Where do you do scaling?

I generally allow for a scaling function to be done via the HMI, behind a password, since I regularly see deviations from one sensor to another. I don't have to worry about the pressure gage not matching the HMI, or whatever... If the customers want them to match and don't want to change the gages, 'we can do that'.

It may not be perfect, but it can make the odd customer happy.

From what I read in the posts I know I am going to be in the minority on this but not unusual I tend to see thing different than most.
But always scale in input module is it has the ability
Let m try to explain things a little.
In old plc’s you had no ability to scale inputs or outputs in the program at all, they had to be scaled in the intelligent modules themselves. You had to do a block transfer write with all the data necessary. I remember on job I did years ago one module I had to do I think around 10 block transfer writes on power up before it could use the module they had to be in a specific order to work. It was a pain but necessary.
With the new PLC’s they are more powerful and have scaling available within the functions of the programs but the plc processor has many things to do and their update time is variable so you really can’t be sure when or if the scale is updated correctly. The modules themselves have a processor built in and it’s only job is to do the scaling. This is a long proven technology that been around long before it was available in a plc program. Given that I would trust the module scaling over the program. And why use unnecessary resources in the plc program to do something that can better be done in the module with far less programming and overhead.
Also that data is available to all programs in the plc for use anywhere you don’t have to think about is it
Is it in a running program updating. With reading or writing it directly I know it’s the most recent data.

I know that one may say what about the guy coming in later and has to edit the program or when there is a problem and the module has to be replaced then they have to set up the module. Well that not true with most modules the module setup information is not stored in the module after power down, it is stored in the plc memory and all the modules are loaded with whatever data necessary on power up first scan. So if it’s is the same module ( and it should be ) the replacement module will be loaded on power up. If it is not the same then it will need to be reconfigured anyway. Also consider that more and more sensors are now intelligent sensors they have to be programmed at the time of installation, should one need to be replaced they would have to be reprogrammed before they can be used, that program is not stored in the plc memory so the technician that is doing the service will need to have at least some minimum skills and understanding of both the plc programming and the sensor programming to get everything back up.
If the technician is qualified there should be no problem, if he is not then he should not be working on it.
I have seen many times where an unqualified technician has cost the entire plant to go down for a full day or more over a problem that was easily solved in a few minutes or less.
Down time is expensive. I have worked in plants where every machine had a hot standby next to it if a machine goes down the operator simply mover to the hot standby while the other is fixed.

I do all scaling in the PLC logic, not in the module, or external devices except for Thermocouples, where I do let the modules handle them. I also provide a nice, consistent faceplate on the HMI's to allow for two-point span calibrations of all analog sensors (aside from Thermocouples)

I too use the PLC to calibrate in the main, however, I do both if the sensor has some form of calibration, for example, A PT100 with a 4-20ma head converter I always calibrate (or check) the 4-20 converter is giving out the correct ma over the range then do the final calibration on the PLC scaling, I do this to ensure that rather than rely on any offset being scaled out in the PLC that the signal into the analogue card is as it should be. If someone changes the head converter most times these will already be calibrated during manufacture.

If you use the Rockwell PlantPAX AOI's and such. For CLX, I set the IO modules as 4-20, for CPX, I use percentage (0-10000). Then the scaling is done in the Faceplates for the Icons, if at some point an instrument has to be respanned, a tech can do it from the faceplate. No PLC connection required.

Always in the PLC.

I usually list the math in ladder logic with step-by-step calculations using Y=MX + B.