Monitor Mode
The large bold value on the top line is the digitally filtered value of the raw ethylene measurements. Next, the relative humidity in percent, of the gas stream in the instrument is displayed followed by the temperature in degrees Celsius. The atmospheric pressure, is displayed. Press the DOWN arrow to view the flow rate in milliliters per minute. The default flow rate is 80 mL per minute. If the unit has a high resolution CO₂ sensor on board, the CO₂ value in ppm will be displayed below the ethylene value. If the unit has a low resolution CO₂ sensor, or O₂ sensor on board, the value will be displayed in percentage (%).
- If Autosave is enabled, “saved” will flash at the top of the display each time the data is automatically saved to the file.
- Press the Stop key to stop the measurement and exit to the Sensor Status display.
- Press the LEFT arrow to enter Graph Mode.
- Press the RIGHT arrow to get to the Measurement Settings menu.
The temperature and relative humidity sensors are located inside the instrument. Therefore, the temperature sensor is subject to heating by the F-900 internal electronics. The temperature reading is often 4-5°C above ambient and in turn, relative humidity readings are affected. The use of water in Chamber In (PolarCept, see page 8), will also affect relative humidity readings.
The detection rate (Det. Rate) in nmol/second is calculated by the following formula, based on ethylene being an ideal gas.
- Detection rate (n’) = (V*P)/(R*T).
- Where volume (V)= flow rate * concentration (L/s)*(ppb) = nL/s
- P = pressure (atm) = (kPa)*0.0098692
- R= constant = 0.0821
- T = absolute t (K)
The detection rate starts calculating the ethylene emission rate from the fruit. Emission rate is typically calculated with the following parameters and the weight of the fruit.
- Flow rate: 0.2L/min = 12.0L/hr
- Sensitivity to C₂H₄: 0.1 µL/L or 100 nL/L
Therefore, the F-900 optimally detects around 1200 nL/hr. The unit is nanoliters of ethylene per gram fresh weight per hour.
Measurement Settings
The Settings screen is accessed by using the RIGHT arrow from Monitor Mode, and provides a short list of variables which may be manipulated while measurements are being made. The exterior Conditioning Chambers are turned On/Off, valves are closed causing gas to circulate internally (Closed Loop On/Off), or the ethylene zero calibration (Set Zero) can be initiated. Chamber Out is primarily used with potassium permanganate (KMnO₄) for cleaning water and setting the instrument’s offset. Changes cannot be made to the settings menu if the F-900 is “correcting offset”. If the measurement is setup for PolarCept (Setup > Chamber > Chamber In = Special) then Chamber In cannot be turned on/off from the Measure > Settings menu. The measurement must be stopped and default Setup Menu parameters changed. It is recommended to use Chamber In with distilled water for most ethylene measurements.
The Set Zero procedure begins by automatically closing the IN and OUT port valves on the front of the instrument (Closed Loop) and enabling Chamber Out (On) with KMnO₄. The instrument will continue to record data to the file, with the mode listed as “offset”. The length of time for the offset can be changed by navigating to Setup > Calibration > C2H4 > Offset Correction. It is recommended to keep at 10 minutes
The ethylene sensor set zero should be performed weekly to compensate for weekly baseline drift. To perform the C₂H₄ Set Zero refer to the Ethylene Sensor Zero Calibration section on pg. 30 of this user manual.
Figure 11: Measurement > Settings Menu display (left); flow path of internal gas stream (right).
Electrochemical Sensor Response
The lag in sensor response is a natural phenomenon for charged porous membrane electrodes. The platinum electrode can be thought of as a series of cylindrical pores. Each of these pores will have a double layer in the radial direction. The interactions between the molecules, convection from air movement, electric fields and the concentration gradient result in a lag following a change in concentration¹. Additionally, a lag is created due to the volume of internal tubing the instrument contains. Because of the volume of internal tubing, it takes about 120 seconds to see a response by the sensor.
Auto-Escape Feature
An upper limit auto-escape feature acts as a safety feature of the unit. This safety feature is always on. If the sensor detects over 200 ppm ethylene, the F-900 will auto-escape from Monitor Mode to prevent poisoning of the electrode. This will stop the measurement. If the sensor becomes poisoned, it will continually auto-escape with the error message "sensor out of bounds". If this happens, use potassium permanganate (KMnO₄) in Chamber Out and set the instrument to Closed Loop. This will begin to clean the gas inside the instrument. If possible, the F-900 ethylene analyzer should be removed from the high concentration environment.
Flow Block Error
The display may show “flow blocked” and a very low flow rate (less than 40 ml/min) for several reasons. If this message appears on the display of the F-900, ensure that the intake on the front of the F-900 is free from obstruction and not covered. If connected to tubing or a fruit chamber, disconnect and reconnect the tubing from the IN port.
If using water in Chamber In, remove the top panel of the F-900 and ensure no water has entered the F-900. There is a white hydrophobic filter directly down the flow path Chamber In to be checked for water, as well as a hydrophobic filter near the in port on the front of the F-900. If water is found inside the tubing or filters, Stop the measurement and allow the instrument to dry out. Be careful no further water damage occurs when turning the pump back on.
No SD card Present
If the autosave is enabled and no SD card is present, the user will be notified by seeing “INSERT SD CARD OR HIT ENTER KEY”. After enter is pressed, “NO SD CARD” is visible on the Sensor Status display where the FILE and filename usually are. Pressing any other button besides Enter, will return to the main menu. Each time the F-900 tries to autosave, the unit will flash “write error” instead of “saved” at the top of the display.
¹ Kontturi et al., “Ionic Transport Processes: In Electrochemistry and Membrane Science.” (Oxford University Press) 2008