Temperature Effects on Pressure Gauges
Effects of Temperature
Changes in ambient temperature affect the accuracy of gauges in several ways. Range shift is caused by the change in modulus of elasticity of the bourdon. This effect increases proportionately as the pressure increases. As a general rule, the loss of accuracy will be an additional 1% of full scale reading for every 50°F change in temperature. Zero shift is created by the change in physical dimensions of the various components brought about by the temperature change. This shift is constant over the entire scale and does not vary with applied pressure.
Minimizing Temperature Effects on Pressure Gauge
Maximum Temperature Limits
To ensure the longest possible life and accurate readings, pressure gauges that have soft-soldered pressure joints (General Equipment gauges with brass internals) should not be exposed to process or ambient temperatures over 120˚F. This is especially true of pressure gauges with liquid filled cases, due to the expansion of the case fill fluid. Long term exposure to temperatures in excess of 120˚ F may cause discoloration of dials and fill fluids, as well as hardening of the case seals and possible fill leakage. Gauges with silver soldered or welded pressure joints (gauges with SST internals / Process Gauges) should not be exposed to process or ambient temperatures over 190˚F.
High and low temperatures affect accuracy on indication. A general rule of thumb for dry gauges is 1% of full scale change for every 50˚F change from 75˚F. This allowance should be doubled for gauges with liquid filled cases.
In order to prevent live steam from entering the bourdon tube, a siphon filled with water should be installed between the gauge and the process line. Siphons should also be used on any application whenever condensing, hot vapors (other than steam) are present.
When a gauge is installed on a process line containing hot liquid or gas, one solution is to simply include a length of capillary tubing between the hot line and the gauge. The slow rate of heat transfer through the added capillary and dead-ended process fluid will generally protect the gauge from damage.
Diaphragm seals with filled, flexible line assemblies are another good solution to the problem of hot liquid and gas lines.
The minimum recommended operating temperature for all gauges is –40˚F. Gauges filled with silicone oil will provide the most resistance to the effects of operating in freezing conditions.
The effective operating temperature of Glycerin Fill Fluid is 40˚F to 140˚F and for Silicone Fill Fluid -40˚F to 190˚F.