Vapro 5520 - 1.4 How the Vapro Works

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SECTION 1

INTRODUCTION

1.4 How the Vapro Works

A 10 microliter specimen is aspirated into a micropipettor tip. The

specimen is then inoculated into a solute-free paper disc in the

sample holder, whereupon the sample holder is pushed into the

instrument and the sample chamber is locked. Locking initiates the

automatic measurement sequence.

The sensing element is a fine-wire thermocouple hygrometer. This

is suspended in a unique, all-metal mount, which when joined with

the sample holder, forms a small chamber enclosing the specimen.

As vapor pressure equilibrates in the chamber airspace, the thermo-

couple senses the ambient air temperature, thus establishing the

reference point for the measurement. Under electronic control, the

thermocouple then seeks the dew point temperature within the

enclosed space, giving an output proportional to the differential tem-

perature.

The difference between the ambient temperature and the dew point

temperature is the dew point temperature depression–an explicit

function of solution vapor pressure.

Dew point temperature depression is measured with a resolution of

0.00031 °C. The microprocessor-controlled measurement cycle

requires 80 seconds.

Appendix C contains the theory of operation of the vapor pressure

osmometer.