Accuracy of SOBO-20 in the measurement of salt on …docs.trb.org/prp/13-2606.pdfAccuracy of SOBO-20 in the measurement of salt on winter pavements . ... PhD-student at the Norwegian

Lysbakken and Lalagüe

Accuracy of SOBO-20 in the measurement of salt on winter pavements

Title: Accuracy of SOBO-20 in the measurement of salt on winter pavements Date: 2012-07-31 Author name and affiliation: Kai Rune Lysbakken (corresponding author) PhD-student at the Norwegian University of Science and Technology Department of Civil and Transport Engineering. Trondheim, Norway [email protected] Telephone: +47 91 36 90 72 Anne Lalagüe PhD student at the Norwegian University of Science and Technology Department of Civil and Transport Engineering Trondheim, Norway [email protected] Telephone: +47 93 44 32 77 Word count: Abstract 225 Text (including abstract): 4076 10 figures: 2500 Total: 6576

TRB 2013 Annual Meeting Paper revised from original submittal.

Lysbakken and Lalagüe 1

ABSTRACT 1 2 The use of chemicals is essential in snow and ice control operations. Sodium chloride 3

(salt) is normally used, as it is efficient, widely available and rather inexpensive. However it 4 is harmful for the environment if encountered in too great quantities, and there is nowadays 5 great attention placed upon the importance of reducing the usage of salt, while still 6 maintaining road serviceability and safety. 7

Optimizing the use of salt requires accurately knowing how much salt is already on a 8 pavement surface. Unfortunately, there is currently no well-documented method available to 9 determine this quantity. 10

SOBO-20 is one of the most common instruments used by the winter maintenance 11 community to calculate salt amounts, although the reliability of the instrument has not yet 12 been shown. 13

This present work aims to fill this gap by carrying out measurements on brine 14 (dissolved salt), dry salt particles and re-crystallized salt. 15

The presented results support the conclusion that SOBO-20 is an accurate and reliable 16 instrument for measuring brine on asphalt pavements. However, it largely underestimates the 17 amount of dry or re-crystallized salt, and more attention should be paid when using SOBO-20 18 on dry pavements. Compliance with the manufacturer's recommendations regarding the 19 proportion of acetone in the measuring fluid is also essential for accurate salt readings. 20

These results on the instrument performance should lead to a better understanding of 21 salt distribution and action time. 22

23



ACCURACY OF SOBO-20 IN THE MEASUREMENT OF SALT ON 24 WINTER PAVEMENTS 25

26 27 28 INTRODUCTION 29 30

Salting is an important part of snow and ice control of winter roads to uphold a high 31 level of accessibility and traffic safety. Due to the costs and environmental drawbacks 32 associated with salting there is a need to optimize the usage of salt by using less amounts, 33 while still securing safe driving conditions. Activities for optimizing the use of chemicals on 34 winter roads include for example the search for best spreading methods, knowledge of salt 35 loss after application and investigations on the effect of salt additives. In these activities the 36 measurements of salt quantity on road surfaces is an essential part. However this can be a 37 challenge and several possible types of instruments and methods, both for research and 38 decision making purposes have been investigated. 39

One of the most common methods in research studies of salting is the use of SOBO-40 20, manufactured by Boschung (1). Despite the wide use of the instrument in the research 41 community on winter maintenance, there is little documentation of the quality of SOBO-20 42 readings. 43

The aim of this work was to determine the accuracy of and to document the possible 44 limitations of the instrument. Several tests of the instrument were performed in a controlled 45 environment in laboratory. Potential sources of error were identified and suggestions are 46 given in this paper improve the validity and reliability of SOBO-20. 47

48 49 MEASURING SALT ON ROAD SURFACES 50 51

There are in principle two possible units of measurement for salt on road surface: 1) 52 salt concentration or 2) salt quantity per unit area (2). The salt concentration can be expressed 53 either as salt concentration in the fluid on the road surface (grams per litre or weight 54 percentage) or as the freezing point, which is a function of the concentration. According to 55 Turunen (3), for historical reasons or for the sake of descriptiveness it has become customary 56 to express salt content on the road surface as freezing point temperature. A possible reason for 57 using concentration as a unit of measurement may be related to the ease of which a direct 58 measurement can be taken. An instrument for measuring road salt has to measure a physical 59 property (optical, electrical, thermodynamical or density related) that depends on the salt 60 concentration (3). To obtain the salt quantity per unit area requires, in addition that the water 61 film thickness has to be measured before the quantity in per unit area can be calculated, or to 62 use the principle of SOBO-20. The instrument itself adds a certain quantity of measuring fluid 63 on a defined area. 64

