User Guide

9
Temperature and humidity recorder (see Fig. 4)
The air circulates around a strip of animal tissue (Goldbeater skin) the length of which varies with the relative
humidity. The membrane is mounted under light spring tension and is connected by means of a bell crank lever
and linkage to a pen arm which records the humidity on a calibrated chart. Temperature is measured by a fluid
filled system which is mounted to the left of the humidity element and mechanically linked to a second pen arm.
Humidity recorder – wet and dry bulb
The relative humidity can be obtained from the temperatures measured by the wet and dry bulb thermometer
using hygrometric or psychrometric tables.
Set up and Overload Protection Springs
To protect the pen mechanics if the measured variable goes significantly outside the span of the instrument the
connecting link may incorporate “set up” and/or “overload” springs (see Fig. 6.).
The set up spring (also shown in Fig. 5.) is used if the measured variable is liable to go significantly below the
chart zero reading, e.g. on a temperature recorder calibrated 100–600
o
C where the measuring element is subject
to normal ambient temperatures during transit. Under these conditions the movement of the Bourdon tube
compensator extends the spring allowing the pen mechanism to rest against the zero stop.
The overload spring is used if the measured variable is liable to go significantly above the maximum chart
reading. Under these conditions the movement of the Bourdon tube compensator extends the spring allowing the
pen mechanism to rest against the top point stop.
Fig. 6
Overload action
Movement of Bourdon Tube compensator
Overload spring
Ratio arm pivot
Set up spring
Set up action