Liquid fert rate incorrect

[Headless chicken]

As above on a househam sprayer. Pressure based as opposed to flow based I believe.
Rate was considerably out compared to what the controller says it should be. Ie the controller said 500 left in tank which was correct for acres but the tank was empty.
Had the mechanics out to it and they can’t find anything wrong.
Suppliers have been very supportive as always and suggested weighing some product( househam we’re fairly sure the SG would have a bit baring) product weighed and although higher is not hugely out.

has anyone had any of these issues before?

 

[T C]

I have a househam AR3000 with flow based controller. Initially I tried setting up different SG but in theory it is measuring volume so shouldn't matter.
Now I just alter the pulse per l for liquid fert. I probably need different ones for each blend.
From memory I have 160 pulses for spray and 152 for fert.