# Error in measurement from two correlated devices

I'll provide physical context just to be able to write my question clearly. In an experiment, we're trying to measure the electric field using a device called an electrofield meter $$EFM$$. The way it works, roughly speaking, is that it converts the field to a potential difference that gets measured by an avometer. To obtain the field, we then multiply the potential difference by a factor, that depends on the calibration of the device, to get the field. I want to know the uncertainty in such a measurement. It results from two devices. But I don't know if they are unrelated/independent, because the desired value is just a multiple of the actual measurement. My teacher said the total error would be $$\sigma^{}_{total} = \sqrt{(\sigma^{}_1)^2 + (\sigma^{}_2)^2}$$ where $$\sigma^{}_1$$ and $$\sigma^{}_2$$ are the errors of the respective devices. But I thought that was when the measurements are made independently, like for example using two devices to measure two different quantities and the desired quantity is their product or their ratio. Am I wrong?