Colorimeter scales and precision

 

The colorimeter absorbance scales come with two different sensitivities. These are given as 0 - 1 and 0 - 2. 1 and 2 would be called the Full Scale Deflection (FSD) because that is the reading when the needle is deflected to the furthest end of the scale. It's easy on the 0 - 2 scale because the numbers are written onto the scale at the bottom.

So the picture above onto the 0 - 2 setting would be 1.83. Absorbance is a logarithm of a ratio so has no units. The scale divisions are 0.05 apart so the way of giving the resolution as half a scale division means we say 1.83 +- 0.025. I'd say by inspection that we can do better than +-0.025. Surely by stating it as 1.83 I really mean +-0.01. 

There are two ways of dealing with the 0 - 1 scale. I have relabeled the scale to show how. I like the top green numbers best. I'm going to say the reading is 0.92 +- 0.01 using the half scale division resolution. But that's the same resolution I was claiming for the top scale. So maybe it's really 0.92 +- 0.005. 

Ergs at the gym

 

I was interested to see this ski trainer at the gym. I knew that ergs are a unit for energy but I hadn't realised what a tiny amount it is. An erg is 10^-7 Joules - and I have always thought that Joules were a disproportionately small amount. I had an idea ergs were American but they were proposed by a European and fit into a metric system. I need to do more work on the C.G.S. system of units.

Absorbance calculations to prove why a colorimeter needs a colour filter

 

I set up a way of thinking about the colour filter. The arrow on the left is the white light made of red, green and blue. Shone at a cuvette full of copper sulfate, let's say half of the red and half of the green is absorbed so the other half is transmitted. The blue light is unaffected so any blue that goes into the cuvette must come out. Io is the abbreviation for the intensity of the light going in and I is the abbreviation for the light coming out. So in my way of doing things, the total going in is 3 and the total coming out is 2.

Now let's insert an orange filter that absorbs all of the blue light.

Then let's apply the formula for the absorbance which is based on logarithms.

Notice that the absorbance value is much higher with the filter in place because the transmitted blue light has been removed. That's why we need a filter in a colorimeter.

Why do colorimeters have colour filters?

 

It has finally occurred to me why a colorimeter has colour filters. Last year I was using copper sulfate solution of different concentrations. A colorimeter shines light through a sample of it placed in a cuvette. It measures the intensity of the light going in and the intensity of the light coming out having been transmitted through the sample. The intensity of the light coming out will be less than the intensity going in because some of the light will have been absorbed. Copper sulfate is blue. That means it absorbs the other colours but all blue light goes through it. Blue is transmitted; it cannot be absorbed by copper sulfate. If we shine white light through, it contains blue as part of its spectrum. That blue light goes straight through. But suppose we use a filter before the sample cuvette that absorbs all of the blue light before it reaches the cuvette. Only the other colours will reach the cuvette and they stand a chance of being absorbed. This time the intensity coming out will be lower. The bigger the difference between intensity in and intensity out, the higher the absorbance. Using the correct filter increases the absorbance.

Brockram in the Cockpit stone circle on Moor Divock?

Brockram is an amazing stone I've seen in the Vale of Eden. Carboniferous limestone from when we were in a coral sea was then eroded when we were in a desert as far north as the Sahara. The stones were not transported far in wadis so they are large and angular and were set into sandstone. Such an amazing story that you can follow down the valley. This stone in a stone circle on Moor Divock between Pooley Bridge and Askham seems almost certain to be brockram, probably getting up there by glacial transport over a few miles.

Magnetic fish tank cleaner

I was interested to see a strong magnet stuck to the glass of this fish tank. It attracts another piece through the glass which can then be dragged up and down the glass to clean it. The glass was about 5mm thick. I can't find any information on the strength of these commercial magnets nor anything about how field strength decreases with distance for pole-faced magnets. For a current-carrying conductor, it it is double the radial distance, halve the field strength. I think I'd need to do some experiments with this to find out the thickness of glass at which it no longer attracts.

Back to rippled stratus

The sky was amazing this morning. The same widely ribbed stratus stretching from Skiddaw to Cross Fell that I blogged about last month. It is definitely higher than Cross Fell at about 900m because I could see the summit but there is cloud above it. I guess it is stratus because stratus is between 1000m and 2000m. A bit of research suggests it is probably the type referred to as Stratus undulatus. Then at right angles in the layer above, seen through the gaps in the undulatus, there was an alto- version of it. That suggests that the winds at that much higher altitude were at 90 degrees to the lower winds.