analysis, the following results have been calculated from the measured values. The temperature of the laboratory was recorded as 21◦ C. The volume concentrations of ethanol and N-heptane, in their respective jars at 21◦ C are given as 6.22% and 4.99%, respectively. where CV1 (%) = CV1 (100)[%] =
1) Ethanol vapour Volume [ml] injected
0
10
Voltage [V (dc)]
4.7
-2.277 -2.498 -2.561 -2.638
-2.78
16.81
33.62
50.43
67.24
134.48
16.81
33.62
50.43
67.24
134.48
Concentration [10−3 %] 0 in the bell Partial pressure [Pa] 0 in the bell
20
30
40
80
Table 4 Ethanol vapour readings and calulation
2) N-heptane vapour Volume [ml] injected
0
10
Voltage [V (dc)]
4.7 -1.01
20
30
40
50
60
70
80
-1.49
-1.76
-1.91
-2.03
-2.07
-2.13
-2.19
Concentration 0 −2 in the bell [10 %]
13.49
26.97
40.46
53.94
67.43
80.92
94.40
107.89
Partial pressure [Pa]
13.49
26.97
40.46
53.94
67.43
80.92
94.40
107.89
0
Table 5 N-heptane vapour readings and calculation
The graph showing the response of the chemical sensor, for both the gases, is mentioned on the next page in Figure 2.
7. Results and Conclusion
In this lab, we could compare temperature and humidity sensors together, and we could evaluate a chemical sensor in different gases environments. The chemical solutions humidity couldn t be evaluated in this lab, because they were dry. First, the two digital sensors gave quite close values one from each other. The temperature was about 21°C, the humidity 50%, and the dew point 10°C. The worst case was an error of 2% between the two devices, which is a really small value in the situation they are used. Furthermore, these units reacted quite fast, with good accuracy, and instant readable result. The mechanical humidity sensor gave quite similar values, but was of course a bit less precise. Indeed, some parts have to be made wet, and it needs to activate a ventilator for performing the humidity measurement. The chemical sensor was used with ethanol and N-heptane gases. It provided a linear response, with accurate values, when used for small amounts of gas volume. In our situation, 10 ml was the limit of the full linearity of the device. Above this value,
the curve can still be evaluated as a linear device, with some approximations of course. There was also some ethanol left on the jar while performing the measurement for the N-heptane, so the procedure has some errors induced by this problem. Finally, we can assume that the digital temperature and humidity sensors will be widely used nowadays, because of their relatively good accuracy and fast response. Mechanical devices tend to disappear because of their moving parts, the necessity of making lot of manipulations, and the slow measure response. The chemical sensor had, in our case, a slow answer too, neccessiting manipulations for each measurement, and thus giving errors in the result. Thus, this device could be used when inflammable gases measurements are needed, or other devices could be as well preferred, depending on the system and the measurement objectives.
