Frequently Asked Questions

Please refer to page 117 in the user manual as well as using these steps to try and resolve any odd operating problems on the AP4:

1. Make sure there are no mobile phones in the vicinity. When a mobile phone is placed near the sensor head of the AP4 it may interferes with the excitation signal. This can affect functionality and therefore the readings taken by it.
   Note: Since 2017, all new heads have a modification that rejects mobile phone signals and stops them being susceptible to interference. Please contact Delta-T if you would like an upgrade.
2. Please double check that the “SET RH” value is a similar value to what the ambient reading is (open the AP4 head and wave it around in the air). If the SET value is a lot higher than the actual ambient RH, the readings will never reach the set value and therefore the pump won’t switch in and a reading won’t be taken (seems like nothing is happening). If the pump is on permanently then it is likely that the air line pipe is disconnected or that the desiccant is worn out.
3. Always use fresh desiccant and check the air line pipes are firmly connected to the desiccant chamber.
4. If you are struggling to get a reading, even with the SET RH set to the ambient conditions, try setting the “SET RH” value, in the calibration menu, to a value ~5% lower than the ambient reading.
5. Check the difference in temperature between the two head thermistors. (Press shift and off together). Allow the reading to stabilise and press the SET key. This should be no more than +/- 0.5degC. More information on this can be found on page 115 of the AP4 user manual (version3) – this can be downloaded from our website if necessary (www.delta-t.co.uk).
6. Check the cup seal is ok, ensure the calibration plate is properly sealed against it and protect it from the wind.
7. Check the calibration plate is clean (no specs of dirt in the holes) and the blotting pad is moist.
8. Extreme temperatures could cause errors, we recommend that it is not exposed to temperatures >50degC (see p94 of user manual). Also avoid the opposite extreme too – falling temperatures can lead to dew or condensation forming – if you are bringing a cold Porometer into a warm greenhouse, give it time to equilibrate.
9. When performing the first calibration, allow the head to sit on one of the plate positions (position 3 for example) for 10-15 minutes to let it acclimatise to the current RH. Then do a calibration run on all positions.
10. Check the barometric pressure you are at when doing the calibration – if you do not have a barometer, use 1000hPa (mbar) at sea level and reduce this by 100hPa for every 1000m altitude.
11. Try a reset on the AP4:Press the help button then when the help screen appears, press the shift and “C” keys together. The screen should go blank and the firmware version number should appear, note: this will reset all settings to the factory default values. The AP4 will then start up as normal.