Even if it has become more common or convenient to measure salt as salt 65 concentration, it is the authors’ opinion not always sufficient to use salt concentration as a 66 unit of measurement. For decision making purposes it is not always enough to know the salt 67 concentration to assess the future development of the road surface condition (2). Also for 68 research activities it is desirable to use quantity per unit area as a unit of measurement, as it 69 describes the movements of salt from, off and on the road surface better than the measurement 70 of salt concentration. 71

The SOBO-20 instrument is relatively unique in its feature of adding measuring fluid 72 on the road surface and thereby measuring the quantity of salt per unit area. 73



SOBO-20 INSTRUMENT 74 75

Operating principle 76 77

SOBO-20 is a well-known and quite old instrument. It has been used in several studies 78 of the salting of roads (4), (5), (6) ,(7), (8), (9). SOBO-20 is produced by Boschung. The 79 principle of SOBO 20 is based on the measurement of the electrical conductivity of a fluid 80 and the known relationship between electrical conductivity and salt concentration. The 81 instrument spreads measuring fluid onto the road surface during the measuring procedure. 82

83 The instrument consists of four main parts as shown in Figure 1: 84 85

Main parts of the instrument: 1. Upper chamber containing the

measuring fluid 2. The middle chamber

containing one “dosage” of fluid.

3. The mouthpiece or measuring chamber containing electrodes and sensors

4. The electronic unit for measuring the conductivity and calculating the salt quantity

86 FIGURE 1 Drawing and picture of SOBO-20 (Nygaard (10)) 87

88 The upper chamber is a cylinder which forms the storage of the measuring fluid which 89 consists of 85 % water and 15 % of acetone. Acetone is added to the measuring fluid for frost 90 protection of the instrument. Electrodes inside the mouthpiece or measuring chamber (marked 91 as 3) are used for conductivity measurements. There is a rubber gasket at the bottom of the 92 mouthpiece. When placing the instrument on the road surface the gasket encloses a certain 93 area of the road surface which forms the measuring area. 94 95 When pushing the instrument against the road, the measuring fluid is sprayed from the middle 96 chamber onto the road surface area enclosed by the measuring chamber. The instrument then 97 measures the electrical conductivity of the fluid inside the measuring chamber. By having a 98 defined area of measurement, a known quantity of measuring fluid and the electrical 99 conductivity, the instrument calculates the quantity of salt on the road surface per unit area 100 (g/m2). The salt quantity is shown on the electronic unit of the instrument on a scale from 1 to 101 15 g/m2 (0.44 oz/yd2). If there are more than 15 g/m2 (0.44 oz/yd2) of salt on the road surface 102 a switch has to be flipped to multiply the scale by 3. The range of the scale is then from 15 to 103 45 g/m2 (1.33 oz/yd2) in steps of 3 g/m2 (15, 18, 21, 24, etc). The use of SOBO-20 in a 104 measuring situation is shown in Figure 1. 105

1

2

3

4



Strengths and limitations 106 107

Despite that SOBO-20 can be described as a low tech and simple instrument, it is still 108 in use in different research activities. The reason for that is that the instrument has some 109 beneficial features. First of all, SOBO-20 measures salt in terms of quantity per unit area. It 110 also adds measuring fluid to the road surface which means that it allows measuring in cases 111 where the quantity of water on road surfaces is small. This is in contrast to road surface 112 sensors that require a certain quantity of water on the road surface to achieve recordings. In 113 principle, SOBO-20 can be used also on dry road surface, but there can be discussed whether 114 SOBO-20 manages to measure dry or re-crystallized salt. Another advantage of the instrument 115 is that it is portable and requires no installation or power supply. This allows measurements at 116 different locations along a road section or anywhere in the cross profile of the road. The 117 measuring procedure is simple, the instrument produces instantaneous readings and no further 118 analysis is required. 119

Blomqvist and Gustafsson (11) published a study of different field techniques for 120 measuring salt and water film thickness - including SOBO-20. The conclusion of the study 121 was that SOBO-20 is suitable to measure salt on wet road surfaces, but shows some 122 underestimations on re-crystallized salt and large underestimations on salt crystals. The tests 123 were all performed on asphalt and brine applications were done with sponge. 124

