SnowSurf IR THERMOMETER FOR THE MEASUREMENT OF SURFACE TEMPERATURE OF THE SNOW COVER (Update Oct.04)

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1 Hansueli Gubler, Richtstattweg 2, CH-7270 Switzerland. Phone / Fax ++41(0) AlpuG@alpug.ch SnowSurf IR THERMOMETER FOR THE MEASUREMENT OF SURFACE TEMPERATURE OF THE SNOW COVER (Update Oct.04) DESCRIPTION BASICS The snow surface temperature is one of the most important parameter to assess the development of the snow structure at the snow surface and in layers close to the surface. In automatic remote networks to measure snow and avalanche parameters, surface temperatures can only be measured contactless. Contactless measurements have the additional advantage that instruments are not exposed to snow pressure. Radiation thermometry is therefore well suited to measure the snow surface temperature. Operating conditions Infrared thermometers are integrated in networks of automatic remote weather stations. Most of these stations are placed at remote locations not accessible during winter. Typical measuring systems consist if a 5 to 8m tower as a carrier of different instruments that measure snow height, wind, radiation, temperatures, IR-thermometer as well as solar-panels and the necessary logging and communication hardware. SnowSurf is compatible to CR510, CR10x and CR23x loggers of Campbell Scientific (SCI) or any programmable logger capable to measure µv to V. The functionality of such stations should not be harmed by harsh weather conditions: Temperature range : -30 C - 40 C rel. humidity : 0-100% kondensierend Wind speed : 70m/s Riming and icing conditions extreme radiation conditions: shortwave 1000W/m 2 from any side, longwave net radiation losses of up to 300W/m 2 (ε =1).

2 2 Emissivity of snow in the far infrared Emissivity of a snow surface in function of grain size and nadir. Measurements in the wave-length range of 7-20 µm result in a typical emissivity of ε= SnoSurf Specs Operating temperature -30 to +5 C humidity 0 to 100% condensating measuring range -35 to +10 C emissivity standard 0.98 accuracy 1 " 0.5 C at 0 C ( ambient temp. 10 C +5 C) " 1.5 C at -10 C " 2.5 C at -20 C " 4 C at -35 C " 5 C for surface temperatures > 0 C (no snow), because emissivity is not known power passive sensor output thermocouple type K, thermistor T107 CSI directivity target diameter / measuring distance = 1/5 mounting at mounting arm 1 to 5m above the snow surface cable shielded cable max. 10m. 1 Irritation by strong fog, snow drift condensation or riming of the instrument window are excluded, instrument temperature <5 C.

3 3 Operating Instructions The IR- thermometer i s a passive sensor ( thermocouples). A CSI 107 thermistor is used to measure the reference temperature. Because the output impedance of the instrument is quite high, measurements are sensitive to input bias currents of the input amplifier used and induced signals. Therefore correct grounding is important. Recommended test: fill the instruments opening with white paper ( paper has a very high emissivity in the far infrared) and wait for equitemperature conditions. The thermocouple output should read 0V. The instrument sensitivity amounts to about 40µV/ C. Corrections of the linearisation are necessary at large differences between instrument and surface temperatures. The instruments are calibrated on snow by AlpuG. Thermocouple and 107 thermistor have to connected to SE inputs of a CR10X. Standard connector female 7 pin type Binder, male Binder , pin 1 NC 2 NC 3 thermocouple output V IR (SE-input), P13,Typ K(3) 4 AG (analog ground) 5 exitation für Campbell 107 NTC 6 NTC output over 1000 Ohm 0.1% 5ppm reference resistor to AG, output to SE-input, P11 7 shield Linearisation (AlpuG): The thermal time constant of the instrument and therefore of the reference temperature at calm (no wind) condition amounts to about 1h. T IR eff = T IRmess + Korr (all temp. T in C)

4 Korr = - [1/230*C*{T IRMess P2* T instr + B} 2 - OFFSET] 4 T IR eff linearized measurement T IRmess original measurement T instr reference temperature of the instrument B calibration constant P2 calibration constant C=1 (standard, value may from 0.5 to 1) OFFSET =1 (standard value may vary from 0.5 to 2.5) Approximation for a correction for emissivities different from the emissivity of snow of about 0.98: T EMKorr = T instr + SIGN(DeltaT) * 1/e * ABS(DeltaT) DeltaT = T IR eff - T instr e emissivity of the surface. code example for CR 10X input : irmv_1 T IR eff linearized measurement: IrResult T IRmess original measurement: irdir_1 T instr Instrumentreferenztemperatur: Ir107_1 B calibration constant: Beta P2 calibration constant: P2 OFFSET = 1 ; ;measuremet ; : Temp (107) (P11) 1: 1 Reps 2: 1 SE Channel 3: 1 Excite all reps w/exchan 1 4: 1 Loc [ Ir107_1 ] 5: 1 Mult 6: 0 Offset 6: Excite-Delay (SE) (P4) attention spezial application if P4, with delay >0 1: 1 Reps 2: mv 50 Hz Rejection Range (Delay must be zero) 3: 2 SE Channel 4: 3 Excite all reps w/exchan 3 5: 1 Delay (units 0.01 sec) 6: 0 mv Excitation 7: 15 Loc [ irmv_1 ]

5 5 8: 1.0 Mult 9: 0.0 Offset 7: Thermocouple Temp (SE) (P13) 1: 1 Reps 2: 31 ñ 2.5 mv 50 Hz Rejection Range 3: SE Channel 4: 3 Type K (Chromel-Alumel) 5: 1 Ref Temp Loc [ Ir107_1 ] 6: 10 Loc [ irdir_1 ] 7: 1 Mult 8: 0 Offset ;IR correction/calibration ; : Z=X*Y (P36) 1: 103 X Loc [P2 ] 2: 155 Y Loc [ Ir107_1 ] 3: Z=X-Y (P35) 1: 150 X Loc [ irdir_1 ] 2: 152 Y Loc [ IRscr_3 ] 8: Z=X (P31) 1: 93 X Loc [ beta ] 2: 151 Z Loc [ IRscr_2 ] 9: Z=F (P30) 1: 1 F 2: 0 Exponent of 10 3: 153 Z Loc [ IRscr_4 ] 11: Z=X+Y (P33) 2: 151 Y Loc [ IRscr_2 ] 17: Z=X*Y (P36) 2: 152 Y Loc [ IRscr_3 ] 18: Z=X*F (P37) 2: F ; 1/230 C * 1/230 19: Z=X*Y (P36) 2: 153 Y Loc [ IRscr_4 ]

6 6 20: Z=X+F (P34) 2: -1 F OFFSET 21: Z=X-Y (P35) 1: 150 X Loc [ IrScr_1 ] 2: 152 Y Loc [ IRscr_3 ] 3: 156 Z Loc [ IrResult ] AlpuG, 2002 H. Gubler

7 4 E= JE 5 D = > A 1I = JE *? 9 M. + A? J H 9 M, JJ K? D 6 O F E? = K JE C 5 5 K H B) F K /

8 1I = JE *? & N & N K JE C 0 A, E= A JA H! 5 5 K H B) F K / & & & $ $