Question

(a) Values of some main engine exhaust temperatures displayed in the engine control room differ from those displayed on the engine for the same cylinders.

(i) Explain how it may be determined which readings are inaccurate

(ii) State possible reasons for these inaccurate readings

(iii) Explain how the location of the faults may be detected

(b) State the periodic checks which should be carried undertaken to ensure that remote engine instrumentation is readings accurately

Answer 1

Part (a)

To ascertain which readings are correct, a comparison would be made between the control room display and the local display. The simplest way to find the correct temperature is to change the local thermometer with a new one. This new reading would be used as the datum to check the control room display.

A portable laser thermometer can also be used to check the local temperature and determine the correctness of readings

Part (b)

Possible reasons for inaccurate readings are:

Vibrations
Physical damage to the mercury in the glass unit
The control room display could be defective due to cable damage
Physical damage to the probe
Incorrect power supply to the display unit
Defective display gauge

Part (c)

The location of the faults may be detected by checking:

In the case of a defective local temperature indication, the fault will be with the thermometer and in most cases, the only remedy available will be to replace the thermometer.
For remote indication, the trouble may lie with the sensor, which consists of the sensing elements, internal conductors, protection tubes, connection boxes or cables, connection terminals, etc. The sensor element is usually protected by a stainless steel tube. Inside the tube, there is a filling material for protection against vibration and shocks. Terminals are located in the connection box, and outer cables are connected to the terminals.
All the above items should be checked visually to locate the fault. A grounding of the sensor element can be checked by measuring the insulation resistance between one of the terminals and its metallic casing

Part (d)

The periodic checks, which should be undertaken to ensure that remote engine instrumentation is working accurately

For local thermometers, the screwed-in thermometer pocket should be taken out and cleaned and visually checked to see if all is in order about once every six months.
The mechanical and electrical parts of the remote temperature-indicating equipment should be inspected approximately once every three months. At this time, the outside of the sensor casing should be cleaned, and it should be ensured that the terminals are secured properly. The general condition of the cable should be checked, and insulation resistance should be taken if any deterioration is suspected.
Makers of remote thermometers often supply data about the resistance of the sensor circuit, which may be checked if the temperature readings are a suspect.
Normally, a periodical recalibration as a routine is not necessary. This may be carried out only if there is a doubt about the temperature readings.
Zero and span setting to be carried out if required for correct reading.

✓ Verified Model Answer (Text Solution)

Possible reasons for inaccurate readings are:

The location of the faults may be detected by checking:

The periodic checks, which should be undertaken to ensure that remote engine instrumentation is working accurately