any air balancer worth his salt would have given you (at a minimum) a complete static pressure profile which would include the delta P across each component in the duct system. To walk away and only furnish air flow and fan pressure is pathetic.
A good balancer hangs in there and helps trouble...
Sometimes the mixed air temp method for determining the percentage of o/a is not practical due to as Mintjulip suggested i.e improper mixing and/or an insufficient temp spread between o/a and return air. What we have done in the past is measure the delta P across the damper, convert that delta...
Our testing found the the freezing point of the prop/water mixture was -15F. From the chart found on the net below the mixture is approximately 46% prop/ 54% water
Freezing Point
Propylene Glycol Solution
(% by mass) 30 40 50 60
Temperature 7 -8 -29 -55
The operating...