There can be identified several potential sources that can make SOBO-20 readings 125 inaccurate or even totally incorrect. SOBO-20 may produce systematic errors that make the 126 instrument over or under estimate the salt quantity. There are also questions regarding the 127 instrument performance on road surfaces where salt grains (solid salt) are present. Klein-Paste 128 (9) showed that it is likely that SOBO-20 underestimates the quantity of re-crystallized salt on 129 road surfaces after they have dried up. Additionally, there is the effect of the road surface 130 texture on the SOBO-20 readings. 131

The presented study strives to go beyond previous studies, investigating and 132 quantifying the underlying factors that may affect the accuracy of the instrument. Those are: 133 the content of acetone in the measuring fluid, the form of salt (grain or brine), and a smooth or 134 coarse surface. 135 136 137 EXPERIMENTAL PROCEDURES 138 139

Six different tests were performed of which four were performed on a smooth surface 140 in order to eliminate the effect of the road surface texture. These four are namely, the 141 calibration test, the test of the acetone content of the measuring fluid, the measurements of 142 salt grains, and the measurements of re-crystallised salt. To study the effect of the road 143 surface texture the calibration test and the measurements of re-crystallised salt were also 144 performed on an asphalt surface. 145

The main challenge of testing SOBO-20 was the proportioning and the application of 146 salt or brine on its relatively small measuring area. The diameter of the measuring area of 147 SOBO-20 is 5.6 cm (2.2 in) which gives a measuring area of less than 1/400 of a square meter 148 (0.027 ft2). That means that very small quantities of brine or salt must be applied within the 149 measuring area of SOBO-20. Performing the test required equipment for dispensing liquid of 150 small volumes and weighing up salt grains. The design and execution of the tests naturally 151 introduce sources of errors: there can be inaccuracies in preparing brines, weighting salt, 152 dispensing a precise volume of solution with the pipette or in placing SOBO-20 onto the area 153 to be measured. Nevertheless precautions were taken to minimize bias, as the use of high 154



precision pipettes and scale. Brines were also always shaken before use to avoid salt 155 decantation and their salt content was regularly checked. 156 157 The calibration test 158 159

The aim of this test was to investigate whether there were errors directly related to the 160 instrument, with all known sources of error eliminated. This could either be a systematic error 161 (or bias) that makes the instrument consistently underestimate or overestimate the salt 162 quantity, or random errors resulting in a high unsystematic variation in SOBO-20 readings. It 163 was therefore desirable to perform these tests with fully dissolved brine, applied with the 164 highest accuracy as possible on a smooth surface to be able to test the “true” performance of 165 the instrument. 166

Small droplets of brine were applied on glass dish with a pipette. The pipette could 167 dispense volumes from 0.5 (0.017 fl oz) to 5 ml (0.017 fl oz). By applying volumes of brines 168 of various concentrations one could simulate different salt quantities in the measuring range 169 of SOBO-20 from 1 (0.03 oz/yd2) to 45 g/m2 (0.44 oz/yd2). Ten repetitions were performed 170 for each salt quantity tested (see Figure 2). 171

172

173 174

FIGURE 2 Applying droplets of salt brine for the calibration tests. 175 176 177 Acetone content in the measuring fluid 178 179

According to a Danish report on SOBO-20 (12), it is recommended to use only 180 distilled water as measuring fluid and not a mix of acetone and water as described by the 181 manufacturer. A test was performed to investigate if the absence of acetone would influence 182 the measuring results. A measuring fluid of distilled water was tested as well as mixes with 183 7.5, 15 and 30 % acetone. Brine was applied on dishes with a pipette, as described for the 184 calibration test. Tests with four different salt quantities in the series of ten repetitions were 185 performed. 186 187 Salt grains 188 189

