Can a -20 C reference point be established in a laboratory?

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Melissa Grant
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1 Can a -20 C reference point be established in a laboratory? Speaker / Author: Yvette Volschenk Company: AFRIQLABS Centurion South Africa Cell: yvette7137@iburst.co.za Abstract: When measuring temperature, a ice point is used as a quick and easy zero reference point. Using this ice point, the laboratory can monitor its reference standard s drift. The ice point is stable and can be made repeatedly with relative ease. Surely then it is a simple task to make a -20 C reference point in a similar manner using a Salt and Ice mix? This purpose of this paper is to show some of the methods and results of the experiments that were conducted by the laboratory. 1. Introduction In the food industry, especially the fish industry, -20 C is a very important temperature to calibrate. Fish must be stored at -20 C, on the fishing boats, after they have been caught. There are Dry Block calibrators on the market that are able to go down to the -20 C point but these are expensive. The laboratory was looking to see if it could find a simple, portable and in-expensive reference point at about -20 C by using a Salt & Ice combination, similar to the regular 0.0 C point commonly used in laboratories. It is obviously necessary that the -20 C point be reproducible, accurate and stable for the process to work properly. 2. Experiment While discussing this approach with several of the laboratories, it became clear that though some of them talked about making a -20 C reference point from a Salt and Ice mix, there was little actual evidence of controlled tests being done and the reproducibility being clearly tracked and documented. Searches on the internet showed a fair amount about the process of getting low temperatures (for all sorts of uses) but it did not produce any discussion or results about making a metrologically sound -20 C reference point. NLA Test and Measurement Conference 2013 Page 1 of 7

2 There are a lot of chemical explanations on a salt and ice mixture. e.g. The lowest temperature obtainable for a brine (from a salt and ice mixture) is about - 22 C. (Walker reports - 21 and Bigelow C.) This temperature is called the cryohydric point. If calcium chloride is used with ice the lowest temperature obtainable is - 55 C. ( Mixtures.html#ixzz2c4CnMBpK) The laboratory then decided to conduct an experiment with the -20 C reference temperature point and see how stable and accurate it could be made and for how long the stable point would be available. It was not just mixing ice and salt together as it was initially thought it should be. Talking to other temperature metrologists, it sounded like that is how it should be made, and bingo you have a -20 C reference temperature point. The first -20 C temperature reference point experiment was made by crushing 3 kilograms of ice, made from distilled water, mixing a half a kilogram of salt together inside a polystyrene cooler box. The very first one was very watery and definitely not stable. The second one the same. It was very disappointing. The immediate conclusion was that the -20 C temperature reference point was not going to be accurate or stable enough. Working with ice and salt is very messy and if one does not clean up properly; it will rust all the metal parts in the laboratory that it comes into contact with. In addition it is dangerous as if you accidentally touch the ice and salt mix with your hands (which you should not do), the mixture will effectively freeze your skin and result in serious frost bite. The next attempt was to place 3 kilograms of ice (shaved very fine) in the polystyrene box. Then the crushed ice inside the cooler box was mixed with a kilogram of coarse salt. It was not easy to stir and was difficult to mix the salt and ice uniformly. Two reference resistance thermometers were used to measure the temperature over a period of time by inserting them in two different holes in the polystyrene box, more or less at the same immersion depth. This was a much better -20 C temperature reference point than the first 2 attempts. The -20 C temperature reference point was however still not stable and did not last long before the temperature started rising. Table 1 and Graph 1 shows the results of the measurements of the two probes over a period of approximately one hour. Probe 1 s temperature changed by C and probe 2 with C. The two reference thermometers temperatures did not change at the same rate either. NLA Test and Measurement Conference 2013 Page 2 of 7

3 Table 1 Time Probe 1 Probe 2 Difference between Probe 1 and Probe 2 10:43-20,85-20,757-0,093 10:49-20,779-20,747-0,032 10:56-20,713-20,689-0,024 11:03-20,644-20,652 0,008 11:10-20,548-20,283-0,265 11:15-20,512-20,258-0,254 11:20-20,467-20,223-0,244 11:24-20,488-20,154-0,334 11:31-20,458-20,114-0,344 11:38-20,397-20,066-0,331 11:41-20,373-20,049-0,324 11:44-20,337-20,009-0, : :48 11:02 11:16 11:31 11:45 12: Graph 1 When the lid of the polystyrene box was opened, it was observed that the general ice mix had not melted except for right next to the resistance thermometers. This resulted in a poor thermal contact and was possibly the cause of the change in temperature. It was decided to compact the ice and salt mixture, and take some readings again. NLA Test and Measurement Conference 2013 Page 3 of 7

