Presented in this paper are a high-performance “photo-tube type frost-point meter for low humidity” which can work at the frost-point below -40°C and a saturator for low humidity to check the frost-point meter, both of which show fairly good result. Satisfactory result was obtained by picking up and solving the following problems :
1) Increase of amplifier gain resulting from decrease of water vapour pressure inclination.
2) Improvement of mirror and mirror body.
3) Error due to water dehydration out of gas passages and test room wall.
4) Error caused by the form of frost attaching the surface of mirror, etc.
As a new type of amplifier, a variant of multi-vibrator was made from the authors own designand good result was obtained even for large input. The characteristics of the frost-point meter made in this way is quick in response (approximately within '30 sec.) and is able to measure approximately from -70°C to 0°C. Its accuracy is within ±1.0° at the temperature lower than -40°C.
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