Li-Cor LI-190 PPFD sensors
These sensors measure PAR, photosynthetically active radiation (wavelengths from 400-700 nm) as photosynthetic photon flux density (PPFD). PPFD is typically measured in μmol s^-1^ m^2^.
Output, wiring, and multipliers
These sensors output μA across two leads and a multiplier is used to relate the μA signal to μmol s^-1^ m^2^. Multipliers are typically in the range of -140 to -170 μmol s^-1^ m^-2^ per μA. The two leads are a bare shield (positive) and a center conductor (negative). This output can be converted to mV (which is read by Campbell dataloggers) using a resistor as long as the multipliers are also converted via Ohm's law. A shunt resistor (roughly 600 Ohms) is placed either between the datalogger terminals or across the leads using an adapter supplied by Li-Cor.
Wiring to Campbell dataloggers
\^ Diff H | Li-Cor Shield (Pos), resistor | \^ Diff L | Li-Cor center conductor (Neg), resistor, ground jumper | \^ gnd | ground jumper |
Note: Li-Cor recommends connecting the wires in the reverse way (Pos-->Diff L, Neg-->Diff H) to reduce interference, but this gives only negative values on a Campbell CR23x, even if the multiplier is entered as a negative value in the datalogger program.
Converting the multiplier to mV
μA current output can be converted to mV using Ohm's law (Voltage =
Current * Resistance). Multipliers (
mult) are given as -μmol s^-1^ m^2^ per
μA. A resistor has a voltage in Ohms. The steps to convert to μmol s^-1^
m^2^ per mV are:
multto μA/1000μmol: 1000μmol/
- Convert multiplier from μA to A:
- Change amps to volts: mult` * R (R = resistance of shunt resistor in Ohms)
- Multiplier is now in V/1000μmol s^-1^m^2^`
- Change multiplier to mV:
- Reciprocal of this number(1000/
mult) is the multiplier in μmol s^-1^m^-2^/mV//
- In short: $
mult/0.001A * R$
Note: Li-Cor and Campbell recommend a 604 Ohm resistor. There may be problems especially with higher resistances.
These sensors are generally read using code like this:
Measure UPWARD looking Quantum sensor - Q27246 Bridged with a 604ohm resistor to convert uA to mV 39: Volt (Diff) (P2) 1: 1 Reps 2: 41 10 mV, 60 Hz Reject, Fast Range 3: 5 DIFF Channel 4: 30 Loc [ PAR_Up ] 5: 1.0 Multiplier 6: 0.0 Offset Set negative values to zero 40: If (X<=>F) (P89) 1: 30 X Loc [ PAR_Up ] 2: 4 < 3: 0.0 F 4: 30 Then Do 41: Z=F x 10^n (P30) 1: 0 F 2: 0 n, Exponent of 10 3: 30 Z Loc [ PAR_Up ] 42: End (P95) Convert mV to umoles m\^-2 s\^-1 using converted multiplier 43: Z=X\*F (P37) 1: 30 X Loc [ PAR_Up ] 2: 243.112 F 3: 30 Z Loc [ PAR_Up ]
\^ Serial no. \^ Location \^ Measurement \^ μA Multiplier \^ Resistor \^ mV Multiplier \^ | Q27246 | Hidden Canyon - Met tower | Upward looking PPFD | -146.84 | 604Ω | 243.112 | | Q33398 | Hidden Canyon - Met tower | Downward looking PPFD | -158.23 | 560Ω | 282.553 |