4 Table 2 Time Probe 1 Probe 2 Difference between Probe 1 and Probe 2 11:46-19,776-18,949-0,827 11:50-20,111-18,817-1,294 11:54-20,197-18,847-1,35 11:59-20,202-18,819-1,383 12:04-20,182-18,799-1, : :45 11:48 11:51 11:54 11:57 12:00 12:02 12: Graph 2 The measured temperatures reduced, as expected, but as can be seen in Table 2 and Graph 2 they are still not acceptable results. Another -20 C temperature reference point was made. A bit different this time. To avoid trying to mix the whole container full of ice and salt in one go, a thin layer of crushed ice was placed on the bottom of the polystyrene cooler box (roughly 4 mm), then the thin layer of crushed ice was covered with a thin layer of salt (roughly 2 spoons). Again a thin layer of crushed ice was placed on top of the first layers of ice and salt and covered with salt again. This method was followed until the polystyrene box was filled to the top with the layers of crushed ice and salt. The two resistance thermometers were placed in the same two holes as previously at more or less the same immersion depth. The results were much better but the crushed ice and salt mixture was not stable for long enough. It was however encouraging. In Table 3 and Graph 3 below, one can see that over a period of a half an hour, the difference between the two resistances thermometers is smaller. Probe 1 s temperature changes over the ± 30 minute period by C and probe 2 by 0.1 C. Stability was still a challenge! NLA Test and Measurement Conference 2013 Page 4 of 7

5 Table 3 Time Probe 1 Probe 2 Difference between Probe 1 and Probe 2 13:31-20,802-21,074 0,272 13:32-20,821-21,078 0,257 13:34-20,833-21,081 0,248 13:35-20,834-21,083 0,249 13:37-20,839-21,083 0,244 13:39-20,842-21,083 0,241 13:42-20,846-21,078 0,232 13:44-20,857-21,051 0,194 13:46-20,862-21,041 0,179 13:47-20,866-21,034 0,168 13"48-20,851-21,031 0,180 13:49-20,871-21,029 0,158 13:52-20,877-21,014 0,137 13:53-20,873-21,006 0,133 13:55-20,871-20,999 0,128 13:57-20,873-20,994 0,121 13:59-20,873-20,989 0,116 14:01-20,874-20,981 0,107 14:03-20,875-20,974 0, Graph 3 Again another -20 C temperature reference point was made using the same method as the previous one. This time after placing a thin layer of crushed ice and covering it with salt, then second layer of ice and salt were applied. After this layer, the mix was compressed very NLA Test and Measurement Conference 2013 Page 5 of 7

6 firmly. This method was repeated until the polystyrene box was filled to the top with thin layers of crushed ice and salt and compressed together. The two resistance thermometers were placed into the same holes at the same depth as before. Much better results were obtained. The -20 C temperature reference point was stable for ± one hour. See results in Table 4 and Graph 4. The difference between the two resistance thermometers over one hour was C. After the initial few minutes to allow the system to stabilize, the -20 C temperature reference point was stable and both probes were drifting at more or less at the same rate. After an half an hour, the resistance thermometer Probe 1, was heating up a bit quicker that Probe :45 12:00 12:14 12:28 12:43 12:57 13: Graph 4 Table 4 Time Probe 1 ( C) Probe 2 ( C) Difference between Probe 1 and Probe 2 11:56-20,903-20,984 0,081 12:01-20,971-21,061 0,090 12:03-20,977-21,063 0,086 12:05-20,988-21,071 0,083 12:07-20,994-21,074 0,080 12:08-20,996-21,076 0,080 12:11-20,998-21,081 0,083 12:13-20,999-21,078 0,079 12:14-21,000-21,081 0,081 12:16-21,000-21,081 0,081 12:17-21,000-21,083 0,083 NLA Test and Measurement Conference 2013 Page 6 of 7

7 12:18-21,001-21,083 0,082 12:19-21,001-21,081 0,080 12:20-21,002-21,083 0,081 12:22-21,001-21,081 0,080 12:23-21,000-21,083 0,083 12:25-20,999-21,081 0,082 12:27-20,998-21,081 0,083 12:29-20,998-21,083 0,085 12:31-20,997-21,083 0,086 12:34-20,996-21,083 0,087 12:37-20,994-21,081 0,087 12:40-20,993-21,081 0,088 12:43-20,990-21,081 0,091 12:49-20,987-21,081 0,094 12:52-20,987-21,081 0,094 12:53-20,987-21,078 0,091 13:00-20,985-21,081 0,096 13:03-20,985-21,081 0,096 Difference from start to finish 0,082 0,097-0, Conclusion The above experiments showed that whilst reasonable results were obtained, the process was not as simple as at first perceived. There are still many aspects that need to be investigated and the procedure refined to ensure that repeatable results can be obtained. Until these have been done, the laboratory is using the latest procedure to make an approximate -20 C transfer medium and referencing the temperature obtained against the laboratory standard thermometers. It is still believed that when these experiments are continued, a procedure will be developed that will allow for a good and in-expensive reference point, with acceptable uncertainties. NLA Test and Measurement Conference 2013 Page 7 of 7

Activity 6.5 From gas to liquid to solid

Activity 6.5 From gas to liquid to solid Activity 6.5 This activity is an extension of Activity 6.4a in which ice is used to make a container cold. As in Activity 6.4a, this activity will work only with sufficient water vapor in the air. Here,