The ASTM D341 charts are log/log charts.
On the spreadsheet the graphs are deliberately shown as linear/linear to show this relationship most visually.
In the ASTM D341 standard the charts are most convenient as straight lines for oil blending purposes.
The relationship between the temperature and viscosity is clearly not linear but a log.log relationship:
V= kinematic viscosity in cSt and T = temp in deg Kelvin.
This means that at low temperatures the rate of change of viscosity with temperature is very much greater than at higher temperatures.
If you want to know the viscosity at 50degC and are measuring at say 100degC and 60degC you can set up the spreadsheet with data which uses these two temperatures as reference temperatures and then make sure that one of the column headings is 50degC.
Next try changing some of the settings to represent measurement errors... 1% for viscosity and 0.5degC say for temperature.
With two measurements you can generate a range of errors and then see what this does to the viscosity at 50deg C.
Then repeat with different reference temperatures.
You should find that the calculation is most sensitive when you are calculating from higher temperatures (60 and 100degC) to lower temperatures (50degC).
The closer the two measurement temperatures the less the accuracy and the greater the difference between the measurement temperatures and the reference temperature the greater the errors.
This may seem alarmist but in practise the dual viscometer system, when properly designed and commissioned, can produce results that are very difficult to discriminate from the lab readings.
Most often, once installed, operators find they have to upgrade their lab equipment or review their sample collection and management procedures and their laboratory procedures, simply because the results are so good.
So don't be off put by my comments, I'm just fussy, but at the same time do be aware that while very good process viscosity measurement is possible and especially with the dual viscometer system, at the same time process viscosity measurement is very sensitive to poor system design, implementation and commissioning.
The temperature viscosity relationships is the main reason why process viscosity measurement has proven so difficult until now, far more so than any consideration about the fluid's rheological properties and whether it is Newtonian or Pseudoplastic or whatever.
It isn't through lack of effort that it has taken so many years to find a way to successfully replace the process capillary viscometer with all its faults, it is because viscosity is a difficult measurement to do well, but when done well, it can be very very good.
Oh yes, and fuel oils are among the most benign applications as it happens, despite being apparently nasty dirty fluids. (Ah, but do be sure you have chosen viscometers with PFA or PTFE non-stick coating and you have decent sample flow rates; when blending residual fuel with diluent (or crude oil with cutter stocks in pipelines), the diluent can dissolve the resins that normally are bonded to asphaltenes causing them to be precipitated which can contaminate the viscometers causing false high readings.
So, anyway, please use the spreadsheet to gain some appreciation for the influence of measurement errors.
You may be surprised how sensitive the calculation is to errors and especially where both measuring temperatures are above the reference temperature and the reference is down in the steep part of the curve.
But, in a good process set up the measurement temperatures will not vary very greatly under normal process conditions because there shouldn't be such significant variation in process temperatures that the heat exchangers can't manage a reasonable control of the measurement temperatures.
The errors can then be addressed as two components:
1) systematic error which is overcome by managing the actual measurement temperatures to be reasonably constant, and calibrating the sensors and system against the lab (where necessary - the errors do not always require this, it depends on what is considered significant) and
2) random errors or noise which are effectively minimised by averaging. Incidentally, most labs are used to taking a sample, managing it indifferently and then running one sample through a singe capillary... it may be an ASTM D445 capillary but one measurement an hour doesn't encourage much fussiness... but in some refineries the laboratory will have multiple capillaries in the temperature bath and will discard highest and lowest values then average the rest.
Finally, the benefits of modern inline viscometer systems are virtually 100% on stream factor, the lack of any need for re-calibration, especially with the dual viscometer system, except under exceptional circumstances and speed of response.
The lab can give very accurate measurements - mainly because the ASTM D445 set up will give very precise temperature control but if using the lab to control the process the time delay between sampling and the result allows plenty of time for product quality deviation which is why this can really only be done with relatively stable processes.
Fuel oil quality varies to much for this which is why process capillaries have been used for heavy fuel ils despite, of all hydrocarbon products this is the least suitable for this technology, and why here is so much maintenance and down time.
The capillary gives something upto 30minutes response times.
The modern viscometer is giving something like 20seconds.
So in terms of product quality control the combination of accuracy and response times gives much improved product quality.
PS I note you say "heavy oil blend with diluent" and not Heavy Fuel Oil, not that it makes any real difference except that it affects the emphasis on accuracy and the need to be fussy.
Dual viscometer systems have been used with both heavy fuel oils and pipeline blending applications where the heavy crude has to be diluted to meet pipeline density and viscosity limits (in such applications a separate fiscal density meter is often used as the primary parameter is density and once inside the density limits the viscosity can be maximised).
In crude oil blending applications fussy is worth while because these systems can pay for themselves in a matter of weeks and even a small accuracy improvement is worth pursuing.