Televac AN 3021: 2A Thermocouple Vacuum Sensor OEM Implementation

1. Scope

1.1 Purpose

The purpose of this document is to explain the basic requirements for acquiring vacuum readings from a Televac 2A thermocouple vacuum sensor.

1.2 Relevant Televac Products

This document applies to the following Televac products:

Description Part Number
2A Thermocouple Sensor – Nickel-Plated Brass 2-2100-10
2A Thermocouple Sensor – Nickel-Plated Brass 2-2100-0XX
2A Thermocouple Sensor – Stainless Steel 2-2126-0XX
2A VacuMini Thermocouple Sensor – Stainless Steel 2-2100-102
2A NASA Mini Thermocouple Sensor – Stainless Steel 2-2100-31

2. Background

Thermal conductivity vacuum sensors, such as the Televac 2A thermocouple, have been in use since the first half of the 20th century. Currently, thermal conductivity sensors such as the thermocouple, pirani, and thermistor types, are the most cost effective solutions for vacuum measurement in the low vacuum range of 1×10-3 Torr to 1×103 Torr. Thermal conductivity sensors rely on the pressure dependent process of heat loss from a heated filament (or multiple filaments) to measure vacuum.

Televac offers a full line of controller and active gauge solutions for the 2A, however, for specific demanding applications these solutions don’t meet all of the necessary requirements. For this reason we provide design support for OEMs to create their own controller solutions for the 2A.

3. Implementing the 2A Control Circuit

The 2A is an easy-to-implement sensor that provides accurate vacuum measurements over a range of 1×10-3 Torr to 1 Torr and indication of pressure up to 20 Torr.

To excite the 2A, provide a constant 95 mA (±0.25 mA) to the heater filament (pins A and D in Figure 1, pins 1 and 8 in Figure 2). The output voltage is measured across a 90.9 Ω (±1%) resistor between pins B and C (Figure 1) or pins 2 and 4 (Figure 2). The conversion from voltage to vacuum values can be found in Section 5.

4. Implementing the 2A Temperature Compensation Circuit

Elements 1 and 2 (Figure 1), or elements 1b and 1c (Figure 2), must be implemented in the circuit to provide temperature compensation of the 2A. These components should be positioned as close to the sensor as possible, optimally at the connection point between the cable and sensor.

Element 1 (Figure 1) and element 1b (Figure 2) is a 33.2 Ω (±1%) 1/8 W resistor. Element 2 (Figure 1) and element 1c (Figure 2) is a proprietary thermistor supplied by Televac.

Figure 1: 2A VacuMini and 2A NASA Mini thermocouple sensor wiring diagram


Figure 2: 2A thermocouple standard sensor wiring diagram

5. Converting 2A Output Voltage to a Vacuum Reading

Table 1 shows the conversion from the voltage measured across pins B and C (Figure 1) or pins 2 and 4 (Figure 2) to vacuum values in Torr.

Output Voltage (V DC) Pressure (Torr)
0.500 100
0.551 20
0.630 1.88
0.667 1.45
0.720 1.00
0.800 0.700
0.905 0.500
1.00 0.400
1.102 0.330
1.148 0.300
1.444 0.200
2.196 0.100
2.538 0.078
2.963 0.056
3.852 0.026
4.293 0.015
4.518 0.010
4.762 0.005
4.970 0.001
5.031 0.000