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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 m2.

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 m2. 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 m2 per μA. A resistor has a voltage in Ohms. The steps to convert to μmol s-1 m2 per mV are:

  • Convert mult to μA/1000μmol: 1000μmol/mult
  • Convert multiplier from μA to A: mult * 1A/106μA
  • Change amps to volts: mult` * R (R = resistance of shunt resistor in Ohms)
  • Multiplier is now in V/1000μmol s-1m2`
  • Change multiplier to mV: mult * 1000
  • Reciprocal of this number(1000/mult) is the multiplier in μmol s-1m-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.

EDLOG instructions

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    ]

Deployed sensors

^ 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 |