This test was performed to quantify the degree of underestimation of SOBO-20 190 readings performed on dry salt grains. The test was carried out like the calibration test, except 191 that salt was applied in the form of crystals instead of brine. Salt crystals with various weights 192 were applied on the dishes and then measured with SOBO-20. The salt crystals were selected 193 in a weight range that represents salt quantities in the range of 2 (0.06 oz/yd2) to above 130 194



g/m2 (3.83 oz/yd2). This test was performed above the measuring range of SOBO-20 since it 195 was shown by Blomqvist (11) that SOBO-20 was largely underestimating the salt quantity 196 with salt crystals. Weighing up salt grains required a precision scale. Therefore, a balance able 197 to measure 1/1000 gram (3.5 x 10-5 oz) was used for this test. One measuring series was 198 performed on single salt grains and another series was performed on the combined weight of 199 two salt grains of smaller size. This was done to study the significance of the interfacial 200 surface area of the grains on the SOBO-20 readings. Small grains are expected to dissolve 201 faster and give better SOBO-20 readings, as more surface is exposed and available for 202 dissolution to occur. 203

204 Re-crystallized salt on smooth surface 205 206

To test and quantify the degree of underestimation on re-crystallized salt, test was also 207 performed on smooth surface on glass dishes. Droplets of brine were applied with the pipette 208 used for the calibration test. The droplets were allowed to dry up in room temperature over a 209 twelve hour period (see Figure 3). After the water has evaporated the salt quantity was 210 measured with SOBO-20. 211

212 213

214 215

FIGURE 3 Re-crystallized salt on glass dishes. 216 217

218 Re-crystallized salt on asphalt pavement 219 220

To test the effect of texture on SOBO-20 measurements, a similar test to the 221 calibration test and the experiment with re-crystallized brine was performed on dense graded 222 asphalt on a parking area. 223

Brine droplets were applied on the asphalt surface using a 100 – 1000 µl (0.0034 – 224 0.034 fl oz) pipette. For testing brine on asphalt, SOBO-20 measurements were performed 225 immediately after application. For the re-crystallized salt, droplets were allowed to dry and 226 the salt to crystallize. Despite the small volumes applied (< 1 ml, 0.034 fl oz), nothing could 227 prevent the brine to spread out widely along the asphalt surface. Therefore, brine was 228 dispensed 200 µl by 200 µl (0.0068 fl oz), with a drying time of about 15 min between each 229 application. This constrained the dispersion of the brine on a surface smaller than the SOBO-230 20 measuring area. 231

232



233 F 234

FIGURE 4 Re-crystallised salt on dense-graded asphalt pavement. 235 236 237 RESULTS AND DISCUSSION 238

239 The calibration test 240 241

The calibration test was measurements of brine on a smooth surface. The results are 242 shown in Figure 5. Each salt quantity was tested in a series of ten measurements. Each data 243 point is therefore an average of ten measurements. The error bars show the standard deviation 244 for the series of ten measurements. 245

246 247 248 249 250 251 252 253 254

255 256 257 258 259 260 261 262 263 264 265

FIGURE 5 Results from the calibration test. 266 267 268 As shown in Figure 5, SOBO-20 is able to measure dissolved salt on a smooth surface 269

quite accurately. There is no significant variation between the applied and detected quantities, 270 as the measurements give a good fit to a straight line (R = 0,9996). 271

272



Acetone content in the measuring fluid 273 274

The results from the test of different acetone contents in the measuring fluid of SOBO-275 20 are shown in Figure 6. Each presented data point is an average of a series of ten 276 measurements. 277

278

0369

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0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

applied [g/m2]

mea

sure

d [g

/m2 ]

water7,50 %15 %30 %

279 280

FIGURE 6 Results from the test of different acetone content in the measuring fluid 281 282

283 From this test it is evident that the correct content of acetone in the measuring fluid is 284

crucial to obtain a correct reading with SOBO-20. As shown in Figure 6, a lower content of 285 acetone than described by the manufacturer means that SOBO-20 will measure salt quantities 286 too high. Decreasing acetone content means increasing the error of SOBO-20 readings. By 287 using only distilled water SOBO-20 measures salt quantities that are in the range of 45 - 66 % 288 greater compared to the applied quantity. 289

It is likely that the Danish recommendations of using only distilled water in SOBO-20 290 have to be followed with a calibration of the electronic unit of the SOBO-20. It should also be 291 mentioned that acetone is originally prescribed by the manufacturer for the frost protection of 292 the instrument. The role of acetone is to avoid the freezing of measuring fluid, especially in 293 the valves of the instrument. 294

