In a strain gauge-strain tube type engine indicator (conventional indicator), zero-shift is caused by the thermal strain of that part of the engine at which the indicator is installed. The amount of zero-shift increases to about 10% of full scale under actual operating conditions.
In this paper, what brings about the zero-shift in the conventional indicator is analyzed into factors. On the basis of the results, we attempted to reduce the amount of zero-shift by the following methods:
a) The spring constant of the diaphragm is reduced in the conventional indicator.
b) The threaded connection for the indicator is structurally modified (new type indicator) .
c) The conventional indicator is used with the water-cooled adapter.
In addition, we ascertained the dynamic characteristic of the indicator with each of the above modifications.
We verified that, for the conventional indicator, the amount of zero-shift varies with the spring constant of the diaphragm, the relationship between the two being nearly linear.
We found also that the new type indicator has a better zero-shift characteristic. The amount of zero-shift can be reduced to about 4% of full scale under actual operating conditions, without deteriorating the dynamic characteristic.
For the conventional indicator with the water-cooled adapter, the dynamic characteristic was worse than that of the conventional indicator but its zero-shift characteristics was far better, the zero-shift being only about 0.7% of full scale under actual operating conditions. It follows that this method can be used where the frequency of pressure cycle is not high.

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