In [Fig. 3364] the line of no pressure is marked at 15 lbs. per square inch below the atmospheric line of the diagram, the atmospheric pressure being for convenience taken as 15 lbs. above a perfect vacuum.
The line of no pressure serves as a guide in showing the effectiveness of the condenser, as well as for computing the volume of steam used, but is not necessary in computing the horse power of a non-condensing engine, because the gauge pressure has its zero marked to correspond with the atmospheric pressure.
In computing the consumption of steam or water from the diagram, therefore, both the clearance line and the line of no pressure must be marked on the diagram, and lines of the diagram extended so as to include them, thus accounting for all the steam that leaves the steam chest from the piston stroke.
Indicator springs are varied in strength to suit the pressure of steam they are to be used for.
The scale of the spring is the number of lbs. pressure per square inch represented by a vertical motion of the pencil; thus, a 40 lb. spring is one in which a pressure of steam of 40 lbs. per square inch would cause the piston to rise an inch above the atmospheric line of the diagram.
The strength or tension of the spring is so adjusted as to cause the diagram to be about 21⁄2 inches high, let the steam pressure be what it may. The following are the scales of springs of the Thompson and Tabor indicator.
THOMPSON INDICATOR. | ||||||
| Scale of spring. | Used for pressure above atmosphere if not more than | |||||
| 15 | lbs. | 21 | lbs. | per | square | inch. |
| 20 | „ | 38 | „ | „ | „ | „ |
| 30 | „ | 94 | „ | „ | „ | „ |
| 40 | „ | 90 | „ | „ | „ | „ |
| 60 | „ | 143 | „ | „ | „ | „ |
TABOR INDICATOR. | ||||||
| 10 | lbs. | 14 | „ | „ | „ | „ |
| 12 | „ | 20 | „ | „ | „ | „ |
| 16 | „ | 30 | „ | „ | „ | „ |
| 20 | „ | 40 | „ | „ | „ | „ |
| 24 | „ | 48 | „ | „ | „ | „ |
| 30 | „ | 60 | „ | „ | „ | „ |
| 32 | „ | 64 | „ | „ | „ | „ |
| 40 | „ | 80 | „ | „ | „ | „ |
| 48 | „ | 96 | „ | „ | „ | „ |
| 50 | „ | 100 | „ | „ | „ | „ |
| 60 | „ | 120 | „ | „ | „ | „ |
| 64 | „ | 128 | „ | „ | „ | „ |
| 80 | „ | 160 | „ | „ | „ | „ |
A spring that is strong enough for a given pressure may be used for any less pressure.
The height of the diagram will, however, be less, and accuracy is best secured by having the diagram up to the limit of about 21⁄2 inches, using a spring that is light enough to secure this result.
Diagrams of high speed engines, however, will have their lines