295 Salt grains 296 297 The results from the measurements of salt grains are presented in Figure 7. 298 299



y = 0,0503xR2 = 0,8895

y = 0,0638xR2 = 0,8972

0

2

4

6

8

10

12

14

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140

applied [g/m2]

mea

sure

d [g

/m2 ]

1 grain

2 grains

linear (1 grain)

linear (2 grains)

300 301

FIGURE 7 Results from the test on measurements of salt grains. 302 303

As shown, SOBO-20 largely underestimates the salt quantity when measuring salt 304 grains. To attain a trend or regression curve it was necessary to apply salt quantities above the 305 scale of SOBO-20. For the one salt grain measurements, SOBO-20 only displays 306 approximately 5 % of the salt quantity. For salt grains corresponding to a quantity of 15 g/m2 307 (0.44 oz/yd2) or less, SOBO-20 does not detect any salt at all. When applying the salt quantity 308 as two salt grains, SOBO-20 measures slightly higher salt quantities. In general, 309 approximately 6 % of the salt is measured. When measuring two grains instead of one grain, 310 the interfacial surface area of the grains per unit weight increases, and therefore the measured 311 salt quantity increases as well. 312

As can be seen from Figure 7, there are substantial variations in the measurements. 313 Some variations can certainly come from uncertainties in weighing up salt grains at an 314 accuracy of 1/1000 grams (3.5 x 10-5 oz). It should also be remembered that the salt grains 315 can have different grain shapes and this can result in differences in surface area per unit 316 weight and thereby contribute to some variation in the results. As shown when comparing the 317 test with one and two salt grains the effect of different surface area can be decisive. 318

319 Re-crystallized salt 320 321

The results of the tests of re-crystallized salt on a smooth surface are shown in Figure 322 8. Each data point is an average of ten measurements. The standard deviation of each series is 323 shown by the error bars. 324

325



326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344

FIGURE 8 Measurements on re-crystallized salt. 345 346 The results confirm that SOBO-20 underestimates the salt quantity when measuring 347

re-crystallised brine. Based on the regression curve one can say that SOBO-20 only detects 348 about 58 % of the salt present when the salt is re-crystallised. The measurement deviation also 349 seems to increase with the salt amount. This can be explained by the measuring scale of 350 SOBO-20 and the fact that SOBO-20 measures salt quantities in a range of 15 (0.44) to 45 351 g/m2 (1.33 oz/yd2) in steps of 3 (15, 18, 21, etc.). This will naturally lead to an increase in the 352 variation of the measurements of salt quantities above 15 g/m2 (0.44 oz/yd2). 353

354 Test on asphalt pavement 355 356

Tests were performed on asphalt surface with both brine and re-crystallized salt. The 357 results are shown in Figures 9 and 10. 358

y = 0,9906xR2 = 0,9987

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0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45

applied [g/m2]

mea

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d [g

/m2 ]

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linear

359 360

FIGURE 9 Brine on asphalt surface. 361



SOBO-20 is able to measure brine on asphalt pavement quite accurately, with a 362 correlation coefficient close to 1. 363

364

365 366

FIGURE 10 Re-crystallized salt on asphalt surface. 367 368

369 In regard to re-crystallised salt, SOBO-20 considerably underestimates the salt 370

quantity on asphalt pavement. While on a smooth surface SOBO-20 detected approximately 371 58 % of the re-crystallised salt, it detects on average only 49 % of the quantity on asphalt 372 pavement. A drop in the measured values when flipping to the 3x scale (from 24 g/m2) was 373 also noticed. Note that there are higher variations in the readings on this test compared to the 374 other tests. 375 376 377 CONCLUSION 378

379 SOBO-20 is a relevant instrument with unique features and it is crucial that the 380

limitations and inaccuracies of the instrument are documented to ensure the correct use and 381 interpretation of the measurements. 382

SOBO-20 does not produce systematic errors. It accurately measures the quantity of 383 salt in brine, on both smooth surfaces and asphalt pavements. However, it only detects 384 between 5 - 6 % of dry salt particles: SOBO-20 gives instantaneous readings and does not 385 allow salt crystals to dissolve. When using SOBO-20 in the measurement of dry or pre-wetted 386 salt, the displayed value has to be interpreted only as the quantity of dissolved salt on the road 387 surface, and not the total salt quantity. The quantity of undissolved salt on a road surface 388 remains unknown, as is the case with other measuring principles. 389

