Mineral-insulated thermocouple sensor for direct insertion through a compression fitting

01

0.15 s at 0.5 mm, 3 s at 6 mm: sheath diameter sets how fast an MI thermocouple reads

JUMO's data sheet 90.1221 for mineral-insulated thermocouples lists response times in water flowing at 0.4 m/s and in air at 2 m/s for each sheath diameter. In water the half-value time t0.5 rises from 0.15 s at 0.5 mm to 3.00 s at 6.0 mm; in air the same sheaths take 3.5 s and 55.0 s. [1]

Editorial calculation: in JUMO's figures the same 6.0 mm sheath takes 55.0 ÷ 3.00 ≈ 18 times longer to reach t0.5 in moving air than in moving water, so a response time quoted without its medium and velocity cannot be compared with another supplier's. [1]

The thinner sheath that reads faster also carries thinner wires. JUMO gives the loop resistance of a single type K element as 126 Ω/m at 0.5 mm and 0.88 Ω/m at 6.0 mm. [1] Editorial calculation: a 1 m run of 0.5 mm type K sheath adds 126 Ω to the loop, while 3 m of 3.0 mm sheath adds 3 × 3.5 = 10.5 Ω; check the input impedance and lead-resistance limits of the controller or transmitter before choosing a long, thin sheath.

JUMO sets a minimum fitting length of 50 mm for 0.5 to 2.0 mm sheaths and 100 mm for 3.0 to 6.0 mm sheaths, with lengths up to 50,000 mm in 50 mm steps. [1]

Give the supplier the medium (liquid, gas or solid), its velocity past the tip, the response time you need and how it is defined (t0.5, t0.9 or 63.2%), the insertion depth, and the instrument the sensor will be wired to.

02

Grounded, insulated or exposed tip: response time traded against electrical isolation

Aircom Instrumentation describes three junction styles in mineral-insulated sheathed thermocouples: a grounded junction is joined to the sheath and gives no electrical isolation with fast response; an ungrounded junction is isolated from the sheath with what Aircom calls "reasonable response time"; an exposed junction sits outside the sheath and gives the fastest response with no isolation. [2]

Aircom's times are for a single element, measured as the time to indicate 63.2% of the change from room temperature to boiling water. [2] Editorial calculation: at these diameters the ungrounded junction takes between 2.0 (0.187 in) and 2.8 (0.375 in) times as long as the grounded one; 0.125 in is about 3.2 mm and 0.250 in about 6.4 mm.

In JUMO's order code the basic version has the thermocouple insulated from the sheath, and an uninsulated assembly, with the thermocouple welded to the tip, is an extra code. JUMO specifies insulation resistance between thermocouple and sheath at ambient temperature of 200 MΩ for lengths up to 1 m and 200 MΩ × m for lengths of 1 m and above. [1] Labfacility's type K fast-response MI sensors are offered with the junction insulated or grounded from the sheath in diameters from 0.5 to 3 mm. [3]

Editorial note: a grounded tip can couple electrical noise or a ground loop from the plant into the measurement, which is why isolated inputs or insulated junctions are asked for where the sheath touches earthed equipment; ask the supplier to state the junction type on the label.

03

Bending the sheath to the port: radius, temperature by diameter and the lead beside the fitting

An MI sheath can be bent on site to reach the measuring point, but the two makers reviewed give different minimum radii: JMS says its sheath can be formed to a radius equal to 2 × the sheath diameter, while JUMO gives a minimum bending radius of 5 × the external diameter. [4] [1] Use the supplier's own figure for the sheath you buy.

Editorial calculation: for a 3 mm sheath the JMS figure gives a 6 mm radius and the JUMO figure 15 mm; for a 6 mm sheath, 12 mm against 30 mm.

Maximum temperature depends on diameter as well as sheath material. Labfacility rates its 310 stainless steel type K sensors from −40 °C to +1100 °C above 1.0 mm diameter and to +750 °C at 1.0 mm and below. [3] JUMO lists type K in Inconel 600 (2.4816) sheath from −200 to +1200 °C and types J and L in stainless steel 1.4541 sheath from −200 to +800 °C. [1]

JMS says its sheath "withstands pressures upwards to 50,000 psi", and lists compression fittings and welded or spring-loaded 1/2 × 1/2 in NPT fittings as process connections in the transition area between the sheath and the cold end. [4] JUMO's sensors are leak-tested with helium at 40 bar at the measurement point. [1] Editorial note: the compression fitting, not the sheath, limits the pressure rating of the seal; ask for the fitting's pressure rating at process temperature and whether the ferrule is metal (permanent) or PTFE (repositionable).

The lead next to the fitting must survive the heat radiating from the pipe or vessel. JUMO lists connecting cables in silicone for −50 to +180 °C, PTFE for −190 to +260 °C and metal braiding for −20 to +350 °C. [1] JMS lists fiberglass, PVC, Teflon, Kapton, high-temperature fiberglass and fiberglass with stainless steel overbraid or flexible armour. [4]

State the process temperature at the tip and the ambient temperature at the fitting, the route and bend radius available, the fitting thread and pressure, the lead type and length, and the termination (bare wires, miniature plug or connector).

04

Send the sheath, junction, fitting and lead together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Thermocouple: [type K, J, T, N; single or dual element; tolerance class]
Sheath: [diameter; material; length; bends]
Junction: [grounded, insulated or exposed]
Response: [t0.5, t0.9 or 63.2%; medium and velocity]
Fitting: [compression or threaded; thread size; pressure; ferrule material]
Lead: [silicone, PTFE, fiberglass or metal braid; length; ambient temperature at the fitting]
Termination: [bare wires, miniature plug, standard plug]
Tests: [insulation resistance; leak test; calibration certificate]

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