Ashford Instrumentation Ltd - Installation & Maintenance of Mechanical Thermometers

Installation & Maintenance of Mechanical Thermometers

INSTALLATION & MAINTENANCE OF MECHANICAL THERMOMETERS

Application

Standard brass and stainless steel thermometer stems and pockets/thermowells are suitable for air, oil, water and other non-corrosive fluids. Where corrosive fluids are present, alternative materials such as PTFE or Hastelloy etc should be specified. Environmental conditions should also be taken into account when considering suitable case or capillary materials. We always recommend stainless steel case thermometers are used on any application outside or where corrosion of steel is likely to occur.

Maximum Ranges

Mechanical thermometers are generally available to measure temperatures between -200 and +800C. The over or under range performance of a particular thermometer should also be checked before exposure as they will vary from design to design. Most thermometers designs have a minimum span of 60C.

Ambient Temperature

Generally mechanical thermometers are designed to operate in ambient temperatures between -10C and +50C. Whilst some thermometer designs have bimetallic temperature compensation built into the case, all thermometers heads and capillary should be protected from localized heat or cold sources as this can lead to indicating errors.

Thermowells

The use of thermowells or pockets is recommended in all applications. The correct specification of thermowell is dependent upon a number of factors such as temperature, flow, process medium, vibration etc and it is recommended that each application is discussed with the manufacturer to ensure the correct selection.

Installation

Care must be taken to ensure the thermometer bulb or sensing element is not damaged during installation. Do not attempt to bend the thermometer stem or bulb under any circumstances.

The sensing bulb or element should be fully immersed in the medium which is being measured. If a thermowell is being used, the heat transfer delay can be improved by filling the thermowell with a heat transfer substance such as graphite paste.

When fitting the thermometer into the thermowell it is essential the bulb or sensing element is not forced against the bottom of the thermowell when tightening the securing nut. Many thermometers are manufactured to match the thermowell they are supplied with to ensure this can not happen however, when the thermometer is supplied with a sliding compression fitting, the correct fitting is achieved by inserting the thermometer into the thermowell until it reaches the bottom when it should be withdrawn by 5mm before tightening he compression nut to hand tight plus a quarter turn.

Check capillary is the correct length by laying along the proposed route. Never attempt to stretch the capillary as this will lead to fracture of the system. The capillary should be securely supported and clipped to wall or other solid surface and must be free from buckling and twisting. Bend radius of less than 60mm should be avoided. Particular care should be taken to the points where the capillary enters the case and bulb. Excess capillary should be coiled and arranged in free swinging loops between the last fixing point and the bulb.

Never tighten the thermometer into the system by grasping or twisting the case as any distortion created will lead to calibration errors. Installation should be made using the instrument spanner flats provided.

Thermometer heads should be mounted in the vertical position unless otherwise agreed with the manufacturer.

Vibration

Mechanical thermometers should always be installed in a vibration free area. The instrument may exhibit excessive wear on the bearing surfaces of the movement. If an installed instrument fails exhibiting these symptons it is almost certain the wrong type of instrument has been used for the application and it is essential the manufacturer is consulted before a replacement device is fitted.

Threads & Joints

Care must be taken to ensure mis-match of threads does not occur. Mating female connections must have a pressure rating that is compatible with the pressure rating of the instrument.

Maintenance

The function of a thermometer does not require any special maintenance procedures but frequent checks must be made to ensure the instrument is working correctly and accurately. Any shift in temperature reading greater than twice the tolerance of the instrument must be investigated and the immediate replacement of the thermometer is recommended if it is found to be faulty.

Repairs

The repair and recalibration of any thermometer should only be undertaken by a competent person who has at their disposal the necessary facilities. It is our recommendation that all calibration equipment used should be traceable to National Standards and be within a recognized calibration/quality system.

Storage

Thermometers should be stored in clean dry conditions and care should be taken to ensure the ambient temperature does not exceed or fall below the measuring range of the instrument. All thermometers must be protected against any impact damage.

Transport

Although care is taken in packing thermometers for shipment it is possible they can sustain transmit damage. All thermometers should be checked for damage before use.