INSTRUMENTS

Provision should be made for temperature measurements at compressor suc­tion and discharge, and at expansion valve liquid entry. Likewise pressure
measurements should be possible at compressor suction and discharge. This can be considered as a minimum requirement and will enable a log to be kept (see Chapter 29) during initial commissioning and also during the life of the plant. The advent of the electronic probe thermometer has simplified commis­sioning work, and the fitting of thermometer wells is less important. Even so, such facilities are worth considering when large pipes are being erected, and it will be necessary with insulated pipes if true temperatures are to be taken with­out damaging the insulation.

Wells should slope downwards into the pipe, so that they can be part filled with liquid to provide better thermal contact. Where a pipe temperature is a crit­ical factor in the operation of a system, it is usually worth fitting a permanent thermometer.

The monitoring of temperatures for electronic control systems is now mainly by thermocouples or resistance thermometers and these can be secured onto the outside of the pipe using a thermal paste and aluminium tape and the pipe then insulated over.

Pressure gauges should always be fitted on the discharge side of liquid pumps, to check performance and give warning of a possible drop in flow resulting from dirty strainers. Manometer pressure gauges are required across air filters.

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