Re-crystallized salt, made of finer grains, is detected at 58 % on smooth surfaces and 390 49 % on asphalt pavements. If correctly calibrated, SOBO-20 could be used in the 391 determination of the residual salt levels on winter pavements. It might be relevant to first pre-392 wet the pavement surface with water, and allow salt to dissolve before performing 393 measurements. The validity of the method is not known, but will be investigated in the near 394 future. 395

y = 0,4906xR2 = 0,8845

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0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45

applied [g/m2]

mea

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d [g

/m2 ]

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linear



Despite Danish advice of using only distilled water as measuring fluid, it has to be 396 stressed that diverging from the userguide’s recipe of a mix of 15 % acetone and 85 % water 397 highly affects the measurements. Using only water will require a recalibration of the 398 instrument. 399

It is also judicious to perform several measurements to receive a reliable average of 400 salt quantity, especially for high values that tend to have greater variations in the SOBO-20 401 readings, due to the use of the x3 scale. 402

403 404

ACKNOWLEDGMENT 405 406

The authors would like to thank the Norwegian Public Road Administration for the 407 financial support of this work, and Elise Balmand who has graciously provided technical 408 assistance to this project. 409 410 411 REFERENCES 412 413 1. Boschung Mecatronic AG: Instruction Manual Salt Quantity Meter SOBO 20. 414

Boschung Mecatronic AG, 2003. 415 416 2. Lysbakken, K. R. Measuring salt on road surfaces - A discussion of salt concentration 417

versus salt amount. Presented at the 14th International Road Weather Conference. 418 Prague, Czech Republic. 2008. 419

420 3. Turunen, M. Measuring salt and freezing temperature on roads. Meteorologiacal 421

Application 4. 1997. pp.11-15. 422 423 4. Raukola, T.; Kuusela, R.; Lappalainen, H.; Rasmussen, P. Anti-Icing Activities in 424

Finland: Field Tests with Liquid and Prewetted Chemicals. In Transportation 425 Research Record: Journal of the Transportation Research Board, No 1387. 426 Washington D.C., 1993, pp. 48-56 427

428 5. Fonnesbech, J. K. Ice Control Technology with 20 Precent Brine on Highways. In 429

Transportation Research Record: Journal of the Transportation Research Board, No 430 1741. Washington D.C., 2001, pp. 54-56 431

432 6. Blomqvist, G., and Gustafsson, M. Patterns of Residual Salt on Road Surface - Case 433

Study. Proceddings from the Sixth International Symposium on Snow Removal and 434 Ice Control. In Transportation Research Circular. Transportation Research Board. 435 No. E-C063. Washington D.C., 2004, pp 602-608. 436

437 7. Hunt, C. L.; Mitchell, G. F.; Ricchardson, W. Field Persistence of Anti-Icing Sodium 438

Chloride Residuals .Proceddings from the Sixth International Symposium on Snow 439 Removal and Ice Control. In Transportation Research Circular. Transportation 440 Research Board. No. E-C063. Washington D.C., 2004, pp 609-622. 441

442 8. Russ, A.; Mitchell, G. F.; Ricchardson, W. Durability of brine applications for winter 443

maintenance on asphalt and PCC pavements. Proceddings from 87th Annual Meeting 444 of the Transportation Research Board. Washington D.C., 2008. 445



446 9. Klein-Paste, A. Reduction in salt use by additatives to salt - field trials during winter 447

2007/08; Teknologirapport nr. 2523. Norwegian Public Roads Administration, 2008. 448 (in Norwegian). 449

450 10. Nygaard, H. Rapport Restsaltmåleren SOBO 20. Vinterudvalget Denmark. 2003. 451 (in Danish) 452 453 11. Blomqvist, G.; Gustafsson, M.Residual Salt and Road Surface Wetness: Comparison 454

of Field Techniques. Proceddings from the International Conference on Winter 455 Maintenance and Surface Transportation Weather. In Transportation Research 456 Circular. Transportation Research Board. No. E-C162. Washington D.C., 2012, pp 457 443-449. 458

459 12. Nygaard, H. Restsaltsmåleren SOBO 20. Vinterudvalget Denmark. 2005. 460 (in Danish) 461


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Accuracy of SOBO-20 in the measurement of salt on …docs.trb.org/prp/13-2606.pdfAccuracy of SOBO-20 in the measurement of salt on winter pavements . ... PhD-student at the Norwegian