Adam:
I believe the "80" rating on the indicator spring is not a constant 80 psi over the entire compression distance of the spring. I.E., as the pressure on the piston of the indicator increases beyond 80 psi, the spring compresses more. The indicator manufacturers often furnished wood scales (rulers) with the pressure rating of the indicator springs on them. Consider the Mean Effective Pressure (MEP) of a steam engine. I am sure you are well familiar with the 'events' that occur in a steam engine cylinder which result in the indicator diagram or 'card'. The indicator spring has to have enough range to handle pressures ranging from at, or near, 'steam at the stop valve' (or steam chest pressure) down to very low steam pressures during the exhaust portion of the cycle. If an engine were run condensing, the indicator spring would need enough range to handle positive pressures as well as partial vacuum. If a spring rating based solely on boiler pressure were used, there might not be enough range, or enough accuracy in it to get a good indicator card. A more everyday example is the selection of pressure gauges. Ideally, when a pressure gauge is selected for a given application, the typical working pressure of whatever the gauge is measuring should fall about midrange on the gauge. Same holds true for indicators and their springs.
Frictional loss of steam pressure between the throttle valve (or steam chest) and the cylinder are a reality. Different designs of engines tried to minimize this loss of pressure. In something like a simple slide valve engine with cored steam passages and the changes of direction for the steam flow, some pressure drop is inevitable. However, I do not think this pressure drop would be significant enough to result in using indicator springs of a lower rating than steam pressure at the throttle or steam chest.
During the latter years of steam engine building, there was quite a rivalry between steam engine builders as to best type of valve and valve motion. Corliss went quite a long ways to reducing frictional losses (and resulting cooling) of the steam thru the valves and steam passages. Poppet valve engines took it quite a bit farther. I am partial to the Skinner Unaflow engines, having designed a powerplant using a 27 1/2" x 36" Unaflow engine and erected it 43 years years ago. Steam went thru a complete expansion in a Unaflow cylinder. A steam engine indicator had to respond accurately over that same pressure range.
Consider any steam engine exhausting to low back pressure (exhaust steam used for heating or process work), or run condensing. You might be seeing 150 psig or thereabouts 'at the throttle' and maybe 18-21 in of Hg at the exhaust. Quite a range for an indicator spring to maintain accuracy and linearity over. Start off with a 150 lb spring and you might find it did not accurately indicate pressures at the lower end of things happening in the cylinder.
