Siemens P 156, Calculation of the temperature error, Example of temperature error calculation

© Siemens AG 2008

SITRANS P measuring instruments for pressure

Remote seals for transmitters and pressure gages

Technical description

Calculation of the temperature error

The following equation is used to calculate the temperature er- ror:

dp = (ϑRS – ϑCal) ⋅ fRS + (ϑCap – ϑCal) ⋅ lCap ⋅fCap + (ϑTR – ϑCal) ⋅ fPF

2	dp	Additional temperature error (mbar)
	ϑRS	Temperature on remote seal diaphragm (generally
		corresponds to temperature of medium)
	ϑCal	Calibration (reference) temperature (20 °C (68 °F))
	fRS	Temperature error of remote seal
	ϑCap	Ambient temperature on the capillaries
	lCap	Capillary length
	fCap	Temperature error of capillaries
	ϑTR	Ambient temperature on pressure transmitter
	fPF	Temperature error of the oil filling in the process
		flanges of the pressure transmitter

Example of temperature error calculation

Existing conditions

SITRANS P pressure transmitter for	fRS = 0.1 mbar/10 K
differential pressure, 250 mbar, set	(0.0014 psi/10 K)
to 0 ... 100 mbar, with DN 80 remote
seal diaphragms without tube, dia-
phragm made of stainless steel,
mat. No. 1.4404/316L
Capillary length	lCap = 6 m (19.7 ft)
Capillaries fitted on both sides	fCap = 0.07 mbar/(10 K ⋅ mCap)
	(0.001 psi/(10 K ⋅ mCap))
Filling liquid silicone M5	fPF = 0.07 mbar/10 K
	(0.001 psi/10 K)
Process temperature	ϑRS = 100 °C (212 °F)
Temperature on the capillaries	ϑCap = 50 °C (122 °F)
Temperature on pressure transmit-	ϑTR = 50 °C (122 °F)
ter
Calibration temperature	ϑCal = 20 °C (68 °F)
Required
Additional temperature error of	dp
remote seals
Calculation
in mbar

dp = (100 °C – 20 °C) ⋅ 0.1 mbar/10 K + (50 °C – 20 °C) ⋅ 6 m ⋅

0.07mbar/(10 K ⋅ m) + (50 °C – 20 °C) ⋅ 0.07 mbar/10 K dp = 0.8 mbar + 1.26 mbar + 0.21 mbar

in psi

dp = (212 °F – 68 °F) ⋅ 0.0014 psi/10 K + (112 °F – 68 °F) ⋅ 19.7 ft ⋅ 0.001 psi/(10 K ⋅ 3.28 ft) + (112 °F – 68 °F) ⋅ (0.001 psi/10 K)

dp = 0.012 psi + 0.018 psi + 0.003 psi

Result

dp = 2.27 mbar (0.033 psi) (corresponds to 2.27% of set span)

Note

The determined temperature error only applies to the error resulting from connection of the remote seal.

The transmission response of the respective transmitter is not included in this consideration.

It must be calculated separately, and the resulting error added to the error determined above from connection of the remote seal.

Dependence of temperature error on diaphragm material

The temperature errors listed in the previous table are based on the use of stainless steel as the diaphragm material. If other dia- phragm materials are used, the temperature errors change as follows:

Diaphragm material	Change in temperature error of
	remote seal
	Increase in values by
Stainless steel	See previous tables
Hastelloy C4, mat. No. 2.4610	50%
Hastelloy C276, mat. No. 2.4819	50%
Monel 400, mat. No. 2.4360	60%
Tantalum	50%
Titanium	50%
PTFE coating on stainless steel dia-	80%
phragm
ECTFE coating or PFA coating on	100%
stainless steel diaphragm
Gold coating on stainless steel dia-	40%
phragm

Maximum temperature of medium

The following maximum temperatures of the medium apply de- pending on the material of the wetted parts:

Material	pabs < 1 bar		pabs > 1 bar
	(14.5 psi)		(14.5 psi)
	°C	(°F)	°C	(°F)
Stainless steel, 316L	200	(392)	400	(662)
PTFE coating	200	(392)	260	(500)
ECTFE coating	100	(212)	150	(302)
PFA coating	200	(392)	260	(500)
Hastelloy C4, mat. No. 2.4610	200	(392)	260	(500)
Hastelloy C276, mat. No.	200	(392)	400	(662)
2.4819
Monel 400, mat. No. 2.4360	200	(392)	400	(662)
Tantalum	200	(392)	300	(572)

Maximum capillary length for diaphragm seals (guidance values)

Nom. diam.		Max. length of capillary
		Diaphragm seal		Clamp-on seal
		m	(ft)	m	(ft)
DN 25	(1 inch)	2.5	(8.2)	2.5	(8.2)
DN 32	(1¼ inch)	2.5	(8.2)	2.5	(8.2)
DN 40	(1½ inch)	4	(13.1)	6	(19.7)
DN 50	(2 inch)	6	(19.7)	10	(32.8)
DN 65	(2½ inch)	8	(26.2)	10	(32.8)
DN 80	(3 inch)	10	(32.8)	10	(32.8)
DN 100	(4 inch)	10	(32.8)	10	(32.8)
DN 125	(5 inch)	10	(32.8)	-	-

2/156

Siemens FI 01 · 2009